Image encoding/decoding method and apparatus, and recording medium for storing bitstream

[0001] This application is a division of U.S. application Ser. No. 17/436,976, filed on Sep. 7, 2021, which is a National Phase Entry Application of PCT Application No. PCT/KR2020/003208, filed on Mar. 6, 2020, which claims priority to Korean Patent Application No. 10-2019-0028930, filed on Mar. 13, 2019, Korean Patent Application No. 10-2019-0030088, filed on Mar. 15, 2019, and Korean Patent Application No. 10-2019-0026970, filed on Mar. 8, 2019, in the Korean Intellectual Property Office, the entire contents of which are hereby incorporated by reference in its entirety.

[0002] The present invention relates to an image encoding/decoding method and apparatus, and a recording medium for storing a bitstream.

[0003] Recently, the demand for high resolution and quality images such as high definition (HD) or ultra-high definition (UHD) images has increased in various applications. As the resolution and quality of images are improved, the amount of data correspondingly increases. This is one of the causes of increase in transmission cost and storage cost when transmitting image data through existing transmission media such as wired or wireless broadband channels or when storing image data. In order to solve such problems with high resolution and quality image data, a high efficiency image encoding/decoding technique is required.

[0004] There are various video compression techniques such as an inter prediction technique of predicting the values of pixels within a current picture from the values of pixels within a preceding picture or a subsequent picture, an intra prediction technique of predicting the values of pixels within a region of a current picture from the values of pixels within another region of the current picture, a transform and quantization technique of compressing the energy of a residual signal, and an entropy coding technique of allocating frequently occurring pixel values with shorter codes and less occurring pixel values with longer codes.

[0005] The present invention may provide a method and apparatus for deriving an MVD by considering an enhanced performance condition, when a symmetric MVD deriving method is used to improve an encoding/decoding efficiency.

[0006] An object of the present invention is to provide a method and apparatus for encoding and decoding a picture, which improve an encoding and decoding efficiency.

[0007] Another object of the present invention is to provide a recording medium storing a bitstream generated by an image decoding method or apparatus according to the present invention.

[0008] An image decoding method according to an embodiment of the present invention comprises obtaining symmetric motion vector difference mode availability information from a bitstream; obtaining first prediction direction's zero motion vector difference information from the bitstream; obtaining symmetric motion vector difference mode information of a current block from the bitstream on the basis of the symmetric motion vector difference mode availability information and the first prediction direction's zero motion vector difference information; obtaining first prediction direction's reference picture index information, second prediction direction's reference picture index information and a first prediction direction's motion vector difference value on the basis of the symmetric motion vector difference mode information; and generating a prediction block of the current block by using at least one of the first prediction direction's reference picture index information, the second prediction direction's reference picture index information and the first prediction direction's motion vector difference value, wherein the obtaining of the first prediction direction's reference picture index information, the second prediction direction's reference picture index information and the first prediction direction's motion vector difference value obtains the first prediction direction's reference picture index information, the second prediction direction's reference picture index information and the first prediction direction's motion vector difference value by deriving rather than decoding from the bitstream when the symmetric motion vector difference mode information of the current block indicates a symmetric motion vector difference mode.

[0009] In the image decoding method of the present invention, wherein the first prediction direction is L1 prediction direction, and the second prediction direction is L0 prediction direction.

[0010] In the image decoding method of the present invention, wherein the first prediction direction's zero motion vector difference information indicates that the motion vector difference value of the first prediction direction is not decoded but derived as (0, 0).

[0011] In the image decoding method of the present invention, wherein the first prediction direction's zero motion vector difference information is obtained at a picture level.

[0012] In the image decoding method of the present invention, wherein the symmetric motion vector difference mode availability information is obtained at a sequence level.

[0013] In the image decoding method of the present invention, wherein, when the symmetric motion vector difference mode information of the current block indicates a symmetric motion vector difference mode, the first prediction direction's motion vector difference value is derived based on a second prediction direction's motion vector difference value of the current block.

[0014] In the image decoding method of the present invention, wherein, when the symmetric motion vector difference mode information of the current block indicates a symmetric motion vector difference mode, the first prediction direction's reference picture index information is derived as an index of a backward reference picture that is nearest to a current picture in a first prediction direction's reference picture list, and the second prediction direction's reference picture index information is derived as an index of a forward reference picture that is nearest to the current picture in a second prediction direction's reference picture list.

[0015] In the image decoding method of the present invention, wherein, when the symmetric motion vector difference mode information of the current block indicates a symmetric motion vector difference mode, the first prediction direction's reference picture index information is derived as an index of a forward reference picture that is nearest to a current picture in a first prediction direction's reference picture list, and the second prediction direction's reference picture index information is derived as an index of a backward reference picture that is nearest to the current picture in a second prediction direction's reference picture list.

[0016] In the image decoding method of the present invention, wherein, when the symmetric motion vector difference mode information of the current block indicates a symmetric motion vector difference mode, the first prediction direction's reference picture index information and the second prediction direction's reference picture index information are derived as an index of a short-term reference picture.

[0017] An image encoding method according to an embodiment of the present invention comprises determining symmetric motion vector difference mode availability information; determining first prediction direction's zero motion vector difference information; encoding symmetric motion vector difference mode information of a current block on the basis of the symmetric motion vector difference mode availability information and the first prediction direction's zero motion vector difference information; and determining, on the basis of the symmetric motion vector difference mode information, whether or not first prediction direction's reference picture index information, second prediction direction's reference picture index information and a first prediction direction's motion vector difference value are encoded, wherein the determining of whether or not the first prediction direction's reference picture index information, the second prediction direction's reference picture index information and the first prediction direction's motion vector difference value are encoded is determined as not encode the first prediction direction's reference picture index information, the second prediction direction's reference picture index information and the first prediction direction's motion vector difference value, when the symmetric motion vector difference mode information of the current block indicates a symmetric motion vector difference mode.

[0018] In the image encoding method of the present invention, wherein the first prediction direction is L1 prediction direction, and the second prediction direction is L0 prediction direction.

[0019] In the image encoding method of the present invention, wherein the first prediction direction's zero motion vector difference information indicates that the motion vector difference value of the first prediction direction is not encoded but derived as (0, 0).

[0020] In the image encoding method of the present invention, wherein the first prediction direction's zero motion vector difference information is encoded at a picture level.

[0021] In the image encoding method of the present invention, wherein the symmetric motion vector difference mode availability information is encoded at a sequence level.

[0022] In the image encoding method of the present invention, wherein, when the symmetric motion vector difference mode information of the current block indicates a symmetric motion vector difference mode, the first prediction direction's motion vector difference value is derived based on a second prediction direction's motion vector difference value of the current block.

[0023] In the image encoding method of the present invention, wherein, when the symmetric motion vector difference mode information of the current block indicates a symmetric motion vector difference mode, the first prediction direction's reference picture index information is derived as an index of a backward reference picture that is nearest to a current picture in a first prediction direction's reference picture list, and the second prediction direction's reference picture index information is derived as an index of a forward reference picture that is nearest to the current picture in a second prediction direction's reference picture list.

[0024] In the image encoding method of the present invention, wherein, when the symmetric motion vector difference mode information of the current block indicates a symmetric motion vector difference mode, the first prediction direction's reference picture index information is derived as an index of a forward reference picture that is nearest to a current picture in a first prediction direction's reference picture list, and the second prediction direction's reference picture index information is derived as an index of a backward reference picture that is nearest to the current picture in a second prediction direction's reference picture list.

[0025] In the image encoding method of the present invention, wherein, when the symmetric motion vector difference mode information of the current block indicates a symmetric motion vector difference mode, the first prediction direction's reference picture index information and the second prediction direction's reference picture index information are derived as an index of a short-term reference picture.

[0026] A non-transitory computer readable recording medium storing a bitstream generated by an image encoding method according to an embodiment of the present invention, wherein the image encoding method comprises determining symmetric motion vector difference mode availability information; determining first prediction direction's zero motion vector difference information; encoding symmetric motion vector difference mode information of a current block on the basis of the symmetric motion vector difference mode availability information and the first prediction direction's zero motion vector difference information; and determining, on the basis of the symmetric motion vector difference mode information, whether or not first prediction direction's reference picture index information, second prediction direction's reference picture index information and a first prediction direction's motion vector difference value are encoded, wherein the determining of whether or not the first prediction direction's reference picture index information, the second prediction direction's reference picture index information and the first prediction direction's motion vector difference value are encoded is determined as not encode the first prediction direction's reference picture index information, the second prediction direction's reference picture index information and the first prediction direction's motion vector difference value, when the symmetric motion vector difference mode information of the current block indicates a symmetric motion vector difference mode.

[0027] According to the present invention, it is possible to provide an image encoding/decoding method and apparatus with improved compression efficiency. The present invention may improve an image encoding/decoding efficiency by providing an MVD deriving method and apparatus considering an enhanced performance condition, when a symmetric MVD deriving method is used.

[0028] According to the present invention, an image encoding/decoding method and apparatus with enhanced encoding and decoding efficiency may be provided.

[0029] In addition, according to the present invention, a recording medium storing a bitstream generated by an image encoding method or apparatus of the present invention may be provided.

[0030] In addition, according to the present invention, a recording medium storing a bitstream, which is received and decoded by a picture decoding apparatus of the present invention and is used to reconstruct a picture, may be provided.

[0046] A variety of modifications may be made to the present invention and there are various embodiments of the present invention, examples of which will now be provided with reference to drawings and described in detail. However, the present invention is not limited thereto, although the exemplary embodiments can be construed as including all modifications, equivalents, or substitutes in a technical concept and a technical scope of the present invention. The similar reference numerals refer to the same or similar functions in various aspects. In the drawings, the shapes and dimensions of elements may be exaggerated for clarity. In the following detailed description of the present invention, references are made to the accompanying drawings that show, by way of illustration, specific embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to implement the present disclosure. It should be understood that various embodiments of the present disclosure, although different, are not necessarily mutually exclusive. For example, specific features, structures, and characteristics described herein, in connection with one embodiment, may be implemented within other embodiments without departing from the spirit and scope of the present disclosure. In addition, it should be understood that the location or arrangement of individual elements within each disclosed embodiment may be modified without departing from the spirit and scope of the present disclosure. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present disclosure is defined only by the appended claims, appropriately interpreted, along with the full range of equivalents to what the claims claim.

[0047] Terms used in the specification, ‘first’, ‘second’, etc. can be used to describe various components, but the components are not to be construed as being limited to the terms. The terms are only used to differentiate one component from other components. For example, the ‘first’ component may be named the ‘second’ component without departing from the scope of the present invention, and the ‘second’ component may also be similarly named the ‘first’ component. The term ‘and/or’ includes a combination of a plurality of items or any one of a plurality of terms.

[0048] It will be understood that when an element is simply referred to as being ‘connected to’ or ‘coupled to’ another element without being ‘directly connected to’ or ‘directly coupled to’ another element in the present description, it may be ‘directly connected to’ or ‘directly coupled to’ another element or be connected to or coupled to another element, having the other element intervening therebetween. In contrast, it should be understood that when an element is referred to as being “directly coupled” or “directly connected” to another element, there are no intervening elements present.

[0049] Furthermore, constitutional parts shown in the embodiments of the present invention are independently shown so as to represent characteristic functions different from each other. Thus, it does not mean that each constitutional part is constituted in a constitutional unit of separated hardware or software. In other words, each constitutional part includes each of enumerated constitutional parts for convenience. Thus, at least two constitutional parts of each constitutional part may be combined to form one constitutional part or one constitutional part may be divided into a plurality of constitutional parts to perform each function. The embodiment where each constitutional part is combined and the embodiment where one constitutional part is divided are also included in the scope of the present invention, if not departing from the essence of the present invention.

[0050] The terms used in the present specification are merely used to describe particular embodiments, and are not intended to limit the present invention. An expression used in the singular encompasses the expression of the plural, unless it has a clearly different meaning in the context. In the present specification, it is to be understood that terms such as “including”, “having”, etc. are intended to indicate the existence of the features, numbers, steps, actions, elements, parts, or combinations thereof disclosed in the specification, and are not intended to preclude the possibility that one or more other features, numbers, steps, actions, elements, parts, or combinations thereof may exist or may be added. In other words, when a specific element is referred to as being “included”, elements other than the corresponding element are not excluded, but additional elements may be included in embodiments of the present invention or the scope of the present invention.

[0051] In addition, some of constituents may not be indispensable constituents performing essential functions of the present invention but be selective constituents improving only performance thereof. The present invention may be implemented by including only the indispensable constitutional parts for implementing the essence of the present invention except the constituents used in improving performance. The structure including only the indispensable constituents except the selective constituents used in improving only performance is also included in the scope of the present invention.

[0052] Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. In describing exemplary embodiments of the present invention, well-known functions or constructions will not be described in detail since they may unnecessarily obscure the understanding of the present invention. The same constituent elements in the drawings are denoted by the same reference numerals, and a repeated description of the same elements will be omitted.

[0053] Hereinafter, an image may mean a picture configuring a video, or may mean the video itself. For example, “encoding or decoding or both of an image” may mean “encoding or decoding or both of a moving picture”, and may mean “encoding or decoding or both of one image among images of a moving picture.”

[0054] Hereinafter, terms “moving picture” and “video” may be used as the same meaning and be replaced with each other.

[0055] Hereinafter, a target image may be an encoding target image which is a target of encoding and/or a decoding target image which is a target of decoding. Also, a target image may be an input image inputted to an encoding apparatus, and an input image inputted to a decoding apparatus. Here, a target image may have the same meaning with the current image.

[0056] Hereinafter, terms “image”, “picture, “frame” and “screen” may be used as the same meaning and be replaced with each other.

[0057] Hereinafter, a target block may be an encoding target block which is a target of encoding and/or a decoding target block which is a target of decoding. Also, a target block may be the current block which is a target of current encoding and/or decoding. For example, terms “target block” and “current block” may be used as the same meaning and be replaced with each other.

[0058] Hereinafter, terms “block” and “unit” may be used as the same meaning and be replaced with each other. Or a “block” may represent a specific unit.

[0059] Hereinafter, terms “region” and “segment” may be replaced with each other.

[0060] Hereinafter, a specific signal may be a signal representing a specific block. For example, an original signal may be a signal representing a target block. A prediction signal may be a signal representing a prediction block. A residual signal may be a signal representing a residual block.

[0061] In embodiments, each of specific information, data, flag, index, element and attribute, etc. may have a value. A value of information, data, flag, index, element and attribute equal to “0” may represent a logical false or the first predefined value. In other words, a value “0”, a false, a logical false and the first predefined value may be replaced with each other. A value of information, data, flag, index, element and attribute equal to “1” may represent a logical true or the second predefined value. In other words, a value “1”, a true, a logical true and the second predefined value may be replaced with each other.

[0062] When a variable i or j is used for representing a column, a row or an index, a value of i may be an integer equal to or greater than 0, or equal to or greater than 1. That is, the column, the row, the index, etc. may be counted from 0 or may be counted from 1.

[0063] Encoder: means an apparatus performing encoding. That is, means an encoding apparatus.

[0064] Decoder: means an apparatus performing decoding. That is, means an decoding apparatus.

[0065] Block: is an M×N array of a sample. Herein, M and N may mean positive integers, and the block may mean a sample array of a two-dimensional form. The block may refer to a unit. A current block my mean an encoding target block that becomes a target when encoding, or a decoding target block that becomes a target when decoding. In addition, the current block may be at least one of an encode block, a prediction block, a residual block, and a transform block.

[0066] Sample: is a basic unit constituting a block. It may be expressed as a value from 0 to 2^Bd−1 according to a bit depth (B_d). In the present invention, the sample may be used as a meaning of a pixel. That is, a sample, a pel, a pixel may have the same meaning with each other.

[0067] Unit: may refer to an encoding and decoding unit. When encoding and decoding an image, the unit may be a region generated by partitioning a single image. In addition, the unit may mean a subdivided unit when a single image is partitioned into subdivided units during encoding or decoding. That is, an image may be partitioned into a plurality of units. When encoding and decoding an image, a predetermined process for each unit may be performed. A single unit may be partitioned into sub-units that have sizes smaller than the size of the unit. Depending on functions, the unit may mean a block, a macroblock, a coding tree unit, a code tree block, a coding unit, a coding block), a prediction unit, a prediction block, a residual unit), a residual block, a transform unit, a transform block, etc. In addition, in order to distinguish a unit from a block, the unit may include a luma component block, a chroma component block associated with the luma component block, and a syntax element of each color component block. The unit may have various sizes and forms, and particularly, the form of the unit may be a two-dimensional geometrical figure such as a square shape, a rectangular shape, a trapezoid shape, a triangular shape, a pentagonal shape, etc. In addition, unit information may include at least one of a unit type indicating the coding unit, the prediction unit, the transform unit, etc., and a unit size, a unit depth, a sequence of encoding and decoding of a unit, etc.

[0068] Coding Tree Unit: is configured with a single coding tree block of a luma component Y, and two coding tree blocks related to chroma components Cb and Cr. In addition, it may mean that including the blocks and a syntax element of each block. Each coding tree unit may be partitioned by using at least one of a quad-tree partitioning method, a binary-tree partitioning method and ternary-tree partitioning method to configure a lower unit such as coding unit, prediction unit, transform unit, etc. It may be used as a term for designating a sample block that becomes a process unit when encoding/decoding an image as an input image. Here, the quad-tree may mean a quarternary-tree.

[0069] When the size of the coding block is within a predetermined range, the division is possible using only quad-tree partitioning. Here, the predetermined range may be defined as at least one of a maximum size and a minimum size of a coding block in which the division is possible using only quad-tree partitioning. Information indicating a maximum/minimum size of a coding block in which quad-tree partitioning is allowed may be signaled through a bitstream, and the information may be signaled in at least one unit of a sequence, a picture parameter, a tile group, or a slice (segment). Alternatively, the maximum/minimum size of the coding block may be a fixed size predetermined in the coder/decoder. For example, when the size of the coding block corresponds to 256×256 to 64×64, the division is possible only using quad-tree partitioning. Alternatively, when the size of the coding block is larger than the size of the maximum conversion block, the division is possible only using quad-tree partitioning. Herein, the block to be divided may be at least one of a coding block and a transform block. In this case, information indicating the division of the coded block (for example, split_flag) may be a flag indicating whether or not to perform the quad-tree partitioning. When the size of the coding block falls within a predetermined range, the division is possible only using binary tree or ternary tree partitioning. In this case, the above description of the quad-tree partitioning may be applied to binary tree partitioning or ternary tree partitioning in the same manner.

[0070] Coding Tree Block: may be used as a term for designating any one of a Y coding tree block, Cb coding tree block, and Cr coding tree block.

[0071] Neighbor Block: may mean a block adjacent to a current block. The block adjacent to the current block may mean a block that comes into contact with a boundary of the current block, or a block positioned within a predetermined distance from the current block. The neighbor block may mean a block adjacent to a vertex of the current block. Herein, the block adjacent to the vertex of the current block may mean a block vertically adjacent to a neighbor block that is horizontally adjacent to the current block, or a block horizontally adjacent to a neighbor block that is vertically adjacent to the current block.

[0072] Reconstructed Neighbor block: may mean a neighbor block adjacent to a current block and which has been already spatially/temporally encoded or decoded. Herein, the reconstructed neighbor block may mean a reconstructed neighbor unit. A reconstructed spatial neighbor block may be a block within a current picture and which has been already reconstructed through encoding or decoding or both. A reconstructed temporal neighbor block is a block at a corresponding position as the current block of the current picture within a reference image, or a neighbor block thereof.

[0073] Unit Depth: may mean a partitioned degree of a unit. In a tree structure, the highest node (Root Node) may correspond to the first unit which is not partitioned. Also, the highest node may have the least depth value. In this case, the highest node may have a depth of level 0. A node having a depth of level 1 may represent a unit generated by partitioning once the first unit. A node having a depth of level 2 may represent a unit generated by partitioning twice the first unit. A node having a depth of level n may represent a unit generated by partitioning n-times the first unit. A Leaf Node may be the lowest node and a node which cannot be partitioned further. A depth of a Leaf Node may be the maximum level. For example, a predefined value of the maximum level may be 3. A depth of a root node may be the lowest and a depth of a leaf node may be the deepest. In addition, when a unit is expressed as a tree structure, a level in which a unit is present may mean a unit depth.

