Image Processing Method and Apparatus

1. Field of the Invention

The present invention relates to an image processing method and apparatus for forming a half-tone image using screen patterns.

2. Related Background Art

A screen process is known as a means for forming a half-tone image. The screen process is characterized in that a stable half-tone image can be formed by using a small number of gradation steps, for an original such as a photograph and an original having a uniform density without a shade difference.

Although an image formed by dots specific to a screen becomes stable by such a screen process, the edge portions of thin lines and characters have notches under the influence of a screen pattern.

Depending upon the number of lines and a screen angle of the screen process, interference occurs with the screen angle of a read original and conspicuous moire is generated. Examples of such moire are shown in FIG. 22.

If a screen pattern is changed to output a smooth image of fine lines and characters without notches, a pictorial image such as a photograph is not formed with dots specific to the screen. Therefore, a stable image reproduction is impossible.

One of conventional screen processes can select a desired screen from a plurality of screen angles and lines.

However, with a screen process having a selected screen angle, although an image formed with dots specific to the screen becomes stable, the images of fine lines and characters have notched edges under the influence of screen patterns, because a method of fattening pixels of screen masks cannot be selected.

It is an object of the present invention to provide an image processing method and apparatus solving the above disadvantages of conventional techniques and being capable of forming the images of fine lines and characters without notched edges even with a screen process.

It is another object of the present invention to provide an image processing method and apparatus capable of suppressing moire to be generated through interference with a dot original when a screen process is performed.

It is a further object of the present invention to provide an image processing method and apparatus capable of forming the images of fine lines and characters without notches and forming stable half-tone images of photographs or the like, by using a screen pattern selected from a plurality of screen patterns prepared in advance.

It is a still further object of the present invention to provide an image processing method and apparatus capable of achieving an image process matching the quality of an input image, by selecting a screen pattern from a plurality of screen patterns in accordance with the state of edges of the input image.

It is another object of the present invention to provide an image processing method and apparatus capable of selecting a desired image forming method by allowing a user to set a desired fattening process of pixels of a screen matrix, the image forming method including a method of forming a stable image with clearly reproduced dots of a screen, a method of forming an image with unclearly reproduced dots of a screen, a method of forming an image with suppressed edge notches, and other methods.

The above and other objects and advantages of the present invention will become apparent from the following detailed description when read in connection with the accompanying drawings.

FIG. 1 is a block diagram showing the structure of an image forming apparatus according to an embodiment of the invention.

FIG. 2 is a block diagram showing the structure of an image processing unit of the embodiment.

FIG. 3 is a block diagram showing the structure of a screen processing unit according to a first embodiment.

FIGS. 4 to 7 are diagrams showing examples of screen patterns used by the screen processing unit.

FIGS. 8a-8c are diagrams illustrating a growth of screen pixels using the screen pattern shown in FIG. 4.

FIGS. 9a-9c are diagrams illustrating a growth of screen pixels using the screen pattern shown in FIG. 5.

FIGS. 10a-10c are diagrams illustrating a growth of screen pixels using the screen pattern shown in FIG. 7.

FIG. 11 is a diagram illustrating a growth of dots using the screen pattern shown in FIG. 4.

FIG. 12 is a diagram illustrating a growth of dots using the screen pattern shown in FIG. 5.

FIG. 13 is a diagram illustrating a growth of dots using the screen pattern shown in FIG. 7.

FIGS. 14A and 14B are diagrams illustrating a repetition or repeat period of screen patterns.

FIG. 15 is a block diagram showing another structure of the screen processing unit.

FIGS. 16a-16c are diagrams illustrating the output results by the screen process according to the embodiment of the invention.

FIG. 17 is a block diagram showing the structure of the screen processing unit of a second embodiment.

FIGS. 18a-18c are diagrams showing examples of screen patterns stored in a screen mask memory.

FIG. 19 is a diagram showing examples of strong and weak screen gradations.

FIG. 20 is a diagram illustrating a screen data generating unit.

FIG. 21 is a diagram illustrating transfer (mask repetition) of memory contents.

FIG. 22 is a diagram showing output results by a conventional screen process.

Preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.

(First Embodiment)

Described first is an example of detecting edges of an input image and selecting in accordance with the detected results a screen associated with a different dot growth method.

