Content-based similarity retrieval system for image data

[0002] 1. Field of the Invention

[0003] The present invention is generally directed to image retrieval systems, and more particularly, to content-based retrieval of similar-looking images by employing multi-instance or relevance feedback to query images.

[0004] 2. Description of the Related Art

[0005] The importance of content-based retrieval techniques for multimedia is assured in view of the widely used and distributed international coding standards, such as JPEG and MPEG, on the Internet. A particular multimedia content description interface, MPEG-7, is currently used to provide normal numerical descriptors for database search engine as matching criteria. For 2D shapes, MPEG-7 uses contour-based and region-based descriptors. Although describing shape contour by Fourier descriptors (FDs) can provide size, rotation and transition invariants for indexing, FDs are sensitive to noises and are more suitable for describing closed contour of shapes. For region-based descriptors, zernike and pseudo-zernike moments (ZMs and PZMs) are efficient features for retrieving similar shapes. Specific features such as edge orientation, aspect ratio, or complexity can be extracted for different databases and applications. Statistics, such as histogram or probability distribution model for the above-described features, are computed and considered as matching criteria for similarity measurement.

[0006] Shape descriptors have also been used to extract shape boundaries which are grouped into families on the basis of perceptual similarity. Visually salient feature is determined using probabilistic distribution model of trademarks in database and then trademarks with similar shape were retrieved according to this salient feature. A Multi-resolution description can be obtained by computing the mean and variance of each wavelet sub-band as to provide a concise description of the shape's texture and shape. Shape features as discussed above can also be brought up according to specific databases and user requirements. Although efficient in retrieving similar shapes for one application, one set of universal descriptors cannot satisfy all specific requirements. Indeed, one set of descriptors may perform well for one database but not the other, and each user may need specific combination of features for their specific retrieval target.

[0007] Accordingly, an object of the present invention is to provide a content-based retrieval method and apparatus which retrieves statistically salient common features among sample query images having different feature sets.

[0008] It is another object of the present invention to update sample query images by multi-instance or relevance feedback.

[0009] It is still another object of the present invention to provide a universal query mechanism (UQM) or process that is plural and flexible in selecting proper features as to meet a user's intent or requirement.

[0010] These and other objects of the present invention are achieved by providing a content-based retrieval method and apparatus which finds the most common features among each set of sample query images from multi-instance or relevance feedback. In particular, each set of sample query images is constructed by finding similar images shapes in the database. The resulting sample query images are statistically similar to query input, i.e. relative instead of absolute similarity. A probability distribution model for the feature vectors is used to dynamically adjust weights such that most common ones among sample query images dominates feedback query. Whenever new feature sets are devised, they could be acquired by the query system. The query unit then searches from all feature sets such that the statistically common features become the new query vector. Accordingly, the UQM accommodates new feature sets easily and adjusts weights for various features dynamically according to a user's query and statistics of the database.

[0011] For similarity retrieval, each user has his definition for shape similarity and no one universal set of shape descriptors could satisfy all specific requirements. Usually, the indexing system provides relevance feedback to learn what user's intention is and generating a new feature vector for next query. Instead of using mean of feature vectors (MFV) from sample query images as the new query vector, the UQM of the present invention finds statistically salient common features among sample query images with different feature sets.

[0012] Related aspects and advantages of the invention will become apparent and more readily appreciated from the following detailed description of the invention, taken in conjunction with the accompanying drawings.

[0013] FIG. 1 is a schematic diagram showing a content-based retrieval method according to a preferred embodiment of the present invention;

[0014] FIG. 2 is a schematic diagram showing a UQM according to the preferred embodiment of the present invention;

[0015] FIG. 3 is a schematic chart showing the measurement of feature saliency; and

[0016] FIGS. 4A-C and 5A-B show results of the content-based retrieval method according to a preferred embodiment of the present invention

[0017] The present invention will now be described by way of preferred embodiments with references to the accompanying drawings. Like numerals refer to corresponding parts of various drawings.