[0074] Bitstream: may mean a bitstream including encoding image information.

[0075] Parameter Set: corresponds to header information among a configuration within a bitstream. At least one of a video parameter set, a sequence parameter set, a picture parameter set, and an adaptation parameter set may be included in a parameter set. In addition, a parameter set may include a slice header, a tile group header, and tile header information. The term “tile group” means a group of tiles and has the same meaning as a slice.

[0076] An adaptation parameter set may mean a parameter set that can be shared by being referred to in different pictures, subpictures, slices, tile groups, tiles, or bricks. In addition, information in an adaptation parameter set may be used by referring to different adaptation parameter sets for a subpicture, a slice, a tile group, a tile, or a brick inside a picture.

[0077] In addition, regarding the adaptation parameter set, different adaptation parameter sets may be referred to by using identifiers of different adaptation parameter sets for a subpicture, a slice, a tile group, a tile, or a brick inside a picture.

[0078] In addition, regarding the adaptation parameter set, different adaptation parameter sets may be referred to by using identifiers of different adaptation parameter sets for a slice, a tile group, a tile, or a brick inside a subpicture.

[0079] In addition, regarding the adaptation parameter set, different adaptation parameter sets may be referred to by using identifiers of different adaptation parameter sets for a tile or a brick inside a slice.

[0080] In addition, regarding the adaptation parameter set, different adaptation parameter sets may be referred to by using identifiers of different adaptation parameter sets for a brick inside a tile.

[0081] Information on an adaptation parameter set identifier may be included in a parameter set or a header of the subpicture, and an adaptation parameter set corresponding to the adaptation parameter set identifier may be used for the subpicture.

[0082] The information on the adaptation parameter set identifier may be included in a parameter set or a header of the tile, and an adaptation parameter set corresponding to the adaptation parameter set identifier may be used for the tile.

[0083] The information on the adaptation parameter set identifier may be included in a header of the brick, and an adaptation parameter set corresponding to the adaptation parameter set identifier may be used for the brick.

[0084] The picture may be partitioned into one or more tile rows and one or more tile columns.

[0085] The subpicture may be partitioned into one or more tile rows and one or more tile columns within a picture. The subpicture may be a region having the form of a rectangle/square within a picture and may include one or more CTUs. In addition, at least one or more tiles/bricks/slices may be included within one subpicture.

[0086] The tile may be a region having the form of a rectangle/square within a picture and may include one or more CTUs. In addition, the tile may be partitioned into one or more bricks.

[0087] The brick may mean one or more CTU rows within a tile. The tile may be partitioned into one or more bricks, and each brick may have at least one or more CTU rows. A tile that is not partitioned into two or more may mean a brick.

[0088] The slice may include one or more tiles within a picture and may include one or more bricks within a tile.

[0089] Parsing: may mean determination of a value of a syntax element by performing entropy decoding, or may mean the entropy decoding itself.

[0090] Symbol: may mean at least one of a syntax element, a coding parameter, and a transform coefficient value of an encoding/decoding target unit. In addition, the symbol may mean an entropy encoding target or an entropy decoding result.

[0091] Prediction Mode: may be information indicating a mode encoded/decoded with intra prediction or a mode encoded/decoded with inter prediction.

[0092] Prediction Unit: may mean a basic unit when performing prediction such as inter-prediction, intra-prediction, inter-compensation, intra-compensation, and motion compensation. A single prediction unit may be partitioned into a plurality of partitions having a smaller size, or may be partitioned into a plurality of lower prediction units. A plurality of partitions may be a basic unit in performing prediction or compensation. A partition which is generated by dividing a prediction unit may also be a prediction unit.

[0093] Prediction Unit Partition: may mean a form obtained by partitioning a prediction unit.

[0094] Reference picture list may refer to a list including one or more reference pictures used for inter prediction or motion compensation. There are several types of usable reference picture lists, including LC (List combined), L0 (List 0), L1 (List 1), L2 (List 2), L3 (List 3).

[0095] Inter prediction indicator may refer to a direction of inter prediction (unidirectional prediction, bidirectional prediction, etc.) of a current block. Alternatively, it may refer to the number of reference pictures used to generate a prediction block of a current block. Alternatively, it may refer to the number of prediction blocks used at the time of performing inter prediction or motion compensation on a current block.

[0096] Prediction list utilization flag indicates whether a prediction block is generated using at least one reference picture in a specific reference picture list. An inter prediction indicator can be derived using a prediction list utilization flag, and conversely, a prediction list utilization flag can be derived using an inter prediction indicator. For example, when the prediction list utilization flag has a first value of zero (0), it means that a reference picture in a reference picture list is not used to generate a prediction block. On the other hand, when the prediction list utilization flag has a second value of one (1), it means that a reference picture list is used to generate a prediction block.

[0097] Reference picture index may refer to an index indicating a specific reference picture in a reference picture list.

[0098] Reference picture may mean a reference picture which is referred to by a specific block for the purposes of inter prediction or motion compensation of the specific block. Alternatively, the reference picture may be a picture including a reference block referred to by a current block for inter prediction or motion compensation. Hereinafter, the terms “reference picture” and “reference image” have the same meaning and can be interchangeably.

[0099] Motion vector may be a two-dimensional vector used for inter prediction or motion compensation. The motion vector may mean an offset between an encoding/decoding target block and a reference block. For example, (mvX, mvY) may represent a motion vector. Here, mvX may represent a horizontal component and mvY may represent a vertical component.

[0100] Search range may be a two-dimensional region which is searched to retrieve a motion vector during inter prediction. For example, the size of the search range may be M×N. Here, M and N are both integers.

[0101] Motion vector candidate may refer to a prediction candidate block or a motion vector of the prediction candidate block when predicting a motion vector. In addition, a motion vector candidate may be included in a motion vector candidate list.

[0102] Motion vector candidate list may mean a list composed of one or more motion vector candidates.

[0103] Motion vector candidate index may mean an indicator indicating a motion vector candidate in a motion vector candidate list. Alternatively, it may be an index of a motion vector predictor.

[0104] Motion information may mean information including at least one of the items including a motion vector, a reference picture index, an inter prediction indicator, a prediction list utilization flag, reference picture list information, a reference picture, a motion vector candidate, a motion vector candidate index, a merge candidate, and a merge index.

[0105] Merge candidate list may mean a list composed of one or more merge candidates.

[0106] Merge candidate may mean a spatial merge candidate, a temporal merge candidate, a combined merge candidate, a combined bi-predictive merge candidate, or a zero merge candidate. The merge candidate may include motion information such as an inter prediction indicator, a reference picture index for each list, a motion vector, a prediction list utilization flag, and an inter prediction indicator.

[0107] Merge index may mean an indicator indicating a merge candidate in a merge candidate list. Alternatively, the merge index may indicate a block from which a merge candidate has been derived, among reconstructed blocks spatially/temporally adjacent to a current block. Alternatively, the merge index may indicate at least one piece of motion information of a merge candidate.

[0108] Transform Unit: may mean a basic unit when performing encoding/decoding such as transform, inverse-transform, quantization, dequantization, transform coefficient encoding/decoding of a residual signal. A single transform unit may be partitioned into a plurality of lower-level transform units having a smaller size. Here, transformation/inverse-transformation may comprise at least one among the first transformation/the first inverse-transformation and the second transformation/the second inverse-transformation.

[0109] Scaling: may mean a process of multiplying a quantized level by a factor. A transform coefficient may be generated by scaling a quantized level. The scaling also may be referred to as dequantization.

[0110] Quantization Parameter: may mean a value used when generating a quantized level using a transform coefficient during quantization. The quantization parameter also may mean a value used when generating a transform coefficient by scaling a quantized level during dequantization. The quantization parameter may be a value mapped on a quantization step size.

[0111] Delta Quantization Parameter: may mean a difference value between a predicted quantization parameter and a quantization parameter of an encoding/decoding target unit.

[0112] Scan: may mean a method of sequencing coefficients within a unit, a block or a matrix. For example, changing a two-dimensional matrix of coefficients into a one-dimensional matrix may be referred to as scanning, and changing a one-dimensional matrix of coefficients into a two-dimensional matrix may be referred to as scanning or inverse scanning.

[0113] Transform Coefficient: may mean a coefficient value generated after transform is performed in an encoder. It may mean a coefficient value generated after at least one of entropy decoding and dequantization is performed in a decoder. A quantized level obtained by quantizing a transform coefficient or a residual signal, or a quantized transform coefficient level also may fall within the meaning of the transform coefficient.

[0114] Quantized Level: may mean a value generated by quantizing a transform coefficient or a residual signal in an encoder. Alternatively, the quantized level may mean a value that is a dequantization target to undergo dequantization in a decoder. Similarly, a quantized transform coefficient level that is a result of transform and quantization also may fall within the meaning of the quantized level.

[0115] Non-zero Transform Coefficient: may mean a transform coefficient having a value other than zero, or a transform coefficient level or a quantized level having a value other than zero.

[0116] Quantization Matrix: may mean a matrix used in a quantization process or a dequantization process performed to improve subjective or objective image quality. The quantization matrix also may be referred to as a scaling list.

[0117] Quantization Matrix Coefficient: may mean each element within a quantization matrix. The quantization matrix coefficient also may be referred to as a matrix coefficient.

[0118] Default Matrix: may mean a predetermined quantization matrix preliminarily defined in an encoder or a decoder.

[0119] Non-default Matrix: may mean a quantization matrix that is not preliminarily defined in an encoder or a decoder but is signaled by a user.

[0120] Statistic Value: a statistic value for at least one among a variable, a coding parameter, a constant value, etc. which have a computable specific value may be one or more among an average value, a sum value, a weighted average value, a weighted sum value, the minimum value, the maximum value, the most frequent value, a median value, an interpolated value of the corresponding specific values.

[0121] FIG. 1 is a block diagram showing a configuration of an encoding apparatus according to an embodiment to which the present invention is applied.

[0122] An encoding apparatus 100 may be an encoder, a video encoding apparatus, or an image encoding apparatus. A video may include at least one image. The encoding apparatus 100 may sequentially encode at least one image.

[0123] Referring to FIG. 1, the encoding apparatus 100 may include a motion prediction unit 111, a motion compensation unit 112, an intra-prediction unit 120, a switch 115, a subtractor 125, a transform unit 130, a quantization unit 140, an entropy encoding unit 150, a dequantization unit 160, an inverse-transform unit 170, an adder 175, a filter unit 180, and a reference picture buffer 190.

[0124] The encoding apparatus 100 may perform encoding of an input image by using an intra mode or an inter mode or both. In addition, encoding apparatus 100 may generate a bitstream including encoded information through encoding the input image, and output the generated bitstream. The generated bitstream may be stored in a computer readable recording medium, or may be streamed through a wired/wireless transmission medium. When an intra mode is used as a prediction mode, the switch 115 may be switched to an intra. Alternatively, when an inter mode is used as a prediction mode, the switch 115 may be switched to an inter mode. Herein, the intra mode may mean an intra-prediction mode, and the inter mode may mean an inter-prediction mode. The encoding apparatus 100 may generate a prediction block for an input block of the input image. In addition, the encoding apparatus 100 may encode a residual block using a residual of the input block and the prediction block after the prediction block being generated. The input image may be called as a current image that is a current encoding target. The input block may be called as a current block that is current encoding target, or as an encoding target block.

[0125] When a prediction mode is an intra mode, the intra-prediction unit 120 may use a sample of a block that has been already encoded/decoded and is adjacent to a current block as a reference sample. The intra-prediction unit 120 may perform spatial prediction for the current block by using a reference sample, or generate prediction samples of an input block by performing spatial prediction. Herein, the intra prediction may mean intra-prediction,

[0126] When a prediction mode is an inter mode, the motion prediction unit 111 may retrieve a region that best matches with an input block from a reference image when performing motion prediction, and deduce a motion vector by using the retrieved region. In this case, a search region may be used as the region. The reference image may be stored in the reference picture buffer 190. Here, when encoding/decoding for the reference image is performed, it may be stored in the reference picture buffer 190.

[0127] The motion compensation unit 112 may generate a prediction block by performing motion compensation for the current block using a motion vector. Herein, inter-prediction may mean inter-prediction or motion compensation.

[0128] When the value of the motion vector is not an integer, the motion prediction unit 111 and the motion compensation unit 112 may generate the prediction block by applying an interpolation filter to a partial region of the reference picture. In order to perform inter-picture prediction or motion compensation on a coding unit, it may be determined that which mode among a skip mode, a merge mode, an advanced motion vector prediction (AMVP) mode, and a current picture referring mode is used for motion prediction and motion compensation of a prediction unit included in the corresponding coding unit. Then, inter-picture prediction or motion compensation may be differently performed depending on the determined mode.

[0129] The subtractor 125 may generate a residual block by using a difference of an input block and a prediction block. The residual block may be called as a residual signal. The residual signal may mean a difference between an original signal and a prediction signal. In addition, the residual signal may be a signal generated by transforming or quantizing, or transforming and quantizing a difference between the original signal and the prediction signal. The residual block may be a residual signal of a block unit.

[0130] The transform unit 130 may generate a transform coefficient by performing transform of a residual block, and output the generated transform coefficient. Herein, the transform coefficient may be a coefficient value generated by performing transform of the residual block. When a transform skip mode is applied, the transform unit 130 may skip transform of the residual block.

[0131] A quantized level may be generated by applying quantization to the transform coefficient or to the residual signal. Hereinafter, the quantized level may be also called as a transform coefficient in embodiments.

[0132] The quantization unit 140 may generate a quantized level by quantizing the transform coefficient or the residual signal according to a parameter, and output the generated quantized level. Herein, the quantization unit 140 may quantize the transform coefficient by using a quantization matrix.

[0133] The entropy encoding unit 150 may generate a bitstream by performing entropy encoding according to a probability distribution on values calculated by the quantization unit 140 or on coding parameter values calculated when performing encoding, and output the generated bitstream. The entropy encoding unit 150 may perform entropy encoding of sample information of an image and information for decoding an image. For example, the information for decoding the image may include a syntax element.

[0134] When entropy encoding is applied, symbols are represented so that a smaller number of bits are assigned to a symbol having a high chance of being generated and a larger number of bits are assigned to a symbol having a low chance of being generated, and thus, the size of bit stream for symbols to be encoded may be decreased. The entropy encoding unit 150 may use an encoding method for entropy encoding such as exponential Golomb, context-adaptive variable length coding (CAVLC), context-adaptive binary arithmetic coding (CABAC), etc. For example, the entropy encoding unit 150 may perform entropy encoding by using a variable length coding/code (VLC) table. In addition, the entropy encoding unit 150 may deduce a binarization method of a target symbol and a probability model of a target symbol/bin, and perform arithmetic coding by using the deduced binarization method, and a context model.

[0135] In order to encode a transform coefficient level (quantized level), the entropy encoding unit 150 may change a two-dimensional block form coefficient into a one-dimensional vector form by using a transform coefficient scanning method.

[0136] A coding parameter may include information (flag, index, etc.) such as syntax element that is encoded in an encoder and signaled to a decoder, and information derived when performing encoding or decoding. The coding parameter may mean information required when encoding or decoding an image. For example, at least one value or a combination form of a unit/block size, a unit/block depth, unit/block partition information, unit/block shape, unit/block partition structure, whether to partition of a quad-tree form, whether to partition of a binary-tree form, a partition direction of a binary-tree form (horizontal direction or vertical direction), a partition form of a binary-tree form (symmetric partition or asymmetric partition), whether or not a current coding unit is partitioned by ternary tree partitioning, direction (horizontal or vertical direction) of the ternary tree partitioning, type (symmetric or asymmetric type) of the ternary tree partitioning, whether a current coding unit is partitioned by multi-type tree partitioning, direction (horizontal or vertical direction) of the multi-type three partitioning, type (symmetric or asymmetric type) of the multi-type tree partitioning, and a tree (binary tree or ternary tree) structure of the multi-type tree partitioning, a prediction mode (intra prediction or inter prediction), a luma intra-prediction mode/direction, a chroma intra-prediction mode/direction, intra partition information, inter partition information, a coding block partition flag, a prediction block partition flag, a transform block partition flag, a reference sample filtering method, a reference sample filter tab, a reference sample filter coefficient, a prediction block filtering method, a prediction block filter tap, a prediction block filter coefficient, a prediction block boundary filtering method, a prediction block boundary filter tab, a prediction block boundary filter coefficient, an intra-prediction mode, an inter-prediction mode, motion information, a motion vector, a motion vector difference, a reference picture index, a inter-prediction angle, an inter-prediction indicator, a prediction list utilization flag, a reference picture list, a reference picture, a motion vector predictor index, a motion vector predictor candidate, a motion vector candidate list, whether to use a merge mode, a merge index, a merge candidate, a merge candidate list, whether to use a skip mode, an interpolation filter type, an interpolation filter tab, an interpolation filter coefficient, a motion vector size, a presentation accuracy of a motion vector, a transform type, a transform size, information of whether or not a primary (first) transform is used, information of whether or not a secondary transform is used, a primary transform index, a secondary transform index, information of whether or not a residual signal is present, a coded block pattern, a coded block flag (CBF), a quantization parameter, a quantization parameter residue, a quantization matrix, whether to apply an intra loop filter, an intra loop filter coefficient, an intra loop filter tab, an intra loop filter shape/form, whether to apply a deblocking filter, a deblocking filter coefficient, a deblocking filter tab, a deblocking filter strength, a deblocking filter shape/form, whether to apply an adaptive sample offset, an adaptive sample offset value, an adaptive sample offset category, an adaptive sample offset type, whether to apply an adaptive loop filter, an adaptive loop filter coefficient, an adaptive loop filter tab, an adaptive loop filter shape/form, a binarization/inverse-binarization method, a context model determining method, a context model updating method, whether to perform a regular mode, whether to perform a bypass mode, a context bin, a bypass bin, a significant coefficient flag, a last significant coefficient flag, a coded flag for a unit of a coefficient group, a position of the last significant coefficient, a flag for whether a value of a coefficient is larger than 1, a flag for whether a value of a coefficient is larger than 2, a flag for whether a value of a coefficient is larger than 3, information on a remaining coefficient value, a sign information, a reconstructed luma sample, a reconstructed chroma sample, a residual luma sample, a residual chroma sample, a luma transform coefficient, a chroma transform coefficient, a quantized luma level, a quantized chroma level, a transform coefficient level scanning method, a size of a motion vector search area at a decoder side, a shape of a motion vector search area at a decoder side, a number of time of a motion vector search at a decoder side, information on a CTU size, information on a minimum block size, information on a maximum block size, information on a maximum block depth, information on a minimum block depth, an image displaying/outputting sequence, slice identification information, a slice type, slice partition information, tile identification information, a tile type, tile partition information, tile group identification information, a tile group type, tile group partition information, a picture type, a bit depth of an input sample, a bit depth of a reconstruction sample, a bit depth of a residual sample, a bit depth of a transform coefficient, a bit depth of a quantized level, and information on a luma signal or information on a chroma signal may be included in the coding parameter.

[0137] Herein, signaling the flag or index may mean that a corresponding flag or index is entropy encoded and included in a bitstream by an encoder, and may mean that the corresponding flag or index is entropy decoded from a bitstream by a decoder.

[0138] When the encoding apparatus 100 performs encoding through inter-prediction, an encoded current image may be used as a reference image for another image that is processed afterwards. Accordingly, the encoding apparatus 100 may reconstruct or decode the encoded current image, or store the reconstructed or decoded image as a reference image in reference picture buffer 190.