FIG. 1 is a block diagram showing the structure of an image forming apparatus illustrating the outline of the embodiment of the invention.

An image reader unit 100 has a lens 101 for converging light from an original 100, a CCD sensor 102 for converting light input through the lens 101 into an electrical signal, an analog signal processing unit 103 for processing a color signal output from the CCD sensor 102, and other necessary elements. The CCD sensor 102 converts an original image focussed thereon via the lens 101 into analog electrical signals for R (red), G (green), and B (blue). The converted RGB image information is input to the analog signal processing unit 103, subjected to a sample-and-hold process for each color R, G, and B and a dark level correction process and the like, and thereafter analog/digital (A/D) converted to be output as a digital image signal. The output full-color digital image signal is input to an image processing unit 104.

The image processing unit 104 executes correction processes necessary for a reader system such as shading correction, color correction, and gamma correction, a smoothing process, edge emphasis, and other processes.

The processed image data is output to a printer unit 105. The printer unit 105 may use a laser beam printer, an LED printer, or the like. If a laser printer is used, it has an exposure control unit with a semiconductor laser, an image forming unit, a transport control unit for a transfer sheet, and the like, and records an image on a transfer sheet in accordance with an input image signal.

The printer 105 controls a record time of one pixel or a record intensity of one pixel, and can record a pixel at 16 gradation levels.

A CPU circuit unit 110 has a CPU 106, a ROM 107, a RAM 108, and other circuit elements. ROM 107 stores control programs to be executed by CPU 106 and various data. RAM 108 is used as a working area for the processes by CPU 106 and temporarily stores various data. The CPU circuit unit 110 controls the image reader unit 109, image processing unit 104, printer unit 105, and other necessary elements to supervise the control sequence of the image forming apparatus of this embodiment. Reference numerals 120 to 122 represent control buses interconnecting the CPU circuit unit 110 and other units of the apparatus.

Next, referring to FIG. 2, the structure of the image processing unit 104 will be described. FIG. 2 is a block diagram showing the functional structure of the image processing unit 104.

RGB digital image signals output from the analog signal processing unit 103 are input to a shading correction unit 201. This shading correction unit 201 corrects a variation of characteristics of the CCD sensor 102 which reads an original and the optical distribution of an illuminating lamp which illuminates an original 100. The image signals processed and corrected by the shading correction unit 201 are input to a gradation correction unit 202 which converts luminance signals (RGB) into density data of C (cyan), M (magenta), and Y (yellow). The gradation correction unit 202 generates CMY image density data. The image signals converted into density data are input to a color/monochrome conversion unit 203 which converts the color image data into monochrome (black and white) image data 300. The image data 300 output from the color/monochrome conversion unit 203 is input to a screen processing unit 204 and converted into a screen pattern with which a half-tone image is formed. In this example, image data of one pixel is converted into image data having one of 16 levels. The present invention is applicable to conversion of data of one pixel into data having one of three levels.

FIG. 3 is a block diagram showing the structure of the screen processing unit 204 which is the main circuit of the first embodiment.

Monochrome image data 300 supplied from the screen processing unit 204 is first input to an edge detection unit 301 which detects an edge portion of the image data 300. An example of a filter used by the edge detection unit 301 is shown at 320, which is a general differential filter.

By using this filter, the edge detection unit 301 performs a matrix calculation of the input image data 300 and outputs the results. If the results are negative, a value "0" is output. Eight-bit data obtained by the edge detection unit 301 is input to a separation unit 302 which performs a threshold value process. With this threshold value process by the separation unit 302, a 2-bit signal 321 of any one of "0/1/2/3" corresponding to the edge intensity is output. If the edge value is large a value "3" is output, and as the edge value becomes smaller, values "2", "1", and "0" gradually becoming smaller are output in this order.

This 2-bit signal 321 is input to a select signal terminal of a selector 303. If the 2-bit signal 321 is "0", an address signal from a main scan 304 and a sub scan 305 is output to a memory 306. If the 2-bit signal 321 is "1", the address signal from the main scan 304 and sub scan 305 is output to a memory 309. If the 2-bit signal 321 is "2", the address signal from the main scan 304 and sub scan 305 is output to a memory 308. If the 2-bit signal 321 is "3", the address signal from the main scan 304 and sub scan 305 is output to a memory 307.