[0018] One embodiment of content-based similarity retrieval process for database images is shown in FIG. 1. During the first phase, common features of multimedia content in a multimedia database are extracted by feature extraction units 12 and 14 and stored in feature databases 16 and 18. In the second phase (i.e., similarity retrieval), a user selects an initial query image either through an on-line editing interface 20 or from one or more sample images stored in the database 10. If a new image is created from the on-line interface 20, features in the new image must be extracted by both feature extraction units 12 and 14 before searching in the feature databases 16 and 18. If one of the sample images stored in the database 10 were used as the initial query image, then only an identification code will be sent to the feature databases 16 and 18, which sends associated feature of the initial query image to a UQM 22 such that both query units 24 and 26 in the UQM can generate a representative feature vector for retrieving a set of sample query images.

[0019] Since more than one feature set may be created and new features can be plugged into the UQM based on a user's requirement, statistically common features are retrieved among sample query images through multi-instance or by relevance feedback. The weight factor for each feature is adjusted by UQM to meet a user's query requirement. The query results are the same when non-related features are involved or excluded in the retrieval process.

[0020] Assume n feature sets are created, then the feature vector for sample s can be represented as {right arrow over (F)}_s=({right arrow over (𝑓)}₁, {right arrow over (𝑓)}₂, . . . , {right arrow over (𝑓)}_n), where the i-th feature set is {right arrow over (𝑓)}_i=(𝑓_i1, 𝑓_i2, . . . 𝑓_iN). Let {right arrow over (N)}_t={ij|i=1, . . . , n, j=1, . . . , N_i}, and the feature vector can be represented as {right arrow over (F)}_s={𝑓_k|k∈{right arrow over (N)}_t}. Let the probability distribution function of feature 𝑓_ibe p(𝑓_i), then the probability density function P(𝑓) would be: $\begin{array}{cc}P\left(f\right)={\int }_0^{f}p\left(f_i\right)d{f}_i& \left(1\right)\end{array}$

[0021] Through multi-instance or relevance feedback, the UQM as shown in FIG. 2 seeks to find common features among sample query images. Weight factors are adjusted such that statistically common features dominate in the representative feature vector for next sample query images. Let there be q sample query images, either for multi-instance or relevance feedback, and the q feature vectors available are {right arrow over (F)}_swhere s=1, . . . , q. Mean and variance of these vectors could be represented as: $\begin{array}{cc}{\left\{m_j\right\}}_{{\stackrel{\rightharpoonup }{N}}_t}=E\left[{\overrightarrow{F}}_s\right]={\left\{E\left[f_j\right]\right\}}_{j\in {\stackrel{\rightharpoonup }{N}}_t}={\left\{E\left\{f_{\mathrm{ij}}❘i=1,\dots ,q\right\}\right\}}_{j\in {\stackrel{\rightharpoonup }{N}}_t}\text{and}& \left(2\right)\\ {\left\{{\sigma }_j^2\right\}}_{{\stackrel{\rightharpoonup }{N}}_t}={\left\{E\left[{\left(f_j-m_j\right)}^2\right]\right\}}_{j\in {\stackrel{\rightharpoonup }{N}}_t}={\left\{E\left\{f_{\mathrm{ij}}^2❘i=1,\dots ,q\right\}-m_j^2\right\}}_{j\in {\stackrel{\rightharpoonup }{N}}_t}.& \left(3\right)\end{array}$

[0022] For similarity measurement in statistics as shown in FIG. 3, positive and negative feedback samples are provided and feature compactness F_C(•) and feature diversity F_D(•) in probability are defined as:

[0023] F_C(m,σ)=P(m+σ)−P(m−σ); and  (4)

[0024] F_D(m₁,σ₁,m₂,σ₂)=P(m₂−σ₂)−P(m₁+σ₁).  (5)

[0025] F_D(•)=1 is set so that if the number of negative feedback samples is zero. The weight, w_j, of j-th feature, 𝑓_j, for the next query are adjusted according to the following relation: $\begin{array}{cc}{\omega }_j\cong \frac{F_D\left(m_j^P,{\sigma }_j^P,m_j^N,{\sigma }_j^N\right)}{F_C\left(m_j^P,{\sigma }_j^P\right)·F_C\left(m_j^N,{\sigma }_j^N\right)}& \left(6\right)\end{array}$