[0139] A quantized level may be dequantized in the dequantization unit 160, or may be inverse-transformed in the inverse-transform unit 170. A dequantized or inverse-transformed coefficient or both may be added with a prediction block by the adder 175. By adding the dequantized or inverse-transformed coefficient or both with the prediction block, a reconstructed block may be generated. Herein, the dequantized or inverse-transformed coefficient or both may mean a coefficient on which at least one of dequantization and inverse-transform is performed, and may mean a reconstructed residual block.

[0140] A reconstructed block may pass through the filter unit 180. The filter unit 180 may apply at least one of a deblocking filter, a sample adaptive offset (SAO), and an adaptive loop filter (ALF) to a reconstructed sample, a reconstructed block or a reconstructed image. The filter unit 180 may be called as an in-loop filter.

[0141] The deblocking filter may remove block distortion generated in boundaries between blocks. In order to determine whether or not to apply a deblocking filter, whether or not to apply a deblocking filter to a current block may be determined based samples included in several rows or columns which are included in the block. When a deblocking filter is applied to a block, another filter may be applied according to a required deblocking filtering strength.

[0142] In order to compensate an encoding error, a proper offset value may be added to a sample value by using a sample adaptive offset. The sample adaptive offset may correct an offset of a deblocked image from an original image by a sample unit. A method of partitioning samples of an image into a predetermined number of regions, determining a region to which an offset is applied, and applying the offset to the determined region, or a method of applying an offset in consideration of edge information on each sample may be used.

[0143] The adaptive loop filter may perform filtering based on a comparison result of the filtered reconstructed image and the original image. Samples included in an image may be partitioned into predetermined groups, a filter to be applied to each group may be determined, and differential filtering may be performed for each group. Information of whether or not to apply the ALF may be signaled by coding units (CUs), and a form and coefficient of the ALF to be applied to each block may vary.

[0144] The reconstructed block or the reconstructed image having passed through the filter unit 180 may be stored in the reference picture buffer 190. A reconstructed block processed by the filter unit 180 may be a part of a reference image. That is, a reference image is a reconstructed image composed of reconstructed blocks processed by the filter unit 180. The stored reference image may be used later in inter prediction or motion compensation.

[0145] FIG. 2 is a block diagram showing a configuration of a decoding apparatus according to an embodiment and to which the present invention is applied.

[0146] A decoding apparatus 200 may a decoder, a video decoding apparatus, or an image decoding apparatus.

[0147] Referring to FIG. 2, the decoding apparatus 200 may include an entropy decoding unit 210, a dequantization unit 220, an inverse-transform unit 230, an intra-prediction unit 240, a motion compensation unit 250, an adder 225, a filter unit 260, and a reference picture buffer 270.

[0148] The decoding apparatus 200 may receive a bitstream output from the encoding apparatus 100. The decoding apparatus 200 may receive a bitstream stored in a computer readable recording medium, or may receive a bitstream that is streamed through a wired/wireless transmission medium. The decoding apparatus 200 may decode the bitstream by using an intra mode or an inter mode. In addition, the decoding apparatus 200 may generate a reconstructed image generated through decoding or a decoded image, and output the reconstructed image or decoded image.

[0149] When a prediction mode used when decoding is an intra mode, a switch may be switched to an intra. Alternatively, when a prediction mode used when decoding is an inter mode, a switch may be switched to an inter mode.

[0150] The decoding apparatus 200 may obtain a reconstructed residual block by decoding the input bitstream, and generate a prediction block. When the reconstructed residual block and the prediction block are obtained, the decoding apparatus 200 may generate a reconstructed block that becomes a decoding target by adding the reconstructed residual block with the prediction block. The decoding target block may be called a current block.

[0151] The entropy decoding unit 210 may generate symbols by entropy decoding the bitstream according to a probability distribution. The generated symbols may include a symbol of a quantized level form. Herein, an entropy decoding method may be an inverse-process of the entropy encoding method described above.

[0152] In order to decode a transform coefficient level (quantized level), the entropy decoding unit 210 may change a one-directional vector form coefficient into a two-dimensional block form by using a transform coefficient scanning method.

[0153] A quantized level may be dequantized in the dequantization unit 220, or inverse-transformed in the inverse-transform unit 230. The quantized level may be a result of dequantizing or inverse-transforming or both, and may be generated as a reconstructed residual block. Herein, the dequantization unit 220 may apply a quantization matrix to the quantized level.

[0154] When an intra mode is used, the intra-prediction unit 240 may generate a prediction block by performing, for the current block, spatial prediction that uses a sample value of a block adjacent to a decoding target block and which has been already decoded.

[0155] When an inter mode is used, the motion compensation unit 250 may generate a prediction block by performing, for the current block, motion compensation that uses a motion vector and a reference image stored in the reference picture buffer 270.

[0156] The adder 225 may generate a reconstructed block by adding the reconstructed residual block with the prediction block. The filter unit 260 may apply at least one of a deblocking filter, a sample adaptive offset, and an adaptive loop filter to the reconstructed block or reconstructed image. The filter unit 260 may output the reconstructed image. The reconstructed block or reconstructed image may be stored in the reference picture buffer 270 and used when performing inter-prediction. A reconstructed block processed by the filter unit 260 may be a part of a reference image. That is, a reference image is a reconstructed image composed of reconstructed blocks processed by the filter unit 260. The stored reference image may be used later in inter prediction or motion compensation.

[0157] FIG. 3 is a view schematically showing a partition structure of an image when encoding and decoding the image. FIG. 3 schematically shows an example of partitioning a single unit into a plurality of lower units.

[0158] In order to efficiently partition an image, when encoding and decoding, a coding unit (CU) may be used. The coding unit may be used as a basic unit when encoding/decoding the image. In addition, the coding unit may be used as a unit for distinguishing an intra prediction mode and an inter prediction mode when encoding/decoding the image. The coding unit may be a basic unit used for prediction, transform, quantization, inverse-transform, dequantization, or an encoding/decoding process of a transform coefficient.

[0159] Referring to FIG. 3, an image 300 is sequentially partitioned in a largest coding unit (LCU), and a LCU unit is determined as a partition structure. Herein, the LCU may be used in the same meaning as a coding tree unit (CTU). A unit partitioning may mean partitioning a block associated with to the unit. In block partition information, information of a unit depth may be included. Depth information may represent a number of times or a degree or both in which a unit is partitioned. A single unit may be partitioned into a plurality of lower level units hierarchically associated with depth information based on a tree structure. In other words, a unit and a lower level unit generated by partitioning the unit may correspond to a node and a child node of the node, respectively. Each of partitioned lower unit may have depth information. Depth information may be information representing a size of a CU, and may be stored in each CU. Unit depth represents times and/or degrees related to partitioning a unit. Therefore, partitioning information of a lower-level unit may comprise information on a size of the lower-level unit.

[0160] A partition structure may mean a distribution of a coding unit (CU) within an LCU 310. Such a distribution may be determined according to whether or not to partition a single CU into a plurality (positive integer equal to or greater than 2 including 2, 4, 8, 16, etc.) of CUs. A horizontal size and a vertical size of the CU generated by partitioning may respectively be half of a horizontal size and a vertical size of the CU before partitioning, or may respectively have sizes smaller than a horizontal size and a vertical size before partitioning according to a number of times of partitioning. The CU may be recursively partitioned into a plurality of CUs. By the recursive partitioning, at least one among a height and a width of a CU after partitioning may decrease comparing with at least one among a height and a width of a CU before partitioning. Partitioning of the CU may be recursively performed until to a predefined depth or predefined size. For example, a depth of an LCU may be 0, and a depth of a smallest coding unit (SCU) may be a predefined maximum depth. Herein, the LCU may be a coding unit having a maximum coding unit size, and the SCU may be a coding unit having a minimum coding unit size as described above. Partitioning is started from the LCU 310, a CU depth increases by 1 as a horizontal size or a vertical size or both of the CU decreases by partitioning. For example, for each depth, a CU which is not partitioned may have a size of 2N×2N. Also, in case of a CU which is partitioned, a CU with a size of 2N×2N may be partitioned into four CUs with a size of N×N. A size of N may decrease to half as a depth increase by 1.

[0161] In addition, information whether or not the CU is partitioned may be represented by using partition information of the CU. The partition information may be 1-bit information. All CUs, except for a SCU, may include partition information. For example, when a value of partition information is a first value, the CU may not be partitioned, when a value of partition information is a second value, the CU may be partitioned

[0162] Referring to FIG. 3, an LCU having a depth 0 may be a 64×64 block. 0 may be a minimum depth. A SCU having a depth 3 may be an 8×8 block. 3 may be a maximum depth. A CU of a 32×32 block and a 16×16 block may be respectively represented as a depth 1 and a depth 2.

[0163] For example, when a single coding unit is partitioned into four coding units, a horizontal size and a vertical size of the four partitioned coding units may be a half size of a horizontal and vertical size of the CU before being partitioned. In one embodiment, when a coding unit having a 32×32 size is partitioned into four coding units, each of the four partitioned coding units may have a 16×16 size. When a single coding unit is partitioned into four coding units, it may be called that the coding unit may be partitioned into a quad-tree form.

[0164] For example, when one coding unit is partitioned into two sub-coding units, the horizontal or vertical size (width or height) of each of the two sub-coding units may be half the horizontal or vertical size of the original coding unit. For example, when a coding unit having a size of 32×32 is vertically partitioned into two sub-coding units, each of the two sub-coding units may have a size of 16×32. For example, when a coding unit having a size of 8×32 is horizontally partitioned into two sub-coding units, each of the two sub-coding units may have a size of 8×16. When one coding unit is partitioned into two sub-coding units, it can be said that the coding unit is binary-partitioned or is partitioned by a binary tree partition structure.

[0165] For example, when one coding unit is partitioned into three sub-coding units, the horizontal or vertical size of the coding unit can be partitioned with a ratio of 1:2:1, thereby producing three sub-coding units whose horizontal or vertical sizes are in a ratio of 1:2:1. For example, when a coding unit having a size of 16×32 is horizontally partitioned into three sub-coding units, the three sub-coding units may have sizes of 16×8, 16×16, and 16×8 respectively, in the order from the uppermost to the lowermost sub-coding unit. For example, when a coding unit having a size of 32×32 is vertically split into three sub-coding units, the three sub-coding units may have sizes of 8×32, 16×32, and 8×32, respectively in the order from the left to the right sub-coding unit. When one coding unit is partitioned into three sub-coding units, it can be said that the coding unit is ternary-partitioned or partitioned by a ternary tree partition structure.

[0166] In FIG. 3, a coding tree unit (CTU) 320 is an example of a CTU to which a quad tree partition structure, a binary tree partition structure, and a ternary tree partition structure are all applied.

[0167] As described above, in order to partition the CTU, at least one of a quad tree partition structure, a binary tree partition structure, and a ternary tree partition structure may be applied. Various tree partition structures may be sequentially applied to the CTU, according to a predetermined priority order. For example, the quad tree partition structure may be preferentially applied to the CTU. A coding unit that cannot be partitioned any longer using a quad tree partition structure may correspond to a leaf node of a quad tree. A coding unit corresponding to a leaf node of a quad tree may serve as a root node of a binary and/or ternary tree partition structure. That is, a coding unit corresponding to a leaf node of a quad tree may be further partitioned by a binary tree partition structure or a ternary tree partition structure, or may not be further partitioned. Therefore, by preventing a coding block that results from binary tree partitioning or ternary tree partitioning of a coding unit corresponding to a leaf node of a quad tree from undergoing further quad tree partitioning, block partitioning and/or signaling of partition information can be effectively performed.

[0168] The fact that a coding unit corresponding to a node of a quad tree is partitioned may be signaled using quad partition information. The quad partition information having a first value (e.g., “1”) may indicate that a current coding unit is partitioned by the quad tree partition structure. The quad partition information having a second value (e.g., “0”) may indicate that a current coding unit is not partitioned by the quad tree partition structure. The quad partition information may be a flag having a predetermined length (e.g., one bit).

[0169] There may not be a priority between the binary tree partitioning and the ternary tree partitioning. That is, a coding unit corresponding to a leaf node of a quad tree may further undergo arbitrary partitioning among the binary tree partitioning and the ternary tree partitioning. In addition, a coding unit generated through the binary tree partitioning or the ternary tree partitioning may undergo a further binary tree partitioning or a further ternary tree partitioning, or may not be further partitioned.

[0170] A tree structure in which there is no priority among the binary tree partitioning and the ternary tree partitioning is referred to as a multi-type tree structure. A coding unit corresponding to a leaf node of a quad tree may serve as a root node of a multi-type tree. Whether to partition a coding unit which corresponds to a node of a multi-type tree may be signaled using at least one of multi-type tree partition indication information, partition direction information, and partition tree information. For partitioning of a coding unit corresponding to a node of a multi-type tree, the multi-type tree partition indication information, the partition direction, and the partition tree information may be sequentially signaled.

[0171] The multi-type tree partition indication information having a first value (e.g., “1”) may indicate that a current coding unit is to undergo a multi-type tree partitioning. The multi-type tree partition indication information having a second value (e.g., “0”) may indicate that a current coding unit is not to undergo a multi-type tree partitioning.

[0172] When a coding unit corresponding to a node of a multi-type tree is further partitioned by a multi-type tree partition structure, the coding unit may include partition direction information. The partition direction information may indicate in which direction a current coding unit is to be partitioned for the multi-type tree partitioning. The partition direction information having a first value (e.g., “1”) may indicate that a current coding unit is to be vertically partitioned. The partition direction information having a second value (e.g., “0”) may indicate that a current coding unit is to be horizontally partitioned.

[0173] When a coding unit corresponding to a node of a multi-type tree is further partitioned by a multi-type tree partition structure, the current coding unit may include partition tree information. The partition tree information may indicate a tree partition structure which is to be used for partitioning of a node of a multi-type tree. The partition tree information having a first value (e.g., “1”) may indicate that a current coding unit is to be partitioned by a binary tree partition structure. The partition tree information having a second value (e.g., “0”) may indicate that a current coding unit is to be partitioned by a ternary tree partition structure.

[0174] The partition indication information, the partition tree information, and the partition direction information may each be a flag having a predetermined length (e.g., one bit).

[0175] At least any one of the quadtree partition indication information, the multi-type tree partition indication information, the partition direction information, and the partition tree information may be entropy encoded/decoded. For the entropy-encoding/decoding of those types of information, information on a neighboring coding unit adjacent to the current coding unit may be used. For example, there is a high probability that the partition type (the partitioned or non-partitioned, the partition tree, and/or the partition direction) of a left neighboring coding unit and/or an upper neighboring coding unit of a current coding unit is similar to that of the current coding unit. Therefore, context information for entropy encoding/decoding of the information on the current coding unit may be derived from the information on the neighboring coding units. The information on the neighboring coding units may include at least any one of quad partition information, multi-type tree partition indication information, partition direction information, and partition tree information.

[0176] As another example, among binary tree partitioning and ternary tree partitioning, binary tree partitioning may be preferentially performed. That is, a current coding unit may primarily undergo binary tree partitioning, and then a coding unit corresponding to a leaf node of a binary tree may be set as a root node for ternary tree partitioning. In this case, neither quad tree partitioning nor binary tree partitioning may not be performed on the coding unit corresponding to a node of a ternary tree.

[0177] A coding unit that cannot be partitioned by a quad tree partition structure, a binary tree partition structure, and/or a ternary tree partition structure becomes a basic unit for coding, prediction and/or transformation. That is, the coding unit cannot be further partitioned for prediction and/or transformation. Therefore, the partition structure information and the partition information used for partitioning a coding unit into prediction units and/or transformation units may not be present in a bit stream.

[0178] However, when the size of a coding unit (i.e., a basic unit for partitioning) is larger than the size of a maximum transformation block, the coding unit may be recursively partitioned until the size of the coding unit is reduced to be equal to or smaller than the size of the maximum transformation block. For example, when the size of a coding unit is 64×64 and when the size of a maximum transformation block is 32×32, the coding unit may be partitioned into four 32×32 blocks for transformation. For example, when the size of a coding unit is 32×64 and the size of a maximum transformation block is 32×32, the coding unit may be partitioned into two 32×32 blocks for the transformation. In this case, the partitioning of the coding unit for transformation is not signaled separately, and may be determined through comparison between the horizontal or vertical size of the coding unit and the horizontal or vertical size of the maximum transformation block. For example, when the horizontal size (width) of the coding unit is larger than the horizontal size (width) of the maximum transformation block, the coding unit may be vertically bisected. For example, when the vertical size (height) of the coding unit is larger than the vertical size (height) of the maximum transformation block, the coding unit may be horizontally bisected.

[0179] Information of the maximum and/or minimum size of the coding unit and information of the maximum and/or minimum size of the transformation block may be signaled or determined at an upper level of the coding unit. The upper level may be, for example, a sequence level, a picture level, a slice level, a tile group level, a tile level, or the like. For example, the minimum size of the coding unit may be determined to be 4×4. For example, the maximum size of the transformation block may be determined to be 64×64. For example, the minimum size of the transformation block may be determined to be 4×4.

[0180] Information of the minimum size (quad tree minimum size) of a coding unit corresponding to a leaf node of a quad tree and/or information of the maximum depth (the maximum tree depth of a multi-type tree) from a root node to a leaf node of the multi-type tree may be signaled or determined at an upper level of the coding unit. For example, the upper level may be a sequence level, a picture level, a slice level, a tile group level, a tile level, or the like. Information of the minimum size of a quad tree and/or information of the maximum depth of a multi-type tree may be signaled or determined for each of an intra-picture slice and an inter-picture slice.

[0181] Difference information between the size of a CTU and the maximum size of a transformation block may be signaled or determined at an upper level of the coding unit. For example, the upper level may be a sequence level, a picture level, a slice level, a tile group level, a tile level, or the like. Information of the maximum size of the coding units corresponding to the respective nodes of a binary tree (hereinafter, referred to as a maximum size of a binary tree) may be determined based on the size of the coding tree unit and the difference information. The maximum size of the coding units corresponding to the respective nodes of a ternary tree (hereinafter, referred to as a maximum size of a ternary tree) may vary depending on the type of slice. For example, for an intra-picture slice, the maximum size of a ternary tree may be 32×32. For example, for an inter-picture slice, the maximum size of a ternary tree may be 128×128. For example, the minimum size of the coding units corresponding to the respective nodes of a binary tree (hereinafter, referred to as a minimum size of a binary tree) and/or the minimum size of the coding units corresponding to the respective nodes of a ternary tree (hereinafter, referred to as a minimum size of a ternary tree) may be set as the minimum size of a coding block.

[0182] As another example, the maximum size of a binary tree and/or the maximum size of a ternary tree may be signaled or determined at the slice level. Alternatively, the minimum size of the binary tree and/or the minimum size of the ternary tree may be signaled or determined at the slice level.

[0183] Depending on size and depth information of the above-described various blocks, quad partition information, multi-type tree partition indication information, partition tree information and/or partition direction information may be included or may not be included in a bit stream.

[0184] For example, when the size of the coding unit is not larger than the minimum size of a quad tree, the coding unit does not contain quad partition information. Thus, the quad partition information may be deduced from a second value.

[0185] For example, when the sizes (horizontal and vertical sizes) of a coding unit corresponding to a node of a multi-type tree are larger than the maximum sizes (horizontal and vertical sizes) of a binary tree and/or the maximum sizes (horizontal and vertical sizes) of a ternary tree, the coding unit may not be binary-partitioned or ternary-partitioned. Accordingly, the multi-type tree partition indication information may not be signaled but may be deduced from a second value.

[0186] Alternatively, when the sizes (horizontal and vertical sizes) of a coding unit corresponding to a node of a multi-type tree are the same as the maximum sizes (horizontal and vertical sizes) of a binary tree and/or are two times as large as the maximum sizes (horizontal and vertical sizes) of a ternary tree, the coding unit may not be further binary-partitioned or ternary-partitioned. Accordingly, the multi-type tree partition indication information may not be signaled but be derived from a second value. This is because when a coding unit is partitioned by a binary tree partition structure and/or a ternary tree partition structure, a coding unit smaller than the minimum size of a binary tree and/or the minimum size of a ternary tree is generated.