The main scan 304 and sub scan 305 are both, for example, a 4-bit counter which counts a pixel clock signal synchronizing with the image data 300. A carry signal of the main scan counter 304 is input to a clock terminal of the sub scan counter 305. Therefore, an 8-bit address signal is output from the main and sub scan counters 304 and 305.

Each of the memories 306 to 309 can write screen data from CPU 106. In this case, the CPU bus 121 is connected to address and data buses of the memories 306 to 309, and screen data is written in each memory at the address supplied from the CPU bus 121. The screen data is a threshold value to be compared with the input image data. Examples of screen pattern data are shown as patterns (A) to (P) in FIG. 4 and patterns (A) to (P) in FIG. 7. In this embodiment, screen data shown in FIG. 4 is stored in the memory 306, screen data shown in FIG. 5 is stored in the memory 307, screen data shown in FIG. 6 is stored in the memory 308, and screen data shown in FIG. 7 is stored in the memory 309. In each of the memories 306 to 309, data of level "0" is written in a memory field 1, data of level "1" is written in a memory field 2, . . . , and data of level "15" is written in a memory field 16.

The screen data shown in FIG. 4 and stored in the memory 306 has a very strong screen tone degree, and is selected when data from the separation unit 302 is "0", i.e., when it is judged that the image data has no edge. The screen data shown in FIG. 5 and stored in the memory 307 has a very weak screen tone degree, and is selected when data from the separation unit 302 is "3", i.e., when it is judged that the image data has a strong edge. The screen data shown in FIG. 6 and stored in the memory 308 has a weak screen tone degree, and is selected when data from the separation unit 302 is "2", i.e., when it is judged that the edge is intermediary (rather strong). The screen data shown in FIG. 7 and stored in the memory 309 has a strong screen tone degree, and is selected when data from the separation unit 302 is "1", i.e., when it is judged that the edge is intermediary (rather weak).

Although the characteristics of each screen data will be later described, the screen data shown in FIG. 4 used when the image data has no edge forms images such as shown in FIGS. 8a-8c and 11, the screen data shown in FIG. 5 used when the image data has a strong edge forms images such as shown in FIGS. 9a-9c and 12, and the screen data shown in FIG. 7 used when the edge is intermediary forms images such as shown in FIGS. 10a-10c and 13. Images formed by the screen data shown in FIG. 6 are not shown.

In this embodiment, an edge is detected from each image and one of the memories 306 to 309 is selected.

For the screen process to be executed by the screen processing unit 204, data is accessed from each of the memories 306 to 309 in accordance with an output from the main scan and sub scan counters 304 and 305 selected by the selector 303. An output from each of the memories 306 to 309 is input via a corresponding OR circuit 310 to a corresponding comparator 311. The comparator 311 compares the input monochrome image data 300 with an output of each of the memories 306 to 309. If the image data 300 (B input) is larger than the output from the memory, the comparator 311 outputs "1". A 16-bit output from each comparator 311 is encoded by an encoder 312 and output the encoded result as 4-bit data.

This encoder 312 outputs, for example, "0" when 16-bit data "0000000000000000" is input, "1" when "0000000000000001" is input, "2" when 16-bit data "0000000000000011" is input, . . . , and "15" when "0111111111111111" is input. The value of the 16-bit data uses the memory field 16 as the most significant bit (MSB).

The main and sub scan counters 304 and 305 generate count signals by using Hsync (horizontal sync signal) and Vsync (vertical sync signal) in accordance with the sizes of a screen mask in the main and sub scan directions. This is because the counters 304 and 305 are required to be reset for each size of the screen mask. The reason why the count signals from the main and sub scan counters 304 and 305 are each four bits is that the number of bits depends on the repetition (cycle) pattern size of the screen mask data value. Specifically, if the mask size is 3.times.4, the mask repetitive pattern size is 10.times.10 as shown in FIGS. 14A and 14B so that the output signals from the main and sub scan counters 304 and 305 are each 4 bits.