[0026] For one feature, say j-th feature, which is statistically similar among query sample images, it would yield a smaller F_C(•), and this feature would get high credit because of larger w_jis assigned. Whenever new feature sets are created, statistically common features among all feature sets would dominate the query process no matter how many feature sets are specified. In short, feature sets could be integrated or excluded from the query according to a user's requirement without affecting query results, i.e., only statistically similar features dominate the query.

[0027] For applications that provide positive and negative samples, the query unit intends to enhance a positive feature while excluding the negative ones. Since F_D(m_j^P,σ_j^P,m_j^N,σ_j^N) evaluates the depletion region between features to be discriminated, features to be excluded needs not be statistically common hence the following is set: F_C(m_j^N,σ_j^N)=1. In addition, to make common features dominate and to exclude unrelated features, the following is set: W_j=1 when the ratio in equation (6) is larger than a predefined threshold and 0 for others.

[0028] In short, the control steps of the invention are summarized as follows:

[0029] (1) the feature vector (𝑓₁,𝑓₂, . . . , 𝑓_N) is extracted for each sample image in the database;

[0030] (2) the probability distribution function is computed and stored for each feature either by {(m_i,σ_i)}_{i=1, . . . , N}or {p(𝑓_ij)|j=1, . . . , M}_{i=1, . . . , N};

[0031] (3) for each query, compute mean (m) and standard deviation (σ) for each feature among query image;

[0032] (4) find the feature commonality for each feature i.e., F_C(m,σ)=P(m+σ)−P(m−σ) and generate new weight by the following condition: i𝑓(F_c<P_T) ω_i=1 else ω_i=0; and

[0033] (5) perform new retrieval by new feature vector (m₁,m₂, . . . , m_N) and new weight vector (ω₁,ω₂, . . . , ω_N).

[0034] Statistical characteristics of features in the database are exploited for similarity retrieval. In cases that most feature values of the same family diverse while they are similar in probability space, the retrieval results may not converge from human perception. It happens when there are very few similar images in the database or too many noises in the images. For this, w_i=0 is set to keep the common feature dominating the query process when ${\sigma }_i^c>\frac{1}{c}·{\sigma }_i^s,$

[0035] where σ_i^cand σ_i^care standard deviation of ith features from relevant and all database samples, respectively.

[0036] Results of the content-based retrieval method according to a preferred embodiment of the present invention are shown in FIGS. 4 and 5. Thirty thousand registered Taiwan trademarks were collected from Taiwan Bell Trademark Company. They contain text pattern, animal, regular geometrical shape and shapes with text et al. These shape patterns were segmented manually from scanned documents in JPEG format. Each one is pre-processed by locating MBC of shapes before further feature extraction. For efficient feature extraction, magnitude of ZM and PZM were computed for each sample by the lookup-table method with order, n, up to ten. The numbers of ZM and PZM are 36 and 66, respectively.

[0037] FIG. 4A is the retrieval results by one query image 42. Image 43 is selected as the relevance feedback for the next retrieval. Mean of feature vectors of the two instances (images 42 and 43) is first used as the input for the second query. FIG. 4B shows the second query results and two visually similar shapes, i.e., images 44 and 45, other than those in FIG. 4A, are retrieved if features were not weighted by the UQM method. In FIG. 4C, three more similar shapes in images 46, 47 and 48 are retrieved when the proposed UQM are used.

[0038] Another retrieval results of query by three-instances are demonstrated in FIGS. 5A and 5B. In FIG. 5A, the proposed UQM searches out two more visually similar images 50 and 51 that are not presented in FIG. 5B in which features are not weighted by UQM.

[0039] The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. Scope on the invention is thus indicated by the claims rather than by the forgoing description, and all changes which come within the meanings and range of equivalency of the claims are therefore intended to be embraced herein.