[0187] Alternatively, the binary tree partitioning or the ternary tree partitioning may be limited on the basis of the size of a virtual pipeline data unit (hereinafter, a pipeline buffer size). For example, when the coding unit is divided into sub-coding units which do not fit the pipeline buffer size by the binary tree partitioning or the ternary tree partitioning, the corresponding binary tree partitioning or ternary tree partitioning may be limited. The pipeline buffer size may be the size of the maximum transform block (e.g., 64×64). For example, when the pipeline buffer size is 64×64, the division below may be limited.

• • [0188] N×M (N and/or M is 128) Ternary tree partitioning for coding units
  • [0189] 128×N (N<=64) Binary tree partitioning in horizontal direction for coding units
  • [0190] N×128 (N<=64) Binary tree partitioning in vertical direction for coding units

[0191] Alternatively, when the depth of a coding unit corresponding to a node of a multi-type tree is equal to the maximum depth of the multi-type tree, the coding unit may not be further binary-partitioned and/or ternary-partitioned. Accordingly, the multi-type tree partition indication information may not be signaled but may be deduced from a second value.

[0192] Alternatively, only when at least one of vertical direction binary tree partitioning, horizontal direction binary tree partitioning, vertical direction ternary tree partitioning, and horizontal direction ternary tree partitioning is possible for a coding unit corresponding to a node of a multi-type tree, the multi-type tree partition indication information may be signaled. Otherwise, the coding unit may not be binary-partitioned and/or ternary-partitioned. Accordingly, the multi-type tree partition indication information may not be signaled but may be deduced from a second value.

[0193] Alternatively, only when both of the vertical direction binary tree partitioning and the horizontal direction binary tree partitioning or both of the vertical direction ternary tree partitioning and the horizontal direction ternary tree partitioning are possible for a coding unit corresponding to a node of a multi-type tree, the partition direction information may be signaled. Otherwise, the partition direction information may not be signaled but may be derived from a value indicating possible partitioning directions.

[0194] Alternatively, only when both of the vertical direction binary tree partitioning and the vertical direction ternary tree partitioning or both of the horizontal direction binary tree partitioning and the horizontal direction ternary tree partitioning are possible for a coding tree corresponding to a node of a multi-type tree, the partition tree information may be signaled. Otherwise, the partition tree information may not be signaled but be deduced from a value indicating a possible partitioning tree structure.

[0195] FIG. 4 is a view showing an intra-prediction process.

[0196] Arrows from center to outside in FIG. 4 may represent prediction directions of intra prediction modes.

[0197] Intra encoding and/or decoding may be performed by using a reference sample of a neighbor block of the current block. A neighbor block may be a reconstructed neighbor block. For example, intra encoding and/or decoding may be performed by using a coding parameter or a value of a reference sample included in a reconstructed neighbor block.

[0198] A prediction block may mean a block generated by performing intra prediction. A prediction block may correspond to at least one among CU, PU and TU. A unit of a prediction block may have a size of one among CU, PU and TU. A prediction block may be a square block having a size of 2×2, 4×4, 16×16, 32×32 or 64×64 etc. or may be a rectangular block having a size of 2×8, 4×8, 2×16, 4×16 and 8×16 etc.

[0199] Intra prediction may be performed according to intra prediction mode for the current block. The number of intra prediction modes which the current block may have may be a fixed value and may be a value determined differently according to an attribute of a prediction block. For example, an attribute of a prediction block may comprise a size of a prediction block and a shape of a prediction block, etc.

[0200] The number of intra-prediction modes may be fixed to N regardless of a block size. Or, the number of intra prediction modes may be 3, 5, 9, 17, 34, 35, 36, 65, or 67 etc. Alternatively, the number of intra-prediction modes may vary according to a block size or a color component type or both. For example, the number of intra prediction modes may vary according to whether the color component is a luma signal or a chroma signal. For example, as a block size becomes large, a number of intra-prediction modes may increase. Alternatively, a number of intra-prediction modes of a luma component block may be larger than a number of intra-prediction modes of a chroma component block.

[0201] An intra-prediction mode may be a non-angular mode or an angular mode. The non-angular mode may be a DC mode or a planar mode, and the angular mode may be a prediction mode having a specific direction or angle. The intra-prediction mode may be expressed by at least one of a mode number, a mode value, a mode numeral, a mode angle, and mode direction. A number of intra-prediction modes may be M, which is larger than 1, including the non-angular and the angular mode. In order to intra-predict a current block, a step of determining whether or not samples included in a reconstructed neighbor block may be used as reference samples of the current block may be performed. When a sample that is not usable as a reference sample of the current block is present, a value obtained by duplicating or performing interpolation on at least one sample value among samples included in the reconstructed neighbor block or both may be used to replace with a non-usable sample value of a sample, thus the replaced sample value is used as a reference sample of the current block.

[0202] FIG. 7 is a diagram illustrating reference samples capable of being used for intra prediction.

[0203] As shown in FIG. 7, at least one of the reference sample line 0 to the reference sample line 3 may be used for intra prediction of the current block. In FIG. 7, the samples of a segment A and a segment F may be padded with the samples closest to a segment B and a segment E, respectively, instead of retrieving from the reconstructed neighboring block. Index information indicating the reference sample line to be used for intra prediction of the current block may be signaled. For example, in FIG. 7, reference sample line indicators 0, 1, and 2 may be signaled as index information indicating reference sample lines 0, 1 and 2. When the upper boundary of the current block is the boundary of the CTU, only the reference sample line 0 may be available. Therefore, in this case, the index information may not be signaled. When a reference sample line other than the reference sample line 0 is used, filtering for a prediction block, which will be described later, may not be performed.

[0204] When intra-predicting, a filter may be applied to at least one of a reference sample and a prediction sample based on an intra-prediction mode and a current block size.

[0205] In case of a planar mode, when generating a prediction block of a current block, according to a position of a prediction target sample within a prediction block, a sample value of the prediction target sample may be generated by using a weighted sum of an upper and left side reference sample of a current sample, and a right upper side and left lower side reference sample of the current block. In addition, in case of a DC mode, when generating a prediction block of a current block, an average value of upper side and left side reference samples of the current block may be used. In addition, in case of an angular mode, a prediction block may be generated by using an upper side, a left side, a right upper side, and/or a left lower side reference sample of the current block. In order to generate a prediction sample value, interpolation of a real number unit may be performed.

[0206] In the case of intra prediction between color components, a prediction block for the current block of the second color component may be generated on the basis of the corresponding reconstructed block of the first color component. For example, the first color component may be a luma component, and the second color component may be a chroma component. For intra prediction between color components, the parameters of the linear model between the first color component and the second color component may be derived on the basis of the template. The template may include upper and/or left neighboring samples of the current block and upper and/or left neighboring samples of the reconstructed block of the first color component corresponding thereto. For example, the parameters of the linear model may be derived using a sample value of a first color component having a maximum value among samples in a template and a sample value of a second color component corresponding thereto, and a sample value of a first color component having a minimum value among samples in the template and a sample value of a second color component corresponding thereto. When the parameters of the linear model are derived, a corresponding reconstructed block may be applied to the linear model to generate a prediction block for the current block. According to a video format, subsampling may be performed on the neighboring samples of the reconstructed block of the first color component and the corresponding reconstructed block. For example, when one sample of the second color component corresponds to four samples of the first color component, four samples of the first color component may be sub-sampled to compute one corresponding sample. In this case, the parameter derivation of the linear model and intra prediction between color components may be performed on the basis of the corresponding sub-sampled samples. Whether or not to perform intra prediction between color components and/or the range of the template may be signaled as the intra prediction mode.

[0207] The current block may be partitioned into two or four sub-blocks in the horizontal or vertical direction. The partitioned sub-blocks may be sequentially reconstructed. That is, the intra prediction may be performed on the sub-block to generate the sub-prediction block. In addition, dequantization and/or inverse transform may be performed on the sub-blocks to generate sub-residual blocks. A reconstructed sub-block may be generated by adding the sub-prediction block to the sub-residual block. The reconstructed sub-block may be used as a reference sample for intra prediction of the sub-sub-blocks. The sub-block may be a block including a predetermined number (for example, 16) or more samples. Accordingly, for example, when the current block is an 8×4 block or a 4×8 block, the current block may be partitioned into two sub-blocks. Also, when the current block is a 4×4 block, the current block may not be partitioned into sub-blocks. When the current block has other sizes, the current block may be partitioned into four sub-blocks. Information on whether or not to perform the intra prediction based on the sub-blocks and/or the partitioning direction (horizontal or vertical) may be signaled. The intra prediction based on the sub-blocks may be limited to be performed only when reference sample line 0 is used. When the intra prediction based on the sub-block is performed, filtering for the prediction block, which will be described later, may not be performed.

[0208] The final prediction block may be generated by performing filtering on the prediction block that is intra-predicted. The filtering may be performed by applying predetermined weights to the filtering target sample, the left reference sample, the upper reference sample, and/or the upper left reference sample. The weight and/or the reference sample (range, position, etc.) used for the filtering may be determined on the basis of at least one of a block size, an intra prediction mode, and a position of the filtering target sample in the prediction block. The filtering may be performed only in the case of a predetermined intra prediction mode (e.g., DC, planar, vertical, horizontal, diagonal, and/or adjacent diagonal modes). The adjacent diagonal mode may be a mode in which k is added to or subtracted from the diagonal mode. For example, k may be a positive integer of 8 or less.

[0209] An intra-prediction mode of a current block may be entropy encoded/decoded by predicting an intra-prediction mode of a block present adjacent to the current block. When intra-prediction modes of the current block and the neighbor block are identical, information that the intra-prediction modes of the current block and the neighbor block are identical may be signaled by using predetermined flag information. In addition, indicator information of an intra-prediction mode that is identical to the intra-prediction mode of the current block among intra-prediction modes of a plurality of neighbor blocks may be signaled. When intra-prediction modes of the current block and the neighbor block are different, intra-prediction mode information of the current block may be entropy encoded/decoded by performing entropy encoding/decoding based on the intra-prediction mode of the neighbor block.

[0210] FIG. 5 is a diagram illustrating an embodiment of an inter-picture prediction process.

[0211] In FIG. 5, a rectangle may represent a picture. In FIG. 5, an arrow represents a prediction direction. Pictures may be categorized into intra pictures (I pictures), predictive pictures (P pictures), and Bi-predictive pictures (B pictures) according to the encoding type thereof.

[0212] The I picture may be encoded through intra-prediction without requiring inter-picture prediction. The P picture may be encoded through inter-picture prediction by using a reference picture that is present in one direction (i.e., forward direction or backward direction) with respect to a current block. The B picture may be encoded through inter-picture prediction by using reference pictures that are preset in two directions (i.e., forward direction and backward direction) with respect to a current block. When the inter-picture prediction is used, the encoder may perform inter-picture prediction or motion compensation and the decoder may perform the corresponding motion compensation.

[0213] Hereinbelow, an embodiment of the inter-picture prediction will be described in detail.

[0214] The inter-picture prediction or motion compensation may be performed using a reference picture and motion information.

[0215] Motion information of a current block may be derived during inter-picture prediction by each of the encoding apparatus 100 and the decoding apparatus 200. The motion information of the current block may be derived by using motion information of a reconstructed neighboring block, motion information of a collocated block (also referred to as a col block or a co-located block), and/or a block adjacent to the co-located block. The co-located block may mean a block that is located spatially at the same position as the current block, within a previously reconstructed collocated picture (also referred to as a col picture or a co-located picture). The co-located picture may be one picture among one or more reference pictures included in a reference picture list.

[0216] The derivation method of the motion information may be different depending on the prediction mode of the current block. For example, a prediction mode applied for inter prediction includes an AMVP mode, a merge mode, a skip mode, a merge mode with a motion vector difference, a subblock merge mode, a geometric partitioning mode, an combined inter intra prediction mode, affine mode, and the like. Herein, the merge mode may be referred to as a motion merge mode.

[0217] For example, when the AMVP is used as the prediction mode, at least one of motion vectors of the reconstructed neighboring blocks, motion vectors of the co-located blocks, motion vectors of blocks adjacent to the co-located blocks, and a (0, 0) motion vector may be determined as motion vector candidates for the current block, and a motion vector candidate list is generated by using the emotion vector candidates. The motion vector candidate of the current block can be derived by using the generated motion vector candidate list. The motion information of the current block may be determined based on the derived motion vector candidate. The motion vectors of the collocated blocks or the motion vectors of the blocks adjacent to the collocated blocks may be referred to as temporal motion vector candidates, and the motion vectors of the reconstructed neighboring blocks may be referred to as spatial motion vector candidates.

[0218] The encoding apparatus 100 may calculate a motion vector difference (MVD) between the motion vector of the current block and the motion vector candidate and may perform entropy encoding on the motion vector difference (MVD). In addition, the encoding apparatus 100 may perform entropy encoding on a motion vector candidate index and generate a bitstream. The motion vector candidate index may indicate an optimum motion vector candidate among the motion vector candidates included in the motion vector candidate list. The decoding apparatus may perform entropy decoding on the motion vector candidate index included in the bitstream and may select a motion vector candidate of a decoding target block from among the motion vector candidates included in the motion vector candidate list by using the entropy-decoded motion vector candidate index. In addition, the decoding apparatus 200 may add the entropy-decoded MVD and the motion vector candidate extracted through the entropy decoding, thereby deriving the motion vector of the decoding target block.

[0219] Meanwhile, the coding apparatus 100 may perform entropy-coding on resolution information of the calculated MVD. The decoding apparatus 200 may adjust the resolution of the entropy-decoded MVD using the MVD resolution information.

[0220] Meanwhile, the coding apparatus 100 calculates a motion vector difference (MVD) between a motion vector and a motion vector candidate in the current block on the basis of an affine model, and performs entropy-coding on the MVD. The decoding apparatus 200 derives a motion vector on a per sub-block basis by deriving an affine control motion vector of a decoding target block through the sum of the entropy-decoded MVD and an affine control motion vector candidate.

[0221] The bitstream may include a reference picture index indicating a reference picture. The reference picture index may be entropy-encoded by the encoding apparatus 100 and then signaled as a bitstream to the decoding apparatus 200. The decoding apparatus 200 may generate a prediction block of the decoding target block based on the derived motion vector and the reference picture index information.

[0222] Another example of the method of deriving the motion information of the current may be the merge mode. The merge mode may mean a method of merging motion of a plurality of blocks. The merge mode may mean a mode of deriving the motion information of the current block from the motion information of the neighboring blocks. When the merge mode is applied, the merge candidate list may be generated using the motion information of the reconstructed neighboring blocks and/or the motion information of the collocated blocks. The motion information may include at least one of a motion vector, a reference picture index, and an inter-picture prediction indicator. The prediction indicator may indicate one-direction prediction (L0 prediction or L1 prediction) or two-direction predictions (L0 prediction and L1 prediction).

[0223] The merge candidate list may be a list of motion information stored. The motion information included in the merge candidate list may be at least one of motion information (spatial merge candidate) of a neighboring block adjacent to the current block, motion information (temporal merge candidate) of the collocated block of the current block in the reference picture, new motion information generated by a combination of the motion information exiting in the merge candidate list, motion information (history-based merge candidate) of the block that is encoded/decoded before the current block, and zero merge candidate.

[0224] The encoding apparatus 100 may generate a bitstream by performing entropy encoding on at least one of a merge flag and a merge index and may signal the bitstream to the decoding apparatus 200. The merge flag may be information indicating whether or not to perform the merge mode for each block, and the merge index may be information indicating that which neighboring block, among the neighboring blocks of the current block, is a merge target block. For example, the neighboring blocks of the current block may include a left neighboring block on the left of the current block, an upper neighboring block disposed above the current block, and a temporal neighboring block temporally adjacent to the current block.

[0225] Meanwhile, the coding apparatus 100 performs entropy-coding on the correction information for correcting the motion vector among the motion information of the merge candidate and signals the same to the decoding apparatus 200. The decoding apparatus 200 can correct the motion vector of the merge candidate selected by the merge index on the basis of the correction information. Here, the correction information may include at least one of information on whether or not to perform the correction, correction direction information, and correction size information. As described above, the prediction mode that corrects the motion vector of the merge candidate on the basis of the signaled correction information may be referred to as a merge mode having the motion vector difference.

[0226] The skip mode may be a mode in which the motion information of the neighboring block is applied to the current block as it is. When the skip mode is applied, the encoding apparatus 100 may perform entropy encoding on information of the fact that the motion information of which block is to be used as the motion information of the current block to generate a bit stream, and may signal the bitstream to the decoding apparatus 200. The encoding apparatus 100 may not signal a syntax element regarding at least any one of the motion vector difference information, the encoding block flag, and the transform coefficient level to the decoding apparatus 200.

[0227] The subblock merge mode may mean a mode that derives the motion information in units of sub-blocks of a coding block (CU). When the subblock merge mode is applied, a subblock merge candidate list may be generated using motion information (sub-block based temporal merge candidate) of the sub-block collocated to the current sub-block in the reference image and/or an affine control point motion vector merge candidate.

[0228] The geometric partitioning mode may mean a mode that derives motion information by partitioning the current block into the predefined directions, derives each prediction sample using each of the derived motion information, and derives the prediction sample of the current block by weighting each of the derived prediction samples.

[0229] The inter-intra combined prediction mode may mean a mode that derives a prediction sample of the current block by weighting a prediction sample generated by inter prediction and a prediction sample generated by intra prediction.

[0230] The decoding apparatus 200 may correct the derived motion information by itself. The decoding apparatus 200 may search the predetermined region on the basis of the reference block indicated by the derived motion information and derive the motion information having the minimum SAD as the corrected motion information.

[0231] The decoding apparatus 200 may compensate a prediction sample derived via inter prediction using an optical flow.

[0232] FIG. 6 is a diagram illustrating a transform and quantization process.

[0233] As illustrated in FIG. 6, a transform and/or quantization process is performed on a residual signal to generate a quantized level signal. The residual signal is a difference between an original block and a prediction block (i.e., an intra prediction block or an inter prediction block). The prediction block is a block generated through intra prediction or inter prediction. The transform may be a primary transform, a secondary transform, or both. The primary transform of the residual signal results in transform coefficients, and the secondary transform of the transform coefficients results in secondary transform coefficients.

[0234] At least one scheme selected from among various transform schemes which are preliminarily defined is used to perform the primary transform. For example, examples of the predefined transform schemes include discrete cosine transform (DCT), discrete sine transform (DST), and Karhunen-Loève transform (KLT). The transform coefficients generated through the primary transform may undergo the secondary transform. The transform schemes used for the primary transform and/or the secondary transform may be determined according to coding parameters of the current block and/or neighboring blocks of the current block. Alternatively, transform information indicating the transform scheme may be signaled. The DCT-based transform may include, for example, DCT-2, DCT-8, and the like. The DST-based transform may include, for example, DST-7.

[0235] A quantized-level signal (quantization coefficients) may be generated by performing quantization on the residual signal or a result of performing the primary transform and/or the secondary transform. The quantized level signal may be scanned according to at least one of a diagonal up-right scan, a vertical scan, and a horizontal scan, depending on an intra prediction mode of a block or a block size/shape. For example, as the coefficients are scanned in a diagonal up-right scan, the coefficients in a block form change into a one-dimensional vector form. Aside from the diagonal up-right scan, the horizontal scan of horizontally scanning a two-dimensional block form of coefficients or the vertical scan of vertically scanning a two-dimensional block form of coefficients may be used depending on the intra prediction mode and/or the size of a transform block. The scanned quantized-level coefficients may be entropy-encoded to be inserted into a bitstream.

[0236] A decoder entropy-decodes the bitstream to obtain the quantized-level coefficients. The quantized-level coefficients may be arranged in a two-dimensional block form through inverse scanning. For the inverse scanning, at least one of a diagonal up-right scan, a vertical scan, and a horizontal scan may be used.

[0237] The quantized-level coefficients may then be dequantized, then be secondary-inverse-transformed as necessary, and finally be primary-inverse-transformed as necessary to generate a reconstructed residual signal.