In the patterns (A) to (P) shown in FIG. 4, data of level "0" (pattern (A) shown in FIG. 4) to data of level "15" (pattern (P) shown in FIG. 4) are stored in the respective memory fields 1 to 16 of the memory 306. Specifically, the data of level "0" of the pattern "A" shown in FIG. 4 is stored in the memory 306 at the memory field 1. Similarly, for the data of respective levels shown in FIGS. 5 to 7, data of the patterns (A) to (P) shown in FIG. 5 are stored in the memory 307, data of the patterns (A) to (P) shown in FIG. 6 are stored in the memory 308, and data of the patterns (A) to (P) shown in FIG. 7 are stored in the memory 309, respectively at the memory fields 1 to 16 starting from the level "0". The memory fields 1 to 16 of the memory 306 have the same address space. For example, when an address "0" is input from the selector 303, each of the memory fields 1 to 16 outputs the first data bit of data of each level. This is also true for the other memories 307 to 309.

Each data in the memories 306 to 309 has 16 gradations (4 bits) from level "0" to level "15", so that the number of memory fields of each memory is 16 (memory fields 1 to 16). The value stored in each memory field of the memory is 8-bit data which is compared with the input monochrome image data (8 bits).

For example, if the value of the output signal 321 of the separation unit 302 is "0", the memory 306 is selected. In this case, if the 6-bit output from the main and sub scan counters is "0", the first data "0" of level "0" from the memory field 1 of the memory 306, the first data "124" from the memory field 2 of the memory 306, . . . , and the first data "250" of level "16" from the memory field 16 of the memory 306, are output at the same time. In this case, if the value of the monochrome image data 300 from the color/monochrome conversion unit 203 is "128", an output of the comparator 311 is (0000000000001111) from MSB at the memory field 16 shown in FIG. 3. This value is encoded by the encoder 312 and a 4-bit signal, for example, "4" is output.

Next, the details of the screen patterns used by the embodiment will be described with reference to FIGS. 8a-8c to 13.

FIGS. 8a-8c are diagrams illustrating general fattening, corresponding to the threshold values shown in FIG. 4.

In FIG. 8a-8c, reference numeral 801 represents the whole screen pattern with the screen angle of about 30 degrees. Reference numeral 802 indicates the order of fattening the screen pattern. Pixels are sequentially painted out in the ascending order of fattening sequential numbers. Reference numeral 803 indicates in a three-dimensional form a state of pixels painted out. A height represents a data value of a pixel. In this example shown at 803, although only three screen masks are shown for the purpose of simplicity, many patterns are actually disposed consecutively. In this screen pattern, the central pixel grows to the maximum value ("256" for 8 bits), and thereafter, the next pixel grows to the maximum value, and so on. The pixels are sequentially grown one after another. With this screen pattern, it is therefore possible to form a dot having a density corresponding to the density of an input image and to reproduce a good half-tone image excepting the edge portion.

FIGS. 9a-9c illustrate another method of growing screen pixels, corresponding to the threshold values shown in FIG. 5.

Reference numerals 901 and 902 shown in FIGS. 9a and 9b correspond to reference numerals 801 and 802 shown in FIGS. 8a and 8b, and so the descriptions thereof are omitted. Similar to reference numeral 803, reference numeral 903 indicates in a three-dimensional form a state of pixels painted out. As different from the state indicated at 803 in FIG. 8c, pixels grow gently as a whole. With this screen mask, the central pixel continues to grow until an input image data exceeds a predetermined level, and after the input image data exceeds the predetermined value, all the adjacent pixels start growing gently at the same time, with the central pixel value being set higher than the adjacent pixels.

Even if the input image data 300 is strong, pixels with some value are present over the whole area of the screen mask even at a highlighted area. Therefore, the images of fine lines and characters can be formed without notches specific to a general screen. In addition, with this method, since there is no significant screen angle, moire can be advantageously suppressed and a good image quality can be obtained with a less number of bits.

This screen pattern can favorably reproduce fine lines and characters at the edge portion.

FIGS. 10a-10c illustrate another method of growing screen pixels, corresponding to the threshold values shown in FIG. 7.

Reference numerals 1001 and 1002 shown in FIGS. 10a-10b are similar to those shown in FIGS. 8A & 8B and 9A & 9B, and so the descriptions thereof are omitted. Reference numeral 1003 indicates a state of pixels wherein the central pixel value of each screen mask is high, and the adjacent pixel values become lower toward the periphery of the mask. This state is an intermediary state between those shown in FIGS. 8c and 9c. With this screen pattern, it is possible to form the stable images without notches of originals containing fine lines and characters as well as half-tone originals such as photographs. This method is characterized in that pixels are grown while maintaining a difference of pixel value between the central pixel and adjacent pixels as constant as possible.

A difference of pixel value between the central pixel and adjacent pixels can be set as desired. If dots specific to a screen are intended not to be conspicuous, the difference is set small. Although a variety of screen pattern growth methods may be considered, two types of growth methods are used in this embodiment.

The screen patterns shown in FIGS. 8a to 10c have the same screen angle and different screen tone intensities.

FIGS. 11 to 13 are diagrams illustrating, from a different viewpoint, the growth of pixels of screen masks shown in FIGS. 8a to 10c.

FIG. 11 shows the screen pattern of FIGS. 8a-8c wherein the horizontal line represents a base value of pixels, the vertical direction indicates a growth state of dots from the upper to the lower in the figure, and a central broken line indicates a boundary between two adjacent screen masks. As seen from FIG. 11, pixels of the adjacent masks do not contact each other immediately after the pixel growth. Accordingly, images of fine lines and characters have notches.

FIGS. 12 and 13 show the screen patterns of FIGS. 9a-9c and 10a-10c wherein the horizontal line represents a base value of pixels, the vertical direction indicates a growth state of dots from the upper to the lower in the figures, and a central broken line indicates a boundary between two adjacent screen masks. As seen from FIGS. 12 and 13, pixels of the adjacent masks contact each other at a relatively earlier stage of the pixel growth. This means that images of fine lines and characters can be formed without notches.

According to the first embodiment of the screen processing method described above, it is possible to output a stable image with concentrated dots for an original having a uniform density such as photographs, and to output an image without notches specific to a general screen pattern for an original containing fine lines and characters.

Without incorporating complicated image area separation for switching between half-tone processing methods for photographs, fine lines, and characters, it is possible to provide an image forming apparatus capable of outputting an image of good quality by using cost effective screen processing.

With the first embodiment described above, a plurality of screen patterns are prepared and a proper one is selected therefrom in accordance with the edge state of input image information. Accordingly, images of fine lines and characters can be formed without notches and half-tone images such as photographs can be reproduced stably.

By preparing a plurality type of screen patterns with different pixel growth and selecting one of them in accordance with the edge state of input image information, it is possible to perform image processing matching the quality of images.

In the above embodiment, the edge detection unit 301 detects an edge of input image information, and in accordance with the detected edge information, a proper screen pattern is selected from a plurality type of screen patterns having different pixel growth stored in advance in the memories. If the screen pattern is selected by an operator, the structure can be simplified.

FIG. 15 shows the structure of a screen processing unit 204 adapted for an operator to select a screen pattern.

The screen pattern data shown in FIGS. 4 to 7 is being stored in RAM 108 shown in FIG. 1. When an operator selects a screen pattern via an unrepresented controller, CPU 106 sets the selected screen pattern data to memories 1303 via the CPU bus 121.

In the screen processing unit 204 shown in FIG. 15, a comparator 1304 compares video data input from the color/monochrome conversion unit 203 with the data in the memories 1303, and the comparison results are encoded by an encoder 1305 to output 4-bit data.

Main and sub scan circuits 1301 and 1302 generate count signals by using Hsync (horizontal sync signal) and Vsync (vertical sync signal) in accordance with the sizes of a screen mask in the main and sub scan directions. This is because the main and sub scan circuits 1301 and 1302 are required to be reset for each size of the screen mask. The reason why the count signals from the main and sub scan circuits 1301 and 1302 are each 3 bits is that the number of bits depends on the repetition (cycle) pattern size of the screen mask data value. Data of a desired screen pattern shown in FIGS. 4 to 7 can be written in each memory 1303 under the control of CPU (FIG. 1). For the screen processing, an access to a particular memory 1303 is achieved by address data sent from the main and sub scan circuits 1301 and 1302 under the control of CPU.