[0238] Inverse mapping in a dynamic range may be performed for a luma component reconstructed through intra prediction or inter prediction before in-loop filtering. The dynamic range may be divided into 16 equal pieces and the mapping function for each piece may be signaled. The mapping function may be signaled at a slice level or a tile group level. An inverse mapping function for performing the inverse mapping may be derived on the basis of the mapping function. In-loop filtering, reference picture storage, and motion compensation are performed in an inverse mapped region, and a prediction block generated through inter prediction is converted into a mapped region via mapping using the mapping function, and then used for generating the reconstructed block. However, since the intra prediction is performed in the mapped region, the prediction block generated via the intra prediction may be used for generating the reconstructed block without mapping/inverse mapping.

[0239] When the current block is a residual block of a chroma component, the residual block may be converted into an inverse mapped region by performing scaling on the chroma component of the mapped region. The availability of the scaling may be signaled at the slice level or the tile group level. The scaling may be applied only when the mapping for the luma component is available and the division of the luma component and the division of the chroma component follow the same tree structure. The scaling may be performed on the basis of an average of sample values of a luma prediction block corresponding to the color difference block. In this case, when the current block uses inter prediction, the luma prediction block may mean a mapped luma prediction block. A value necessary for the scaling may be derived by referring to a lookup table using an index of a piece to which an average of sample values of a luma prediction block belongs. Finally, by scaling the residual block using the derived value, the residual block may be switched to the inverse mapped region. Then, chroma component block restoration, intra prediction, inter prediction, in-loop filtering, and reference picture storage may be performed in the inverse mapped area.

[0240] Information indicating whether the mapping/inverse mapping of the luma component and chroma component is available may be signaled through a set of sequence parameters.

[0241] The prediction block of the current block may be generated on the basis of a block vector indicating a displacement between the current block and the reference block in the current picture. In this way, a prediction mode for generating a prediction block with reference to the current picture is referred to as an intra block copy (IBC) mode. The IBC mode may be applied to M×N (M<=64, N<=64) coding units. The IBC mode may include a skip mode, a merge mode, an AMVP mode, and the like. In the case of a skip mode or a merge mode, a merge candidate list is constructed, and the merge index is signaled so that one merge candidate may be specified. The block vector of the specified merge candidate may be used as a block vector of the current block. The merge candidate list may include at least one of a spatial candidate, a history-based candidate, a candidate based on an average of two candidates, and a zero-merge candidate. In the case of an AMVP mode, the difference block vector may be signaled. In addition, the prediction block vector may be derived from the left neighboring block and the upper neighboring block of the current block. The index on which neighboring block to use may be signaled. The prediction block in the IBC mode is included in the current CTU or the left CTU and limited to a block in the already reconstructed area. For example, a value of the block vector may be limited such that the prediction block of the current block is positioned in an area of three 64×64 blocks preceding the 64×64 block to which the current block belongs in the coding/decoding order. By limiting the value of the block vector in this way, memory consumption and device complexity according to the IBC mode implementation may be reduced.

[0242] Hereinafter, embodiments of the present invention will be described with reference to FIG. 8 to FIG. 20.

[0243] FIG. 8 is a flowchart illustrating an image encoding method according to an embodiment of the present invention, and FIG. 9 is a flowchart illustrating an image decoding method according to an embodiment of the present invention.

[E1/D1] Deriving a Motion Vector Candidate

[0244] Deriving a motion vector candidate [E1/D1] may include at least one or more among deriving a spatial motion vector candidate [E1-1/D1-1], deriving a temporal motion vector candidate [E1-2/D1-2], and deriving a motion vector candidate based on history [E1-3/D1-3]. Here, when a current block is an IBC (Intra Block Copy) mode, a motion vector may mean a block vector.

[E1-1/D1-1] Deriving a Spatial Motion Vector Candidate

[0245] An encoder/decoder may derive a motion vector candidate from a reconstructed block that is spatially adjacent to an encoding/decoding target block.

[0246] For example, as shown in FIG. 10, an encoder/decoder may determine a spatial motion vector candidate for an encoding/decoding target block by deriving a spatial motion vector in the block B1 adjacent to the top of the encoding/decoding target block X, the block A1 adjacent to the left of the encoding/decoding target block, the block B0 adjacent to the top right corner of the encoding/decoding target block, the block B2 adjacent to the top left corner of the encoding/decoding target block, and the block A0 adjacent to the left bottom corner of the encoding/decoding target block.

[0247] In addition, an encoder/decoder may judge whether or not there is a motion vector in each block according to a predetermined order of blocks included in positions A0, A1, B0, B1 and B2. In case there is a motion vector, the encoder/decoder may determine the motion vector of a corresponding block as a spatial motion vector candidate.

[0248] When an encoding/decoding target block is an IBC (Intra Block Copy) mode referring to a current picture, only a neighbor block that is encoded into an IBC mode may be determined as a spatial motion vector candidate. Here, a spatial motion vector candidate may be a spatial block vector candidate.

[0249] When an encoding/decoding target block is an inter prediction mode but not an IBC mode, if a neighbor block is encoded into an IBC mode, an encoder/decoder may not use the motion vector of a corresponding block as a spatial motion vector candidate.

[0250] In other words, only when a spatial neighbor block has the same prediction mode as a current block, an encoder/decoder may determine the motion vector of the spatial neighbor block as a spatial motion vector candidate.

[0251] In addition, when a reference picture of at least one or more blocks included in positions A0, A1, B0, B1 and B2 is different from a reference picture of an encoding/decoding target block, an encoder/decoder may scale a motion vector of a corresponding block and determine it as a spatial motion vector candidate by using a distance between an encoding/decoding target picture and a reference picture of the corresponding block and a distance between the encoding/decoding target picture and a reference picture of the encoding/decoding target block. Herein, an encoder/decoder may scale a motion vector based on a reference picture index instead of a reference picture.

[0252] In addition, when performing the scaling, an encoder/decoder may determine a spatial motion vector candidate by scaling at least one or more motion vectors of blocks included in positions A0, A1, B0, B1 and B2 on the basis of a reference picture corresponding to a reference picture index with a specific value. Here, the specific value may be a positive integer including 0.

[0253] In addition, an encoder/decoder may derive a spatial motion vector candidate based on at least one or more coding parameters.

[0254] In addition, an encoder/decoder may determine a motion vector of a spatial neighbor block as a spatial motion vector candidate on the basis of the size or area of a current block.

[0255] For example, an encoder/decoder may determine a motion vector of a neighbor block as a spatial motion vector candidate only when the area of a current block is larger than a predefined value.

[0256] For another example, if a current block is an IBC mode, an encoder/decoder may determine a block vector of a neighbor block as a spatial block vector candidate, only when the area of the current block is larger than a predefined value.

[0257] Here, the predefined value may be 16.

[E1-2/D1-2] Deriving a Temporal Motion Vector Candidate

[0258] An encoder/decoder may derive a motion vector candidate from a block that is reconstructed in a co-located picture temporally adjacent to an encoding/decoding target block.

[0259] For example, as shown in FIG. 11, an encoder/decoder may derive a temporal motion vector candidate in the order of a block in the position H, which is outside the co-located block C spatially corresponding to an encoding/decoding target block X, and a block in the position C3, in a co-located picture of an encoding/decoding target picture. Here, when a motion vector can be derived from a block in the position H, an encoder/decoder may derive a temporal motion vector candidate in the block located at H. On the other hand, when no motion vector can be derived from the block located at H, the encoder/decoder may derive a temporal motion vector candidate in the position C3. If H or C3 as a predetermined position is intra-encoded or encoded into an IBC (Intra Block Copy) mode using a current picture as a reference picture, an encoder/decoder may not derive a temporal motion vector candidate. In such a case, a temporal motion vector candidate may mean a motion vector of a co-located block.

[0260] In other words, only when a temporal neighbor block has the same prediction mode as a current block, an encoder/decoder may determine a motion vector of the temporal neighbor block as a temporal motion vector candidate.

[0261] In addition, an encoder/decoder may derive the temporal motion vector candidate based on at least one or more coding parameters.

[0262] When a distance between a picture including an encoding/decoding target block and a reference block of the encoding/decoding target block is different from a distance between a picture including a co-located block and a reference picture of the co-located block, an encoder/decoder may derive a temporal motion vector candidate by scaling a motion vector of the co-located block. Herein, an encoder/decoder may scale a motion vector based on a reference picture index instead of a reference picture.

[E1-3/D1-3] Deriving a History-Based Motion Vector Candidate

[0263] At least one piece of information that is used for an encoding/decoding process in an encoder/decoder or is generated after the encoding/decoding process may be included in a history-based candidate list (that is, an HMVP candidate list).

[0264] Here, the information of the block may be at least one among coding parameters like an intra prediction mode and motion information.

[0265] When the current block is not an affine mode or does not use a temporal motion vector candidate in sub blocks, at least one piece of block information of the current block may be included in an HMVP candidate list.

[0266] When the current block is an IBC (Intra Block Copy) mode using a current picture as a reference picture, it may be included in a separate HMVP candidate list. Here, a separate candidate list may be an IBC HMVP candidate list.

[0267] Unlike conventional candidate lists configured in block units (motion vector candidate lists and merge candidate lists), the HMVP candidate list are maintained while being encoded/decoded in units of picture, slice, tile, CTU, CTU row and CTU column. Accordingly, it may be used within units of picture, slice, tile, CTU, CTU row and CTU column. In addition, the HMVP candidate list may include at least one among pieces of block information of a block that is encoded/decoded in units of picture, slice, tile, CTU, CTU row and CTU column before a current block. In addition, the HMVP candidate list may include at least one among pieces of block information of a block that is previously encoded/decoded in units of picture, slice, tile, CTU, CTU row and CTU column.

[0268] As shown in the example of FIG. 12, an encoder/decoder may determine at least one piece of block information of candidate in an HMVP candidate list for an encoding/decoding process of a current block. An encoder/decoder may perform an encoding/decoding process of a current block by using at least one piece of block information of a selected candidate.

[0269] An encoder/decoder may include at least one piece of block information used during an encoding/decoding process of a current block or at least one piece of block information used after the encoding/decoding process of the current block in an HMVP candidate list. Here, including at least one among block information, a candidate and a block in a candidate list may mean adding at least one among block information, a candidate and a block to an HMVP candidate list.

[0270] When at least one piece of block information of the current block is included in an HMVP candidate list, the piece of block information of the current block may be added first or last to the HMVP candidate list.

[0271] The maximum number of candidates in the HMVP candidate list may be determined as P. Here, P may be a positive integer including 0. The P may be determined based on at least one of a coding parameter of a current block and a coding parameter of a candidate. In addition, the P may be a value that is already set in an encoder/decoder or is signaled from an encoder to a decoder.

[0272] A candidate in the HMVP candidate list may be used to configure at least one among an intra prediction mode candidate list, a first MPM (Most Probable Mode) list, a second MPM list, a residual intra prediction mode candidate list, a motion vector candidate list, a merge candidate list, and an IBC candidate list.

[0273] Among pieces of block information, an intra prediction encoding mode may be included in the HMVP candidate list. A candidate in the HMVP candidate list including an intra prediction encoding mode may be used to configure an intra prediction mode candidate list (for example, a first MPM list, a second MPM list, a residual mode list, etc.). The candidate may be included in an intra prediction mode candidate list.

[0274] Among pieces of block information, inter-coding information (for example, a motion vector, a reference picture index (reference picture index), reference picture list information, bi-prediction weight information (bcwIdx), ½ interpolation filter information (HpelIfIdx) etc.) may be included in the HMVP candidate list. A candidate in the candidate list including inter-coding information may be used to configure a motion vector candidate list. The candidate may be included in a motion vector candidate list.

[0275] Among pieces of block information, inter-coding information (for example, a motion vector, a reference picture index (reference picture index), reference picture list information, bi-prediction weight information (bcwIdx), ½ interpolation filter information (HpelIfIdx) etc.) may be included in the HMVP candidate list. A candidate in the HMVP candidate list including inter-coding information may be used to configure a merge candidate list. The candidate may be included in a merge candidate list.

[0276] In this step, a history-based motion vector candidate list may be configured which includes inter-coding information (for example, a motion vector, a reference picture index (reference picture index), reference picture list information, bi-prediction weight information (bcwIdx), ½ interpolation filter information (HpelIfIdx) etc.) among pieces of block information.

[E2/D2] Configuring a Motion Vector Candidate List

[0277] Configuring a motion vector candidate list [E2/D2] may include generating and adding a combined motion vector candidate to a motion vector candidate list [E2-1/D2-1].

[0278] An encoder/decoder may configure a motion vector candidate list either by adding a derived motion vector candidate to a motion vector candidate list or by removing a motion vector candidate included in a motion vector candidate list.

[0279] The derived spatial motion vector candidates, temporal motion vector candidates, and a history-based motion vector candidate list may be added to a motion vector candidate list mvpListLX in a predetermined order. mvpListLX may mean a motion vector candidate list corresponding to at least one or more reference picture lists like L0, L1, L2 and L3. For example, a motion vector candidate list corresponding to the reference picture list L0 may be expressed as mvpListL0.

[0280] Hereinafter, embodiments of configuring a motion vector candidate list will be described.

[0281] For example, an encoder/decoder may sequentially add the derived spatial motion vector candidates, a temporal motion vector candidate and a history-based motion vector candidate list to a motion vector candidate list.

[0282] For example, an encoder/decoder may sequentially add the derived spatial motion vector candidates, a history-based motion vector candidate list, and a temporal motion vector candidate to a motion vector candidate list.

[0283] For example, an encoder/decoder may add a history-based motion vector candidate list among the derived spatial motion vector candidates and then add a temporal motion vector candidate to a motion vector candidate list. Specifically, an encoder/decoder may sequentially add N spatial motion vector candidates, a history-based motion vector candidate list, M spatial motion vector candidates and temporal motion vector candidates to a motion vector candidate list. Here, N and M may be positive integers equal to or greater than 0.

[0284] Alternatively, an encoder/decoder may add spatial motion vector candidates, temporal motion vector candidates, and a history-based motion vector candidate list to a motion vector candidate list in a predetermined order.

[0285] When configuring the motion vector candidate list, an encoder/decoder may judge whether or not motion information is identical only among spatial motion vector candidates. In case the number of candidates in a list does not satisfy the maximum number of motion vector candidates and thus a temporal motion vector candidate or a history-based motion vector candidate is added, the encoder/decor may not check redundancy with preceding candidates.

[0286] In case an encoder/decoder adds a history-based motion vector candidate while configuring the motion vector candidate list, the encoder/decoder may add information of a maximum number of N candidate blocks in a history-based candidate list to the motion vector candidate list. N may be greater than 0 and be equal to a maximum number of candidates in a history-based candidate list.

[0287] In case an encoder/decoder adds a history-based motion vector candidate while configuring the motion vector candidate list, the encoder/decoder may add information of a maximum number of N candidate blocks in a history-based candidate list to the motion vector candidate list. N may be greater than 0 and be equal to a maximum number of candidates in a history-based candidate list. In case the maximum number of candidates in a history-based candidate list is greater than 4, information of as many as 4 candidate blocks may be added to a motion vector candidate list.

[0288] In case an encoder/decoder adds a history-based motion vector candidate while configuring an IBC candidate list, the encoder/decoder may add information of P candidate blocks in a history-based candidate list to the IBC candidate list. Here, P may be 1, and the motion vector candidate may be a block vector candidate.

[0289] Apart from the derived spatial motion vector candidate, a history-based motion vector candidate list and a temporal motion vector candidate, an encoder/decoder may add a vector with a predetermined value to a motion vector candidate list mvpListLX.

[E2-1/D2-1] Generating and Adding a Combined Motion Vector Candidate to a Motion Vector Candidate List

[0290] An encoder/decoder may generate a combined motion vector candidate by using at least one or more among a spatial motion vector candidate in a motion vector candidate list, a history-based motion vector candidate list, a temporal motion vector candidate and a zero motion vector candidate and may also add the combined motion vector candidate to a motion vector candidate list.

[0291] In addition, an encoder/decoder may generate the combined motion vector candidate based on at least one or more coding parameters. In addition, an encoder/decoder may add the combined motion vector candidate to a motion vector candidate list, based on at least one or more coding parameters.

[E3/D3] Determining a Predicted Motion Vector from a Motion Vector Candidate List

[0292] An encoder/decoder may determine a motion vector candidate in a motion vector candidate list mvpListLX corresponding to a motion vector candidate index as a predicted motion vector.

[0293] An encoder may work out a motion vector difference by calculating a difference between a motion vector and a predicted motion vector, and a decoder may work out a motion vector by adding a predicted motion vector and a motion vector difference.

[E4/D4] Performing Motion Compensation

[0294] In an encoder/decoder, inter prediction or motion compensation may be performed by using a motion vector thus determined.

[E5/D5] Entropy Encoding/Decoding of Information about Motion Compensation of a Current Encoding/Decoding Target Block.

[0295] An encoder/decoder may entropy encode/decode information of motion compensation from a bitstream. Here, information of motion compensation may include at least one among the following pieces of information.

• • [0296] inter_pred_idc indicating an inter prediction indicator
  • [0297] Reference picture indexes (ref_idx_10, ref_idx_11, ref_idx_12, ref_idx_13)
  • [0298] Motion vector candidate indexes (mvp_10_flag, mvp_11_flag, mvp_12_flag, mvp_13_flag)
  • [0299] Motion vector difference values
  • [0300] cu_skip_flag indicating whether or not a skip mode is used
  • [0301] merge_flag indicating whether or not a merge mode is used
  • [0302] merge_idx (merge index) indicating a merge candidate
  • [0303] Weighting factors (wf_10, wf_11, wf_12, wf_13)
  • [0304] Offset values (offset_100, offset_11, offset_12, offset_13)

[0305] The inter prediction indicator may refer to a direction of inter prediction (uni-directional prediction, bi-directional prediction, tri-directional prediction, quad-directional prediction, etc.) of a current block during inter prediction. This may mean the number of reference pictures used by a current block to generate a prediction block. Alternatively, one reference picture may be used for multi-directional prediction. In this case, M-directional prediction may be performed by using N reference pictures (N<M). Alternatively, the inter prediction indicator may refer to the number of prediction blocks used by a current block to perform inter prediction or motion compensation. In addition, it may mean the number of prediction blocks that a current block uses when performing inter prediction or motion compensation through at least one or more reference picture lists like L0, L1, L2 and L3. Here, L0, L1, L2 and L3 may refer to List 0, List 1, List 2 and List 3 respectively. In addition, the inter prediction indicator may be information about whether or not a current block refers to a maximum number of N reference picture lists. Here, N may be 1, 2, 3, 4 and above or be a positive integer equal to or greater than 1. A current block may perform motion compensation by using the one or more reference picture lists.

[0306] For example, an encoder/decoder may perform motion compensation by using reference picture lists L0 and L1 and thus generating at least one or more prediction blocks.

[0307] For example, an encoder/decoder may perform motion compensation by using reference picture lists L0, L1 and L2 and thus generating at least one prediction block.

[0308] For example, an encoder/decoder may perform motion compensation by using reference picture lists L0, L1 and L2 and thus generating at least one or more prediction blocks.

[0309] For example, an encoder/decoder may perform motion compensation by using reference picture lists L0, L1 and L2 and thus generating at least one or more prediction blocks and a maximum number of N prediction blocks. Here, N may be 3 or be a positive integer equal to or greater than 2.

[0310] For example, an encoder/decoder may perform motion compensation by using reference picture lists L0, L1, L2 and L3 and thus generating one prediction block.

[0311] For example, an encoder/decoder may perform motion compensation by using reference picture lists L0, L1, L2 and L3 and thus generating at least one or more prediction blocks.

[0312] For example, an encoder/decoder may perform motion compensation by using reference picture lists of L0, L1, L2 and L3 and thus generating at least one or more prediction blocks and a maximum number of N prediction blocks. Here, N may be 4 or be a positive integer equal to or greater than 2.

[0313] An available inter prediction direction may be determined based on an inter prediction indicator. Some or all of available inter prediction directions may be selectively used based on the size and/or shape of a current block.

[0314] An inter prediction indicator may be information indicating whether inter prediction is a uni-prediction using L0 (List 0), a uni-prediction using L1 (List 1) or a bi-prediction using both L0 and L1. The inter prediction indicator may be expressed by a prediction list utilization flag form that includes a flag indicating whether or not L0 direction prediction is used and a flag indicating whether or not L1 direction prediction is used.

[0315] Here, a prediction list utilization flag may indicate whether or not a prediction block is generated using a corresponding reference picture list. For example, when a prediction list utilization flag indicates a first value (1), it may mean that a corresponding reference picture list is used to generate a prediction block. On the other hand, when the prediction list utilization flag indicates a second value (0), it may mean that a corresponding reference picture list is not used to generate a prediction block. In other words, a prediction block for a current block may be generated only by using motion information of a prediction list utilization flag indicating that a corresponding reference picture list may be used to generate a prediction block. In addition, only when a prediction list utilization flag has a first value, a prediction block for a current block may be generated using the corresponding motion information. In addition, a prediction list utilization flag may be set based on an inter prediction indicator, and an inter prediction indicator may be set based on a prediction list utilization flag.

[0316] In addition, num_ref_idx_10_active_minus1, num_ref_idx_11_active_minus1, num_ref_idx_12_active_minus1, and num_ref_idx_13_active_minus1 may refer to the number of reference pictures for reference picture lists L0, L1, L2 and L3 respectively.

[0317] The reference picture index may mean a reference picture referred to by a current block in each reference picture list. For each reference picture list, one or more reference picture indexes may be entropy decoded. An encoder/decoder may perform motion compensation by using the one or more reference picture indexes.

[0318] For example, an encoder/decoder may perform motion compensation by using one reference picture index and thus generating at least one or more prediction blocks.

[0319] For example, an encoder/decoder may perform motion compensation by using two reference picture indexes and thus generating at least one or more prediction blocks and a maximum number of N prediction blocks. Here, N may be 2 or be a positive integer equal to or greater than 2.

[0320] For example, an encoder/decoder may perform motion compensation by using three reference picture indexes and thus generating at least one or more prediction blocks and a maximum number of N prediction blocks. Here, N may be 3 or be a positive integer equal to or greater than 3.

[0321] For example, an encoder/decoder may perform motion compensation by using four reference picture indexes and thus generating at least one or more prediction blocks and a maximum number of N prediction blocks. Here, N may be 4 or be a positive integer equal to or greater than 4.

[0322] The motion vector candidate index may indicate a motion vector candidate used by a current block in a motion vector candidate list that is generated from each reference picture list and/or each reference picture index. For each reference picture list and/or each reference picture index, one or more motion vector candidate indexes may be entropy decoded. An encoder/decoder may perform motion compensation by using the one or more motion vector candidate indexes.

[0323] For example, an encoder/decoder may perform motion compensation by using one motion vector candidate index and thus generating at least one or more prediction blocks.

[0324] For example, an encoder/decoder may perform motion compensation by using two motion vector candidate indexes and thus generating at least one or more prediction blocks and a maximum number of N prediction blocks. Here, N may be 2 or be a positive integer equal to or greater than 2.

[0325] For example, an encoder/decoder may perform motion compensation by using three motion vector candidate indexes and thus generating at least one or more prediction blocks and a maximum number of N prediction blocks. Here, N may be 3 or be a positive integer equal to or greater than 3.

[0326] For example, an encoder/decoder may perform motion compensation by using four motion vector candidate indexes and thus generating at least one or more prediction blocks and a maximum number of N prediction blocks. Here, N may be 4 or be a positive integer equal to or greater than 4.

[0327] The motion vector difference may mean a difference value between a motion vector and a predicted motion vector. For a current block, one or more motion vector differences in each reference picture list and/or each reference picture index may be entropy decoded. An encoder/decoder may perform motion compensation by using the one or more motion vector differences.

[0328] For example, an encoder/decoder may perform motion compensation by using one motion vector difference and thus generating at least one or more prediction blocks.

[0329] For example, an encoder/decoder may perform motion compensation by using two motion vector differences and thus generating at least one or more prediction blocks and a maximum number of N prediction blocks. Here, N may be 2 or be a positive integer equal to or greater than 2.

[0330] For example, an encoder/decoder may perform motion compensation by using three motion vector differences and thus generating at least one or more prediction blocks and a maximum number of N prediction blocks. Here, N may be 3 or be a positive integer equal to or greater than 3.

[0331] For example, an encoder/decoder may perform motion compensation by using four motion vector differences and thus generating at least one or more prediction blocks and a maximum number of N prediction blocks. Here, N may be 4 or be a positive integer equal to or greater than 4.

[0332] The cu_skip_flag may mean information about whether or not to use a skip mode and may be entropy encoded/decoded in at least one or more units of an encoding block and a prediction block. For example, when information about whether or not to use a skip mode has a first value (1), the use of skip mode may be indicated. When the information about whether or not to use a skip mode has a second value (0), the use of skip mode may not be indicated.

[0333] The merge_flag may mean information about whether or not to use a merge mode and may be entropy encoded/decoded in at least one or more units of an encoding block and a prediction block. For example, when information about whether or not to use a merge mode has a first value (1), the use of merge mode may be indicated. When the information about whether or not to use a merge mode has a second value (0), the use of merge mode may not be indicated.

[0334] The merge_idx may mean information indicating a merge candidate within a merge candidate list and may be entropy encoded/decoded in at least one or more units of an encoding block and a prediction block. In addition, merge_idx may mean merge index information. In addition, merge_idx may indicate a block from which a merge candidate is derived, among reconstructed blocks spatially/temporally adjacent to a current block. In addition, merge_idx may indicate at least one or more pieces of motion information of a merge candidate. For example, when merge index information has a first value (0), it may indicate a first merge candidate within a merge candidate list. When merge index information has a second value (1), it may indicate a second merge candidate within a merge candidate list. When merge index information has a third value (2), it may indicate a third merge candidate within a merge candidate list. Likewise, when it has a fourth or N-th value, it may indicate a merge candidate with a corresponding value according to an order in a merge candidate list. Here, N may be a positive integer including 0.

[0335] When motion compensation is performed, at least two or more prediction blocks may be generated and then a weighted sum may be calculated by using at least one or more among weighting factors and offsets for each prediction block. The weighted sum thus calculated may be used for inter prediction or motion compensation of a current block. At least one or more of a weighting factor and an offset for the prediction block may be entropy encoded/decoded in at least one or more units of an encoding block and a prediction block. Herein, at least one or more of a weighting factor and an offset of each prediction block may be entropy encoded/decoded for at least one or more among a reference picture list, a reference picture, a motion vector candidate index, a motion vector difference, a motion vector, information about whether or not to use a skip mode, information about whether or not to use a merge mode, and merge index information. In addition, at least one or more of a weighting factor and an offset value of each prediction block may be entropy encoded/decoded based on an inter prediction indicator.

[0336] At least one or more among the above-described pieces of information about motion compensation may be entropy encoded/decoded in at least one or more units of CTU and sub-CTU. Here, a sub-CTU may include at least one or more among a sub-CTU, a CU and a PU.

[0337] For example, when at least one or more pieces of information about motion compensation are entropy encoded/decoded in CTUs, motion compensation may be performed using at least one or more pieces of information about the motion compensation in all the blocks existing in CTUs.

[0338] For example, when at least one or more pieces of information about motion compensation are entropy encoded/decoded in CTUs or sub-CTUs, at least one or more pieces of information about the motion compensation in a specific block size or at a specific block depth may be entropy encoded/decoded. Here, information about a specific block size or a specific block depth may be additionally entropy encoded/decoded. In addition, a block size or a block depth, which is set in an encoder/decoder beforehand, may be used as a specific block size or a specific block depth. In addition, information about a specific block size or a specific block depth may be determined based on a coding parameter. In addition, information about a specific block size or a specific block depth may be determined based on another encoded/decoded syntax element value. A block in a sub-CTU may have a square or non-square shape.

[0339] Here, in a block including a specific block size or having a larger block size than a specific block size, at least one or more pieces of information about motion compensation may be entropy encoded/decoded. In a block having a smaller block size than a specific block size, at least one or more pieces of information about motion compensation may not be entropy encoded/decoded.

[0340] In a block having a smaller block size within a specific block size, motion compensation may be performed based on at least one or more pieces of entropy-encoded/decoded information about motion compensation in the specific block size. Here, in blocks having a smaller block size within a specific block size, at least one or more among a motion vector candidate, a motion vector candidate list, a merge candidate, a merge candidate list, or other information included in information about motion compensation may be shared.

[0341] Here, in a block including a specific block depth or having a shallower block depth than a specific block depth, at least one or more pieces of information about motion compensation may be entropy encoded/decoded. In a block having a deeper block depth than a specific block depth, at least one or more pieces of information about motion compensation may not be entropy encoded/decoded.

[0342] In a block having a deeper block depth below a specific block depth, motion compensation may be performed based on at least one or more pieces of entropy-decoded information about motion compensation at the specific block depth. In a block having a deeper block depth below a specific block depth, motion compensation may be performed based on at least one or more pieces of information about motion compensation at the specific block depth, and information about motion compensation at the specific block depth may be entropy encoded. Here, in blocks having a deeper block depth below a specific block depth, at least one or more among a motion vector candidate, a motion vector candidate list, a merge candidate, a merge candidate list, or other information included in information about motion compensation may be shared.

[0343] For example, when the block size of a CTU is 64×64 and at least one or more pieces of information about motion compensation in a 32×32 block, which is a sub-CTU, are entropy encoded/decoded, motion compensation in a block belonging to the 32×32 block but having a smaller size may be performed based on at least one or more pieces of entropy-encoded/decoded information about motion compensation in the 32×32 block unit.

[0344] For example, when the block size of a CTU is 128×128 and at least one or more pieces of information about motion compensation in a 16×16 block, which is a sub-CTU, are entropy encoded/decoded, motion compensation in a block belonging to the 16×16 block but having a smaller or equal size to the 16×16 block unit may be performed based on at least one or more pieces of entropy-encoded/decoded information about motion compensation in the 16×16 block unit.

[0345] For example, when the block depth of a CTU is 0 and at least one or more pieces of information about motion compensation at a block depth of 1, that is in a sub-CTU, are entropy encoded/decoded, motion compensation in a block belonging to the block depth of 1 but having a deeper block depth may be performed based on at least one or more pieces of entropy-encoded/decoded information about motion compensation at the block depth of 1.

[0346] For example, when the block depth of a CTU is 0 and at least one or more pieces of information about motion compensation at a block depth of 2, that is in a sub-CTU, are entropy encoded/decoded, motion compensation in a block belonging to the block depth of 2 but having a deeper or equal block depth may be performed based on at least one or more pieces of entropy-encoded/decoded information about motion compensation at the block depth of 2.

[0347] Here, as the positive integer for depth increases, the depth may increase. As the depth value decreases, the depth may become shallower. In addition, as a depth increases, a block size may decrease. On the other hand, as a depth decreases, a block size may increase. In addition, a sub-depth of a specific block depth may mean a deeper depth than the specific block depth. A sub-depth of a specific block depth may mean a deeper depth within a block size corresponding to the specific block depth.

[0348] In addition, at least one or more pieces of the information about motion compensation may be calculated by using index information within sets that are determined beforehand in an encoder and a decoder.

[0349] Also, in at least one or more among a video parameter set, a sequence parameter set, a picture parameter set, an adaptation parameter set, a picture header and a slice header, at least one or more pieces of information about motion compensation may be entropy encoded/decoded.

[0350] Also, a difference value for at least one or more pieces of information about motion compensation in at least one or more units among CTU, sub-CTU, CU and PU may be entropy encoded/decoded by using at least one or more pieces of information about motion compensation at higher levels, which are the parameter sets, the picture header and the slice header, as prediction values. A value for at least one or more pieces of information about motion compensation may be calculated by adding a prediction value for at least one or more pieces of information about motion compensation and a difference value for at least one or more pieces of information about motion compensation.

[0351] Alternatively, at least one or more pieces of information about motion compensation in at least one or more among CTU, sub-CTU, CU and PU may be entropy encoded/decoded by using at least one or more pieces of information about motion compensation in a specific region within a picture, a slice, a tile or a CTU as a prediction value. A value for at least one or more pieces of information about motion compensation may be calculated by adding a prediction value for at least one or more pieces of information about motion compensation and a difference value for at least one or more pieces of information about motion compensation.

[0352] In addition, at least one or more pieces of information about motion compensation for a prediction block may be entropy encoded/decoded in at least one or more specific regions within a picture, a slice, a tile or a CTU.

[0353] In addition, an encoder/decoder may entropy encode/decode a difference value for at least one or more pieces of information about motion compensation by using at least one or more pieces of information about motion compensation as a prediction value for at least one or more pieces of information about motion compensation of an encoded/decoded neighbor block. An encoder/decoder may calculate a value for at least one or more pieces of information about motion compensation by adding a prediction value for at least one or more pieces of information about motion compensation and a difference value for at least one or more pieces of information about motion compensation.

[0354] In addition, an encoder/decoder may use at least one or more pieces of information about motion compensation of an encoded/decoded neighbor block as a value for at least one or more pieces of information about motion compensation for a current block without performing entropy encoding/decoding for at least one or more pieces of information about motion compensation.

[0355] In addition, an encoder/decoder may derive the at least one or more pieces of information about motion compensation on the basis of at least one or more among coding parameters.

[0356] In addition, an encoder/decoder may entropy decode the at least one or more pieces of information about motion compensation from a bitstream on the basis of at least one or more among coding parameters. An encoder/decoder may entropy encode the at least one or more pieces of information about motion compensation into a bitstream on the basis of at least one or more among coding parameters.

[0357] In addition, information about motion compensation may further include at least one or more among a motion vector, motion vector resolution information, overlapped block motion compensation information, local illumination compensation information, affine motion compensation information, decoder-side motion vector derivation information, and bi-directional optical flow information. Here, the decoder-side motion vector derivation may mean pattern matched motion vector derivation.

[0358] Motion vector resolution information may be information about whether or not a specific resolution is used for at least one or more of a motion vector and a motion vector difference. Here, a resolution may mean a precision. In addition, a specific resolution may be at least one or more among 16-pixel (16-pel) unit, 8-pixel (8-pel) unit, 4-pixel (4-pel) unit, integer-pixel (integer-pel) unit, ½-pixel (½-pel) unit, ¼-pixel (¼-pel) unit, ⅛-pixel (⅛-pel) unit, 1/16-pixel ( 1/16-pel) unit, 1/32-pixel ( 1/32-pel) unit, and 1/64-pixel ( 1/64-pel) unit.

[0359] Overlapped block motion compensation information may be information about whether or not a sub-block, which is constructed by using motion information of a spatially neighbor block of an encoding/decoding target block while motion compensation of the encoding/decoding target block, is used to construct a prediction block of the encoding/decoding target block.

[0360] Local illumination compensation information may be information about whether or not at least one or more of a weighting factor and an offset value are applied when a prediction block of an encoding/decoding target block is constructed. Here, a weighting factor and an offset value may be values that are calculated based on a reference block.

[0361] Affine motion compensation information may be information about whether or not an affine motion model is used for motion compensation for an encoding/decoding target block. Here, an affine motion model may be a method of partitioning one block into a multiplicity of sub-blocks through a multiplicity of parameters and calculating a motion vector of a partitioned sub-block from representative motion vectors.

[0362] Decoder-side motion vector derivation information may be information about whether or not a motion vector necessary for motion compensation is derived from a decoder and is used. Here, information about a motion vector may not be entropy encoded/decoded. Here, when a merge mode is used, decoder-side motion vector derivation may be performed.

[0363] Bi-directional optical flow information may be information about whether or not motion compensation is performed by correcting a motion vector in pixels. Here, a motion vector in pixels may not be entropy encoded/decoded. In addition, motion vector correction may change a motion vector value in blocks into pixels.

[0364] A current block may perform motion compensation by using at least one or more pieces of entropy-decoded information about motion compensation. A current block may perform motion compensation by using at least one or more pieces of information about motion compensation and may entropy encode at least one or more pieces of information about motion compensation.

[0365] When entropy encoding/decoding information about motion compensation of a current encoding/decoding target block, an encoder/decoder may not perform entropy encoding/decoding for at least one or more pieces of information about the motion compensation by using encoding information of a current picture, to which the current encoding/decoding target block belongs, and reference pictures in a reference picture list.

[0366] Here, encoding information of a current picture and reference pictures may be POC (Picture Order Count) information. An encoder/decoder may not perform entropy encoding/decoding for at least one or more pieces of information about the motion compensation by using POC information of a current picture and reference pictures in a reference picture list.

[0367] When POC information among a current picture and reference pictures in a reference picture list satisfies the following conditions, an encoder/decoder may entropy encode/decode information (for example, sym_mvd_flag) indicating whether or not entropy encoding/decoding is performed for L0 and L1 reference picture index information (ref_idx_10 and ref_idx_11) and uni-directional (L0 or L1) motion vector difference information (MvdL0 or MvdL1) for a current encoding/decoding target block.

[0368] [Condition 1] A current encoding/decoding target block is a bi-prediction mode and there are L0 reference picture and L1 reference picture.

[0369] [Condition 2-1] At least one or more reference picture POCs among N reference pictures in L0 reference picture list are smaller than a current picture POC, and at least one or more reference picture POCs among M reference pictures in L1 reference picture list are larger than a current picture POC (N and M may be natural numbers greater than 0 and may be equal to or different from each other).

[0370] [Condition 2-2] At least one or more reference picture POCs among N reference pictures in L0 reference picture list are larger than a current picture POC, and at least one or more reference picture POCs among M reference pictures in L1 reference picture list are smaller than a current picture POC (N and M may be natural numbers greater than 0 and may be equal to or different from each other).

[0371] The [Condition 2-2] may be performed when there is no reference picture satisfying the [Condition 2-1]. On the contrary, when there is no reference picture satisfying [Condition 2-2] after [Condition 2-2] is performed, [Condition 2-1] may be performed.

[0372] In case the conditions are satisfied, L0 and L1 reference picture index information and uni-directional (L0 or L1) motion vector difference (MVD) information may be always derived without entropy encoding/decoding, irrespective of information indicating whether or not entropy encoding/decoding is performed (for example, ‘sym_mvd_flag’).

[0373] When the conditions are satisfied and an encoded/decoded ‘sym_mvd_flag’ has a first value (0), entropy encoding/decoding for L0 and L1 reference picture index information and MVD (Motion Vector Difference) information of L0 and L1 may be performed.

[0374] When the conditions are satisfied and an encoded/decoded ‘sym_mvd_flag’ has a second value (1), L0 and L1 reference picture index information and MVD information of one direction (L0 or L1) may not be entropy encoded/decoded but be derived as follows.

[0375] When the [Condition 2-1] is satisfied, if POCs of at least one or more reference pictures among N reference pictures of L0 reference picture list are smaller than a current picture POC, position information of a reference picture being a short-term reference picture and having a POC, which is smaller than and has the smallest difference from the current picture POC, within L0 reference picture list may be derived as a L0 reference picture index value.

[0376] For example, when a second reference picture POC of L0 reference picture list has the smallest difference from a current picture POC, the value ‘1’ indicating a second reference picture in L0 reference picture list may be derived as L0 reference picture index.

[0377] When the [Condition 2-1] is satisfied, if POCs of at least one or more reference pictures among M reference pictures of L1 reference picture list are larger than a current picture POC, position information of a reference picture being a short-term reference picture and having a POC, which is larger than and has the smallest difference from the current picture POC, within L1 reference picture list may be derived as a L1 reference picture index value.

[0378] For example, when a third reference picture POC of L1 reference picture list has the smallest difference from a current picture POC, the value ‘2’ indicating a third reference picture in L1 reference picture list may be derived as L1 reference picture index.