The features of the screen patterns shown in FIGS. 4 to 7 have already been described in connection to the first embodiment, and so the description thereof is omitted. The output results after image data is processed by the screen patterns shown in FIGS. 4, 5, and 7 are shown in FIGS. 16a-16c.

FIG. 16a shows the results processed by the screen pattern of FIGS. 4, 8, and 11, FIG. 16B shows the results processed by the screen pattern of FIGS. 5, 9, and 12, and FIG. 16c shows the results processed by the screen pattern of FIGS. 7, 10a-10c, and 13.

These output results of FIGS. 16a-16c are shown enlarged so as to distinguish between differences of the results. As described above, with the screen processing using the screen patterns shown in FIGS. 5 to 7, the images of fine lines and characters can be output without notches, and moire to be generated by interference with original dots can be reduced.

Furthermore, by selecting one of the screen patterns shown in FIGS. 4 to 7 in accordance with the detected edge information of input image, it becomes possible to perform an optimum screen process most suitable for the quality of input images.

(Second Embodiment)

Next, the second embodiment will be described in which an operator can set a desired method of fattening pixels of a screen.

The structure of the image forming apparatus of this embodiment is the same as shown in FIG. 1, and the structure of an image processing unit 104 is also the same as FIG. 2. FIG. 17 is a block diagram showing the details of a screen processing unit of the second embodiment. A screen mask memory unit 2301 stores data which allows an operator to select a desired screen via an operation unit. Each screen to be selected has the number of screen lines and a screen angle. Data stored in the screen mask memory 2301 will be described with reference to FIGS. 18a-18c which illustratively show three matrices. Numerals in each matrix indicates the sequential order of growing pixels of the matrix. Pixels are grown in the ascending order of these sequential numbers. For example, in the matrix at the upper left in FIG. 18a, the pixel at the center grows first, the pixel under the central pixel grows next, the pixel to the left of the central pixel grows, and so on. This sequential growth order is represented by "Mseq". The screen angle and the number of screen lines encircled by a right frame in FIG. 18a are displayed on the operation unit for the selection of a screen by a user. As the user selects a screen by referring to the displayed data, a mask size (Msize), a sequential growth order (Mseq) of pixels of the mask, and the number (Mmax) of gradations of the screen are output from the screen mask memory unit 2301.

A peak value setting unit 2302 shown in FIG. 17 is provided for a user to set a desired intensity of a screen. The intensity of a screen determines whether dots of a screen are formed clearly, whether the dost are formed not too much clearly, or whether the dots are formed unclearly. The concept of the peak value setting unit 2302 will be described with reference to FIG. 19. For example, in the case of the upper left drawing of FIG. 19, the mask size of the screen is 3.times.(3+1), and the height of the central pixel corresponds to a threshold value of the screen matrix. In this example, the matrix is constituted by 10 pixels. Each pixel becomes low as the input image value becomes small, and high as the input image value becomes large. In setting a peak value, which is a main issue of this embodiment, from the operation unit, a threshold value for the central pixel of the matrix is determined as illustrated in the third mask on the left column and the second mask on the right column of FIG. 19. That is, this setting means how a difference of threshold value between the central pixel and adjacent pixels is set. If this difference is made large, an image can be formed with dots of a screen being made clear. If this difference is made small as illustrated in the right column of FIG. 19, an image can be formed with dots of a screen being made unclear. A peak value set by the user via the operation unit is output from the peak value setting unit 2302.

Input to a screen data generation unit 2303 are Msize (matrix size), Mmax (gradation number of screen), and Mseq (sequential pixel growth order in mask) respectively output from the screen mask memory unit 2301, and Mpeak output from the peak value setting unit 2303. This screen data generation unit 2303 will be detailed with reference to a program shown in FIG. 20. First, in order to set a change rate of pixels of each screen matrix, a step value is calculated from the following equation.

Next, after the counter value is reset to "0", at the first double loop (for statement), a pixel value of at each gradation level is grown to the peak value set by the user while outputting X, Y, and counter value (data). Next, at the second double loop (for statement), the remaining pixel values at each gradation level are grown while outputting X, Y, and counter value (data). Mseq used is a signal output from the screen mask memory unit 2301 and indicates the sequential pixel growth order in the matrix. Screen data thus output is stored in a memory 2305. At this time, a selector 2304 is on the B side upon reception of a selector signal (sell). After all the screen data set is stored in the memory 2305, a memory counter 2307 starts operating so that the contents of the memory 2305 are reallocated and transferred to a memory 2306. At this time, upon reception of selector signals (sel1 and sel2), the selector 2304 and a selector 2308 are on the A side. This data reallocation will be described with reference to FIG. 21. FIG. 21 shows a matrix group (matrices of periodical repetition) at respective gradation levels. Matrices each constituted by 10 pixels are sequentially disposed as shown in the upper drawing of FIG. 21, and the same matrix pattern such as shown in a bold line square is periodically repeated. In transferring the data in the memory 2305 to the memory 2306 shown in FIG. 17, the transfer timings are adjusted basing upon the memory addresses X1, Y1, X2, and Y2 to achieve data reallocation and store the screen threshold value data in the memory 2306.

With the structure described above, after the screen threshold value data set by the user as desired is stored in the memory 2306, the screen process is performed for input image data.

For the screen process, an access to the memory 2306 is enabled by the selector 2308 which switches a memory counter 2307 to a screen counter 2311. A signal from the screen counter 2311 is generated by Hsync and Vsync synchronously with the period as illustrated in FIG. 21. In response to the signal from the screen counter 2311, the memory 2306 outputs all X data at each Y column at the same time. This output data is compared with input video or image data by comparators 2312. If the output of the memory 2306 is smaller than the video data, the comparator 2312 outputs "1". Outputs of the comparators 2312 are encoded by an encoder 2313, and the encoded results are output as a 4-bit signal. This 4-bit data corresponds to the gradation number Mmax of the screen. For example, if Mmax is "8", an output of the encoder 1313 is a 3-bit signal. The encoder 1313 outputs "0" when data (0111111111111111) is input, "2" when data (0011111111111111) is input, . . . , and "15" when data (0000000000000001) is input.

A screen signal value output from the encoder 2313 is supplied to the printer unit 105 whereat 4-bit data is converted into print density data and an image is output.

In the second embodiment described above, a user can set via the operation unit a desired method for fattening pixels of each matrix of a screen.

Therefore, a desired method can be selected by a user from a variety type of image forming methods, such as a method for forming an image with clear dots in a screen, a method for forming an image with rather unclear dots in a screen, and a method for forming an image with unclear dots in a screen with suppressed edge notches.

In the first and second embodiments described above, image data is supplied to the color/monochrome conversion unit 203 of FIG. 2 after it is passed through the gradation correction unit 202. The invention is not limited thereto, but the gradation correction may be performed after the color/monochrome conversion, with the same pattern processing being expected.

A full-color image may be formed without the color/monochrome conversion, by performing the screen process for each C, M, and Y data.

Furthermore, in FIG. 19 illustrating the peak value setting of the second embodiment, the peak value of the central pixel when the adjacent pixels start changing is set. The invention is not limited thereto, but the gradation number Xn when the adjacent pixels start changing may be designated. With this setting method, it is possible to make the screen tone strong up to the density set by the user and to make the screen tone weak from the gradation number set by the user.

Still further, in FIG. 19, although the peak value of the central pixel when the adjacent pixels start changing is set, the invention is not limited thereto. For example, a user may set a desired difference of pixel value between the central pixel and adjacent pixels of a matrix and set a desired gradation number Xn until which the set difference is retained. In this manner, the screen tone can be set as desired.

With this arrangement, the central pixel and adjacent pixels can be gradually changed not by starting the change from only the central pixel.

The invention is applicable not only to a system constituted by a plurality of apparatuses including a host computer, interface, printer, and the like, but also to a single apparatus such as a copier. Obviously, the invention is applicable to a system and an apparatus by supplying them with a program. In this case, a medium storing this program constitutes the present invention. The system or apparatus can operate in a predetermined manner by reading the program from the medium.

The present invention has been described with reference to the preferred embodiments. The invention is not intended to be limited only to those embodiments, but various modifications are possible without departing from the spirit of the invention as recited in the appended claims.