[0379] When the [Condition 2-2] is satisfied, if POCs of at least one or more reference pictures among N reference pictures of L0 reference picture list are larger than a current picture POC, position information of a reference picture being a short-term reference picture and having a POC, which is larger than and has the smallest difference from the current picture POC, within L0 reference picture list may be derived as a L0 reference picture index value.

[0380] For example, when a second reference picture POC of L0 reference picture list has the smallest difference from a current picture POC, the value ‘1’ indicating a second reference picture in L0 reference picture list may be derived as L0 reference picture index.

[0381] When the [Condition 2-2] is satisfied, if POCs of at least one or more reference pictures among M reference pictures of L1 reference picture list are smaller than a current picture POC, position information of a reference picture being a current short-term reference picture and having a POC, which is smaller than and has the smallest difference from the current picture POC, within L1 reference picture list may be derived as a L1 reference picture index value.

[0382] For example, when a third reference picture POC of L1 reference picture list has the smallest difference from a current picture POC, the value ‘2’ indicating a third reference picture in L1 reference picture list may be derived as L1 reference picture index.

[0383] The check for the conditions [2-1] and [2-2] and the derivation process of L0/L1 reference picture indexes may be performed after a reference picture list for a current picture or slice including an encoding/decoding target block is configured.

[0384] Without considering a POC difference among a current picture POC, a L0 reference picture POC and a L1 reference picture POC, which are indicated by the derived L0 reference picture index and L1 reference picture index information, an encoder/decoder may always derive a motion vector difference value of a direction that is not entropy encoded/decoded as follows.

[0385] When a motion vector difference value in the direction L0 (MVD0) is entropy encoded/decoded, a motion vector difference value in the direction L1 (MVD1) may be derived as −MVD0. In other words, the horizontal and vertical motion vector difference values in the direction L1 are as follows. (MVD1_x=−MVD0_x, MVD1_y=−MVD0_y)

[0386] When a motion vector difference value in the direction L1 (MVD1) is entropy encoded/decoded, a motion vector difference value in the direction L0 (MVD0) may be derived as −MVD1. In other words, the horizontal and vertical motion vector difference values in the direction L0 are as follows. (MVD0_x=−MVD1_x, MVD0_y=−MVD1_y)

[0387] An encoder/decoder may derive a motion vector difference value of a direction, which is not encoded/decoded, by considering a current picture POC, a L0 reference picture POC indicated by the derived L0 reference picture index information, and a L1 reference picture POC indicated by the derived L1 reference picture index information.

[0388] For example, when a motion vector difference value in the direction L0 (MVD0) is entropy encoded/decoded, a motion vector difference value in the direction L1 may be derived as a scaled MVD0 value through Equation 1 below.

[0389] $$\begin{gathered} \begin{array}{cc}\mathrm{currPocDiffL}0=\mathrm{DiffPicOrderCnt}\left(\mathrm{currPic},\mathrm{RefPicList}0\left[\mathrm{refIdxL}0\right]\right) \\ \mathrm{currPocDiffL}1=\mathrm{DiffPicOrderCnt}\left(\mathrm{currPic},\mathrm{RefPicList}1\left[\mathrm{refIdxcL}1\right]\right) \\ \mathrm{td}=\mathrm{Clip}3\left(-128,127,\mathrm{currPocDiffL}0\right) \\ \mathrm{tb}=\mathrm{Clip}3\left(-128,127,\mathrm{currPocDiffL}1\right) \\ \mathrm{tx}=\left(16384+\left(\mathrm{Abs}\left(\mathrm{td}\right)>>1\right)\right)/\mathrm{td} \\ \mathrm{distScaleFactor}=\mathrm{Clip}3\left(-4096,4095,\left(\mathrm{tb}*\mathrm{tx}+32\right)>>6\right) \\ \mathrm{MVD1\_x}=\mathrm{Clip}3(-2^{15},2^{15}-1,\mathrm{Sign}\left(\mathrm{distScaleFactor}*\mathrm{MVD0\_x}\right)*\left(\left(\mathrm{Abs}\left(\mathrm{distScaleFactor}*\mathrm{MVD0\_x}\right)+127\right)>>8\right)\mathrm{MVD1\_y}=\mathrm{Clip}3(-2^{15},2^{15}-1,\mathrm{Sign}\left(\mathrm{distScaleFactor}*\mathrm{MVD0\_y}\right)*\left(\left(\mathrm{Abs}\left(\mathrm{distScaleFactor}*\mathrm{MVD0\_y}\right)+127\right)>>8\right)& \mathrm{Equation}1\end{array} \end{gathered}$$

[0390] currPic is a POC of a current picture, RefPicList0[refIdxL0] is a reference picture POC indicated by the derived L0 reference picture index information, and RefPicList1 [refIdxL1] is a reference picture POC indicated by the derived L1 reference picture index information.

[0391] DiffPicOrderCnt( ) is a POC difference value between a current picture POC and a reference picture POC.

[0392] For example, when a motion vector difference value in the direction L1 (MVD01 is entropy encoded/decoded, a motion vector difference value in the direction L0 may be derived as a scaled MVD1 value through Equation 1.

[0393] An encoder/decoder may determine a direction of an entropy-encoded/decoded motion vector difference value by considering a POC difference among a current picture POC, a L0 reference picture POC and a L1 reference picture POC, which are indicated by the derived L0 reference picture index and L1 reference picture index information.

[0394] For example, when a difference between a current picture POC and the derived L0 reference picture POC is larger than a difference between a current picture POC and the derived L1 reference picture POC, a motion vector difference value in the direction L0 (MVD0) may be entropy encoded/decoded, and a motion vector difference value in the direction L1 may be derived as a scaled MVD0 value.

[0395] For example, when a difference between a current picture POC and the derived L1 reference picture POC is larger than a difference between a current picture POC and the derived L0 reference picture POC, a motion vector difference value in the direction L1 (MVD1) may be entropy encoded/decoded, and a motion vector difference value in the direction L0 may be derived as a scaled MVD1 value.

[0396] For another example, when POC information among a current picture and reference pictures in a reference picture list satisfies the following conditions, an encoder/decoder may entropy encode/decode information (for example, sym_mvd_flag) indicating whether or not entropy encoding/decoding is performed for L0 and/or L1 reference picture index information and uni-directional (L0 or L1) motion vector difference (MVD) information for a current encoding/decoding target block.

[0397] [Condition 1] A current encoding/decoding target block is a bi-prediction mode and there are L0 reference picture and L1 reference picture.

[0398] [Condition 2-1] At least one or more reference picture POCs among N reference pictures in L0 reference picture list are smaller than a current picture POC, and at least one or more reference picture POCs among M reference pictures in L1 reference picture list are larger than a current picture POC (N>0, M>0).

[0399] [Condition 2-2] At least one or more reference picture POCs among N reference pictures in L0 reference picture list are larger than a current picture POC, and at least one or more reference picture POCs among M reference pictures in L1 reference picture list are smaller than a current picture POC (N>0, M>0).

[0400] The [Condition 2-2] may be performed when there is no reference picture satisfying the [Condition 2-1]. On the contrary, when there is no reference picture satisfying [Condition 2-2] after [Condition 2-2] is performed, [Condition 2-1] may be performed.

[0401] [Condition 3] A POC difference between L0 reference picture and L1 reference picture, which have the smallest POC difference from a current picture POC, satisfies Equation 2 below.

[0402] $\begin{array}{cc}\mathrm{DiffPicOrderCnt}\left(\mathrm{currPic},\mathrm{RefPicList}0\left[\mathrm{refIdx}0\right]\right)==\mathrm{DiffPicOrderCnt}\left(\mathrm{RefPicList}1\left[\mathrm{refIdx}1\right],\mathrm{currPiC}\right)& \mathrm{Equation}2\end{array}$

[0403] currPic is a POC of a current picture, RefPicList0[refIdx0] is a reference picture POC having the smallest POC difference from a current picture in L0 reference picture list, and RefPicList1 [refIdx1] is a reference picture POC having the smallest POC difference from a current picture in L1 reference picture list.

[0404] When the conditions are satisfied and an encoded/decoded ‘sym_mvd_flag’ has a first value (0), an encoder/decoder may perform entropy encoding/decoding for L0 and L1 reference picture index information and MVD (Motion Vector Difference) information of L0 and L1.

[0405] When the conditions are satisfied and an encoded/decoded ‘sym_mvd_flag’ has a second value (1), an encoder/decoder may not entropy encode/decode but derive L0 and/or L1 reference picture index information and MVD information of one direction (L0 or L1), as follows.

[0406] In case the conditions are satisfied, an encoder/decoder may not entropy encode/decode but always derive L0 and/or L1 reference picture index information and uni-directional (L0 or L1) motion vector difference (MVD) information, irrespective of information indicating whether or not entropy encoding/decoding is performed (for example, ‘sym_mvd_flag’).

[0407] A L0 reference picture index may be derived from the position information of a reference picture having the smallest POC difference from a current picture in a L0 reference picture list.

[0408] For example, when a second reference picture POC of L0 reference picture list has the smallest difference from a current picture POC, an encoder/decoder may derive the value ‘1’ indicating the second reference picture in L0 reference picture list as L0 reference picture index.

[0409] A L1 reference picture index may be derived from the position information of a reference picture having the smallest POC difference from a current picture in a L1 reference picture list.

[0410] For example, when a second reference picture POC of L0 reference picture list has the smallest difference from a current picture POC, an encoder/decoder may derive the value ‘1’ indicating the second reference picture in L0 reference picture list as L0 reference picture index.

[0411] The checking for the conditions [2-1], [2-2] and [3] and the derivation process of L0/L1 reference picture indexes may be performed after a reference picture list for a current picture or slice including an encoding/decoding target block is configured.

[0412] When a motion vector difference value in the direction L0 (MVD0) is entropy encoded/decoded, a motion vector difference value in the direction L1 (MVD1) may be derived as −MVD0. In other words, the horizontal and vertical motion vector difference values in the direction L1 are as follows. (MVD1_x=−MVD0_x, MVD1_y=−MVD0_y)

[0413] When a motion vector difference value in the direction L1 (MVD1) is entropy encoded/decoded, a motion vector difference value in the direction L0 (MVD0) may be derived as −MVD1. In other words, the horizontal and vertical motion vector difference values in the direction L0 are as follows. (MVD0_x=−MVD1_x, MVD0_y=−MVD1_y)

[0414] For another example, when POC information among a current picture and reference pictures in a reference picture list satisfies the following conditions, an encoder/decoder may entropy encode/decode information (for example, sym_mvd_flag) indicating whether or not entropy encoding/decoding is performed for L0 and/or L1 reference picture index information and uni-directional (L0 or L1) motion vector difference (MVD) information for a current encoding/decoding target block.

[0415] [Condition 1] A current encoding/decoding target block is a bi-prediction mode and there are L0 reference picture and L1 reference picture.

[0416] [Condition 2] A POC difference among a current picture POC, a N-th reference picture POC of L0 reference picture list, and a M-th reference picture POC of L1 reference picture list satisfies Equation 3 below.

[0417] $\begin{array}{cc}\mathrm{DiffPicOrderCnt}\left(\mathrm{currPic},\mathrm{RefPicList}0\left[N-1\right]\right)==\mathrm{DiffPicOrderCnt}\left(\mathrm{RefPicList}1\left[M-1\right],\mathrm{currPiC}\right)& \mathrm{Equation}3\end{array}$

[0418] currPic is a POC of a current picture, RefPicList0 [N−1] is a N-th reference picture POC of L0 reference picture list, and RefPicList1 [M−1] is a M-th reference picture POC of L1 reference picture list.

[0419] Here, N and M may be natural numbers greater than 0 and have the same value or different values.

[0420] For example, when N and M have a value of ‘1’, they may mean a first reference picture POC in a L0/L1 reference picture list.

[0421] When the conditions are satisfied and an encoded/decoded ‘sym_mvd_flag’ has a first value (0), an encoder/decoder may perform entropy encoding/decoding for L0 and L1 reference picture index information and MVD (Motion Vector Difference) information of L0 and L1.

[0422] When the conditions are satisfied and an encoded/decoded ‘sym_mvd_flag’ has a second value (1), an encoder/decoder may not entropy encode/decode but derive L0 and/or L1 reference picture index information and MVD information of one direction (L0 or L1), as follows.

[0423] An L0 reference picture index may be derived as a value of ‘N−1’, thereby indicating a N-th reference picture of L0 reference picture list.

[0424] For example, in the case of N=1, a L0 reference picture index may be derived as ‘0’, thereby indicating a first reference picture of L0 reference picture list.

[0425] An L1 reference picture index may be derived as a value of ‘M−1’, thereby indicating a M-th reference picture of L1 reference picture list.

[0426] For example, in the case of M=1, a L1 reference picture index may be derived as ‘0’, thereby indicating a first reference picture of L1 reference picture list.

[0427] The check for the condition [2] and the derivation process of L0/L1 reference picture indexes may be performed after a reference picture list for a current picture or slice including an encoding/decoding target block is configured.

[0428] When a motion vector difference value in the direction L0 (MVD0) is entropy encoded/decoded, a motion vector difference value in the direction L1 (MVD1) may be derived as −MVD0. In other words, the horizontal and vertical motion vector difference values in the direction L1 are as follows. (MVD1_x=−MVD0_x, MVD1_y=−MVD0_y)

[0429] When a motion vector difference value in the direction L1 (MVD1) is entropy encoded/decoded, a motion vector difference value in the direction L0 (MVD0) may be derived as −MVD1. In other words, the horizontal and vertical motion vector difference values in the direction L0 are as follows. (MVD0_x=−MVD1_x, MVD0_y=−MVD1_y)

[0430] In case the conditions are satisfied, L0 and/or L1 reference picture index information and uni-directional (L0 or L1) motion vector difference (MVD) information may be always derived without entropy encoding/decoding, irrespective of information indicating whether or not entropy encoding/decoding is performed (for example, ‘sym_mvd_flag’).

[0431] When a current picture is a B slice and the POC information among a current picture POC and reference pictures in a reference picture list satisfies at least one or more conditions described above, an encoder may entropy encode information (for example, smvd_enabled_flag) indicating whether or not information (for example, sym_mvd_flag) indicating the possibility of entropy decoding for ‘L0 and/or L1 reference picture index information and uni-directional (L0 or L1) motion vector difference (MVD) information’ for all the encoding target blocks in a current slice can be transmitted, and transmit the information to a decoder at levels like sequence parameter set (SPS), picture parameter set (PPS), adaptation parameter set (APS), picture header, tile group header, slice header and CTU.

[0432] A decoder may entropy decode the corresponding information (for example, smvd_enabled_flag), which is transmitted from an encoder, and may decode ‘sym_mvd_flag’ information for a current decoding target block on the basis of the corresponding information. Here, smvd_enabled_flag indicating that sym_mvd_flag can be transmitted may be signaled at levels like sequence parameter set (SPS), picture parameter set (PPS), adaptation parameter set (APS), picture header, tile group header, slice header and CTU.

[0433] Accordingly, when the following conditions are satisfied, an encoder/decoder may entropy encode/decode information (for example, sym_mvd_flag) indicating whether or not entropy encoding/decoding is performed for L0 and L1 reference picture index information (ref_idx_10 and ref_idx_11) and uni-directional (L0 or L1) motion vector difference information (MvdL0 or MvdL1) for a current encoding/decoding target block.

[0434] [Condition 1] A current encoding/decoding target block is a bi-prediction mode and there are L0 reference picture and L1 reference picture.

[0435] [Condition 2-1] At least one or more reference picture POCs among N reference pictures in L0 reference picture list are smaller than a current picture POC, and at least one or more reference picture POCs among M reference pictures in L1 reference picture list are larger than a current picture POC (N and M may be natural numbers greater than 0 and may be equal to or different from each other).

[0436] [Condition 2-2] At least one or more reference picture POCs among N reference pictures in L0 reference picture list are larger than a current picture POC, and at least one or more reference picture POCs among M reference pictures in L1 reference picture list are smaller than a current picture POC (N and M may be natural numbers greater than 0 and may be equal to or different from each other).

[0437] [Condition 3] smvd_enabled_flag has a value indicating that sym_mvd_flag can be transmitted (for example, smvd_enabled_flag is ‘1’)

[0438] The [Condition 2-2] may be performed when there is no reference picture satisfying the [Condition 2-1]. On the contrary, when there is no reference picture satisfying [Condition 2-2] after [Condition 2-2] is performed, [Condition 2-1] may be performed.

[0439] For example, an encoder/decoder may decode ‘sym_mvd_flag’ information only when ‘tile_group_smvd_enabled_flag’ transmitted from a tile group header has a second value (1) and a decoding target block is a bi-prediction mode. When ‘sym_mvd_flag’ has a second value (1), the encoder/decoder may derive L0 and/or L1 reference picture index information and uni-directional motion vector difference (MVD) information for the corresponding block by using at least one or more among the above-described methods.

[0440] For example, when ‘tile_group_smvd_enabled_flag’ transmitted from a tile group header has a first value (0), an encoder/decoder may not decode ‘sym_mvd_flag’ information for all the decoding target blocks but infer the corresponding information as the first value (0) and thus perform entropy decoding for ‘L0 and L1 reference picture index information and L0 motion vector difference/L1 motion vector difference’ information.

[0441] A first value and a second value are not limited to the above example. The present invention may include a case where the same definition as in the above-described embodiment is applied while the first value is 1 and the second value is 0.

[0442] When a current picture is a B slice and the POC information among a current picture POC and reference pictures in a reference picture list satisfies at least one or more conditions described above, an encoder may entropy encode information (for example, ‘tile_group_smvd_enabled_flag’) indicating whether or not information (for example, ‘sym_mvd_flag’) indicating the possibility of entropy decoding for ‘L0 and/or L1 reference picture index information and uni-directional (L0 or L1) motion vector difference (MVD) information’ for all the encoding target blocks in a current slice can be transmitted, at at least one level among sequence parameter set (SPS), picture parameter set (PPS), adaptation parameter set (APS), picture header, tile group header, slice header, CTU and CU, and may transmit the information to a decoder.

[0443] A decoder may entropy decode the corresponding information (for example, ‘tile_group_smvd_enabled_flag’), which is transmitted from an encoder, and may decode ‘sym_mvd_flag’ information for a current decoding target block on the basis of the corresponding information.

[0444] In addition, when at least one or more among the above-described POC conditions are satisfied, an encoder may entropy encode position information of L0 and/or L1 reference pictures at at least one level among picture header, tile group header, slice header and CTU and transmit the information to a decoder.

[0445] For example, as shown in FIG. 13, when ‘tile_group_smvd_enabled_flag’ that is entropy decoded in a tile group header has a second value (‘1’), a decoder may entropy decode reference picture index information (for example, smvd_ref_idx0, smvd_ref_idx1) commonly applicable to all the target blocks included in a tile group. When ‘sym_mvd_flag’ information that is entropy decoded in a decoding target block has a second value (‘1’), the decoder may derive a reference picture for the corresponding block from reference picture index information (for example, smvd_ref_idx0 or smvd_ref_idx1) that is entropy decoded in a tile group header. The tile group in FIG. 13 may mean a slice.

[0446] A first value and a second value are not limited to the above example. The present invention may include a case where the same definition as in the above-described embodiment is applied while the first value is 1 and the second value is 0.

[0447] When a current picture is a B slice and the POC information among a current picture POC and reference pictures in a reference picture list satisfies at least one or more conditions described above, an encoder may entropy encode information (for example, ‘tile_group_smvd_enabled_flag’) indicating whether or not information (for example, ‘sym_mvd_flag’) indicating the possibility of entropy decoding for ‘L0 and/or L1 reference picture index information and uni-directional (L0 or L1) motion vector difference (MVD) information’ for all the encoding target blocks in a current tile group can be transmitted, at at least one level among sequence parameter set (SPS), picture parameter set (PPS), adaptation parameter set (APS), picture header, tile group header, slice header, CTU and CU, and may transmit the information to a decoder.

[0448] A decoder may decode ‘sym_mvd_flag’ information for a current decoding target block on the basis of the corresponding information (for example, ‘tile_group_smvd_enabled_flag’) transmitted from an encoder.

[0449] In addition, when at least one or more among the above-described POC conditions are satisfied, an encoder may entropy encode position information of L0 and/or L1 reference pictures at at least one level among tile group header, slice header and CTU and transmit the information to a decoder.

[0450] For example, as shown in FIG. 14, when ‘tile_group_smvd_enabled_flag’ that is entropy decoded in a tile group header has a second value (‘1’), a decoder may entropy decode information (for example, ‘default_smvd_refIdx_flag’) indicating whether or not default reference picture index information is used which is commonly applicable to all the target blocks included in a tile group. In addition, when ‘default_smvd_refIdx_flag’ has a second value (‘1’), a decoder may apply a reference picture index, which indicates a L0/L1 reference picture in a predetermined position defined by an encoder/decoder, to all the target blocks. The tile group in FIG. 14 may mean a slice.

[0451] For example, a reference picture index indicating a predetermined position that is defined in an encoder/decoder may be ‘0’ indicating a first reference picture of an L0 reference picture list and an L1 reference picture list. When the corresponding flag (‘default_smvd_refIdx_flag’) has a first value (0), reference picture index information (for example, smvd_ref_idx0 and/or smvd_ref_idx1) commonly applicable to all the target blocks in a tile group may be additionally entropy decoded. When ‘sym_mvd_flag’ information that is entropy decoded in a decoding target block has a second value (‘1’), a reference picture for the corresponding block may be derived from a reference picture index (for example, smvd_ref_idx0 and/or smvd_ref_idx1) that is entropy decoded in a tile group header.

[0452] sps_smvd_enable_flag of FIG. 15 is information that is transmitted at a sequence level. This may be information, indicating whether or not a symmetric MVD mode is used, which is derived without entropy decoding of uni-directional motion vector difference information and bi-directional reference picture index information of an encoding/decoding target block.

[0453] mvd_11_zero_flag of FIG. 16 may be information indicating that a motion vector difference value of L1 direction is not decoded but derived as (0, 0). When mvd_11_zero_flag has a first value (1), it may indicate that a motion vector difference value of L1 direction is not decoded but derived as (0, 0). When mvd_11_zero_flag has a second value (0), it may indicate that a motion vector difference value of L1 direction is not derived as (0, 0).

[0454] Accordingly, when the following conditions are satisfied, an encoder/decoder may entropy encode/decode information (for example, sym_mvd_flag) indicating whether or not entropy encoding/decoding is performed for L0 and L1 reference picture index information (ref_idx_10 and ref_idx_11) and uni-directional (L0 or L1) motion vector difference information (MvdL0 or MvdL1) for a current encoding/decoding target block.

[0455] [Condition 1] A current encoding/decoding target block is a bi-prediction mode and there are L0 reference picture and L1 reference picture.

[0456] [Condition 2-1] At least one or more reference picture POCs among N reference pictures in L0 reference picture list are smaller than a current picture POC, and at least one or more reference picture POCs among M reference pictures in L1 reference picture list are larger than a current picture POC (N and M may be natural numbers greater than 0 and may be equal to or different from each other).

[0457] [Condition 2-2] At least one or more reference picture POCs among N reference pictures in L0 reference picture list are larger than a current picture POC, and at least one or more reference picture POCs among M reference pictures in L1 reference picture list are smaller than a current picture POC (N and M may be natural numbers greater than 0 and may be equal to or different from each other).

[0458] [Condition 3] sps_smvd_enabled_flag has a value indicating that sym_mvd_flag can be transmitted (for example, sps_smvd_enabled_flag is ‘1’).

[0459] [Condition 4] mvd_11_zero_flag has a value indicating that a motion vector difference value of L1 direction is not derived as (0, 0) (for example, mvd_11_zero_flag is ‘0’).

[0460] The [Condition 2-2] may be performed when there is no reference picture satisfying the [Condition 2-1]. On the contrary, when there is no reference picture satisfying [Condition 2-2] after [Condition 2-2] is performed, [Condition 2-1] may be performed.

[0461] Meanwhile, when the ‘mvd_11_zero_flag’ information has a second value (1), at least one or more among ‘tile_group_smvd_enabled_flag’, ‘default_smvd_refIdx_flag’, ‘smvd_ref_idx0’, ‘smvd_ref_idx1’, and ‘sym_mvd_flag’, which are described above, may not be entropy encoded/decoded. In addition, although the ‘mvd_11_zero_flag’ was described to be encoded/decoded in a tile group header, it may be encoded/decoded at at least one level among sequence parameter set (SPS), picture parameter set (PPS), adaptation parameter set (APS), picture header, slice header, CTU and CU.

[0462] FIG. 16 to FIG. 18 are other embodiments of symmetric MVD mode.

[0463] In FIG. 16, ‘sym_mvd_ref_idx [i]’ that is entropy decoded in a tile group header may be signaled only when each reference picture list has at least one or more reference pictures.

[0464] For example, when there is one reference picture in L0 reference picture list and there are two or more reference pictures in L1 reference picture list, ‘sym_mvd_ref_idx [1]’ may be entropy decoded only for L1 direction, and ‘sym_mvd_ref_idx [0]’ may be inferred as 0 for L0 direction.

[0465] For another example, only one ‘sym_mvd_ref_idx’ that is applied to both L0/L1 reference picture lists may be entropy decoded.

[0466] For example, when ‘tile_group_smvd_enabled_flag’ has a second value (1), a decoder may entropy decode only one ‘sym_mvd_ref_idx’ and use L0 and L1 reference pictures indicated by a corresponding value.

[0467] When a decoder needs to generate a bitstream satisfying at least one or more among [Condition A], [Condition B] and [Condition C], which are described in FIG. 17, the decoder should be able to decode a bitstream satisfying at least one or more among the conditions.

[Condition A]

[0468] A POC difference between a current picture and a reference picture indicated by ‘sym_mvd_ref_idx [0]’ and a POC difference between a current picture and a reference picture indicated by ‘sym_mvd_ref_idx [1]’ should have different encoding, a POC difference between a current picture and a reference picture indicated by ‘sym_mvd_ref_idx [0]’ in L0 reference picture list should be smallest, and a POC difference between a current picture and a reference picture indicated by ‘sym_mvd_ref_idx [1]’ in L1 reference picture list should be smallest.

[Condition B]

[0469] A POC difference between a current picture and a reference picture indicated by ‘sym_mvd_ref_idx [0]’ and a POC difference between a current picture and a reference picture indicated by ‘sym_mvd_ref_idx [1]’ should have different encoding.

[Condition C]

[0470] A POC difference between a current picture and a reference picture indicated by ‘sym_mvd_ref_idx [0]’ and a POC difference between a current picture and a reference picture indicated by ‘sym_mvd_ref_idx [1]’ should be equal to each other, and a reference picture indicated by ‘sym_mvd_ref_idx [0]’ and a reference picture indicated by ‘sym_mvd_ref_idx [1]’ should be located in opposite directions from the current picture.

[0471] Meanwhile, the tile group in FIG. 16 and FIG. 17 may mean a slice.

[0472] FIG. 19 is a view for explaining an image decoding method according to an embodiment of the present invention.

[0473] A decoder may obtain symmetric motion vector difference mode availability information from a bitstream (S1901).

[0474] Here, the symmetric motion vector difference mode availability information may be sps_smvd_enable flag that is obtained at a sequence level. Detailed description of sps_smvd_enable flag is omitted here since it is already described above.

[0475] In addition, a decoder may obtain first prediction direction's zero motion vector difference information from a bitstream (S1902). Specifically, the first prediction direction's zero motion vector difference information may indicate that the motion vector difference value of the first prediction direction is not decoded but derived as (0, 0).

[0476] Here, the first prediction direction's zero motion vector difference information may be mvd_11_zero_flag obtained at a picture level. Detailed description of mvd_11_zero_flag is omitted here since it is already described above.

[0477] In addition, a decoder may obtain symmetric motion vector difference mode information of a current block from the bitstream on the basis of the symmetric motion vector difference mode availability information and the first prediction direction's zero motion vector difference information (S1903).

[0478] Here, the motion vector difference mode information may be the above-described sym_mvd_flag.

[0479] In addition, a decoder may obtain first prediction direction's reference picture index information, second prediction direction's reference picture index information and a first prediction direction's motion vector difference value on the basis of the symmetric motion vector difference mode information (S1904). Specifically, when the symmetric motion vector difference mode information of the current block indicates a symmetric motion vector difference mode, a decoder may obtain the first prediction direction's reference picture index information, the second prediction direction's reference picture index information and the first prediction direction's motion vector difference value by deriving rather decoding from the bitstream.

[0480] When the symmetric motion vector difference mode information of the current block indicates a symmetric motion vector difference mode, the first prediction direction's motion vector difference value may be derived based on a second prediction direction's motion vector difference value of the current block.

[0481] When the symmetric motion vector difference mode information of the current block indicates a symmetric motion vector difference mode, the first prediction direction's reference picture index information may be derived as an index of a backward reference picture that is nearest to a current picture in a first prediction direction's reference picture list, and the second prediction direction's reference picture index information may be derived as an index of a forward reference picture that is nearest to the current picture in a second prediction direction's reference picture list.

[0482] When the symmetric motion vector difference mode information of the current block indicates information may be derived as an index of a forward reference picture that is nearest to a current picture in a first prediction direction's reference picture list, and the second prediction direction's reference picture index information may be derived as an index of a backward reference picture that is nearest to the current picture in a second prediction direction's reference picture list.

[0483] Meanwhile, when the symmetric motion vector difference mode information of the current block indicates a symmetric motion vector difference mode, the first prediction direction's reference picture index information and the second prediction direction's reference picture index information may be derived as an index of a short-term reference picture.

[0484] In addition, a decoder may generate a prediction block of the current block by using at least one of the first prediction direction's reference picture index information, the second prediction direction's reference picture index information, and the first prediction direction's motion vector difference value (S1905).

[0485] Here, the first prediction direction may be L1 prediction direction, and the second prediction direction's may be L0 prediction direction.

[0486] FIG. 20 is a view for explaining an image encoding method according to an embodiment of the present invention.

[0487] An encoder may determine symmetric motion vector difference mode availability information (S2001).

[0488] Here, the symmetric motion vector difference mode availability information may be sps_smvd_enable flag that is encoded at a sequence level. Detailed description of sps_smvd_enable flag is omitted here since it is already described above.

[0489] In addition, an encoder may determine first prediction direction's zero motion vector difference information (S2002). Specifically, the first prediction direction's zero motion vector difference information may indicate that the motion vector difference value of the first prediction direction is not encoded but derived as (0, 0).

[0490] Here, the first prediction direction's zero motion vector difference information may be mvd_11_zero_flag encoded at a picture level. Detailed description of mvd_11_zero_flag is omitted here since it is already described above.

[0491] In addition, an encoder may encode symmetric motion vector difference mode information of a current block on the basis of the symmetric motion vector difference mode availability information and the first prediction direction's zero motion vector difference information (S2003). Here, the motion vector difference mode information may be the above-described sym_mvd_flag.

[0492] In addition, an encoder may determine, based on the symmetric motion vector difference mode information, whether or not first prediction direction's reference picture index information, second prediction direction's reference picture index information and a first prediction direction's motion vector difference value are encoded (S2004). Specifically, when the symmetric motion vector difference mode information of the current block indicates a symmetric motion vector difference mode, an encoder may determine as not encode the first prediction direction's reference picture index information, the second prediction direction's reference picture index information and the first prediction direction's motion vector difference value.

[0493] When the symmetric motion vector difference mode information of the current block indicates a symmetric motion vector difference mode, the first prediction direction's motion vector difference value may be derived based on a second prediction direction's motion vector difference value of the current block.

[0494] When the symmetric motion vector difference mode information of the current block indicates a symmetric motion vector difference mode, the first prediction direction's reference picture index information may be derived as an index of a backward reference picture that is nearest to a current picture in a first prediction direction's reference picture list, and the second prediction direction's reference picture index information may be derived as an index of a forward reference picture that is nearest to the current picture in a second prediction direction's reference picture list.

[0495] When the symmetric motion vector difference mode information of the current block indicates information may be derived as an index of a forward reference picture that is nearest to a current picture in a first prediction direction's reference picture list, and the second prediction direction's reference picture index information may be derived as an index of a backward reference picture that is nearest to the current picture in a second prediction direction's reference picture list.

[0496] Meanwhile, when the symmetric motion vector difference mode information of the current block indicates a symmetric motion vector difference mode, the first prediction direction's reference picture index information and the second prediction direction's reference picture index information may be derived as an index of a short-term reference picture.

[0497] Here, the first prediction direction may be L1 prediction direction, and the second prediction direction may be L0 prediction direction.

[0498] A bitstream generated by an image encoding method that is described in FIG. 20 may be stored in a non-transitory computer readable recording medium.

[0499] When at least one or more pieces of information about the motion compensation are entropy encoded/decoded, at least one or more among the following binarization methods may be used.

• • [0500] Truncated rice binarization method
  • [0501] K-th order Exp_Golomb binarization method
  • [0502] Limited K-th order Exp_Golomb binarization method
  • [0503] Fixed-length binarization method
  • [0504] Unary binarization method
  • [0505] Truncated unary binarization method

[0506] When entropy encoding/decoding at least one or more pieces of information about the motion compensation, an encoder/decoder may determine a context model by using at least one or more pieces of information about motion compensation of a neighbor block, at least one or more pieces of information about motion information that is previously encoded/decoded, information about a current unit/block depth, or information about a current unit/block size.

[0507] An encoder/decoder may entropy encode/decode at least one or more pieces of information about the motion compensation by using at least one or more pieces of information about motion compensation of a neighbor block, at least one or more pieces of information about motion information that is previously encoded/decoded, information about a current unit/block depth, or information about a current unit/block size as a prediction value for information about motion compensation of a current block.

[0508] As described in the embodiment of the present invention, a reference picture set used for reference picture list construction and reference picture list modification may use at least one or more reference picture lists among L0, L1, L2 and L3.

[0509] When a boundary strength in a deblocking filter is calculated according to the embodiment of the present invention, 1 to N motion vectors of an encoding/decoding target block may be used. Here, N represents a positive integer of 1 or more and may be 2, 3, 4, or the like.

[0510] The embodiments of the present invention may also be applied when a motion vector for motion vector prediction has at least one or more among 16-pixel (16-pel) unit, 8-pixel (8-pel) unit, 4 pixel (4-pel) unit, integer-pixel (integer-pel) unit, ½-pixel (½-pel) unit, ¼-pixel (¼-pel) unit, ⅛-pixel (⅛-pel) unit, 1/16-pixel ( 1/16-pel) unit, 1/32-pixel ( 1/32-pel) unit, and 1/64-pixel ( 1/64-pel) unit. In addition, when motion vector prediction is performed, a motion vector may be selectively used for each of the pixel units.

[0511] A slice type to which the embodiments of the present invention are applied may be defined, and the embodiments of the present invention may be applied according to the corresponding slice type.

[0512] For example, when a slice type is T (Tri-predictive)-slice, a prediction block may be generated using at least three or more motion vectors, and a weighted sum of at least three or more prediction blocks may be calculated and used as a final prediction block of an encoding/decoding target block. For example, when a slice type is Q (Quad-predictive)-slice, a prediction block may be generated using at least four or more motion vectors, and a weighted sum of at least four or more prediction blocks may be calculated and used as a final prediction block of an encoding/decoding target block.

[0513] The above embodiments of the present invention may be applied not only to inter prediction and motion compensation methods using motion vector prediction but also to inter prediction and motion compensation methods using a skip mode and a merge mode.

[0514] The above embodiments may be performed in the same method in an encoder and a decoder.

[0515] At least one or a combination of the above embodiments may be used to encode/decode a video.

[0516] A sequence of applying to above embodiment may be different between an encoder and a decoder, or the sequence applying to above embodiment may be the same in the encoder and the decoder.

[0517] The above embodiment may be performed on each luma signal and chroma signal, or the above embodiment may be identically performed on luma and chroma signals.

[0518] A block form to which the above embodiments of the present invention are applied may have a square form or a non-square form.

[0519] The above embodiment of the present invention may be applied depending on a size of at least one of a coding block, a prediction block, a transform block, a block, a current block, a coding unit, a prediction unit, a transform unit, a unit, and a current unit. Herein, the size may be defined as a minimum size or maximum size or both so that the above embodiments are applied, or may be defined as a fixed size to which the above embodiment is applied. In addition, in the above embodiments, a first embodiment may be applied to a first size, and a second embodiment may be applied to a second size. In other words, the above embodiments may be applied in combination depending on a size. In addition, the above embodiments may be applied when a size is equal to or greater that a minimum size and equal to or smaller than a maximum size. In other words, the above embodiments may be applied when a block size is included within a certain range.

[0520] For example, the above embodiments may be applied when a size of current block is 8×8 or greater. For example, the above embodiments may be applied when a size of current block is 4×4 only. For example, the above embodiments may be applied when a size of current block is 16×16 or smaller. For example, the above embodiments may be applied when a size of current block is equal to or greater than 16×16 and equal to or smaller than 64×64.

[0521] The above embodiments of the present invention may be applied depending on a temporal layer. In order to identify a temporal layer to which the above embodiments may be applied, a corresponding identifier may be signaled, and the above embodiments may be applied to a specified temporal layer identified by the corresponding identifier. Herein, the identifier may be defined as the lowest layer or the highest layer or both to which the above embodiment may be applied, or may be defined to indicate a specific layer to which the embodiment is applied. In addition, a fixed temporal layer to which the embodiment is applied may be defined.

[0522] For example, the above embodiments may be applied when a temporal layer of a current image is the lowest layer. For example, the above embodiments may be applied when a temporal layer identifier of a current image is 1. For example, the above embodiments may be applied when a temporal layer of a current image is the highest layer.

[0523] A slice type or a tile group type to which the above embodiments of the present invention are applied may be defined, and the above embodiments may be applied depending on the corresponding slice type or tile group type.

[0524] In the above-described embodiments, the methods are described based on the flowcharts with a series of steps or units, but the present invention is not limited to the order of the steps, and rather, some steps may be performed simultaneously or in different order with other steps. In addition, it should be appreciated by one of ordinary skill in the art that the steps in the flowcharts do not exclude each other and that other steps may be added to the flowcharts or some of the steps may be deleted from the flowcharts without influencing the scope of the present invention.

[0525] The embodiments include various aspects of examples. All possible combinations for various aspects may not be described, but those skilled in the art will be able to recognize different combinations. Accordingly, the present invention may include all replacements, modifications, and changes within the scope of the claims.

[0526] The embodiments of the present invention may be implemented in a form of program instructions, which are executable by various computer components, and recorded in a computer-readable recording medium. The computer-readable recording medium may include stand-alone or a combination of program instructions, data files, data structures, etc. The program instructions recorded in the computer-readable recording medium may be specially designed and constructed for the present invention, or well-known to a person of ordinary skilled in computer software technology field. Examples of the computer-readable recording medium include magnetic recording media such as hard disks, floppy disks, and magnetic tapes; optical data storage media such as CD-ROMs or DVD-ROMs; magneto-optimum media such as floptical disks; and hardware devices, such as read-only memory (ROM), random-access memory (RAM), flash memory, etc., which are particularly structured to store and implement the program instruction. Examples of the program instructions include not only a mechanical language code formatted by a compiler but also a high level language code that may be implemented by a computer using an interpreter. The hardware devices may be configured to be operated by one or more software modules or vice versa to conduct the processes according to the present invention.

[0527] Although the present invention has been described in terms of specific items such as detailed elements as well as the limited embodiments and the drawings, they are only provided to help more general understanding of the invention, and the present invention is not limited to the above embodiments. It will be appreciated by those skilled in the art to which the present invention pertains that various modifications and changes may be made from the above description.

[0528] Therefore, the spirit of the present invention shall not be limited to the above-described embodiments, and the entire scope of the appended claims and their equivalents will fall within the scope and spirit of the invention.

[0529] The present invention may be used to encode or decode an image.