Abstract:
A method of significance group flag coding is disclosed. The method includes: receiving one or more significance group flags associated with a TU (transform unit), wherein the TU is divided into one or more sub-blocks; and coding said one or more significance group flags based on context set selection, wherein the context set selection is associated with significance map coding of the sub-block, and the context set selection depends on horizontal sub-block index, vertical sub-block index, or both the horizontal sub-block index and the vertical sub-block index.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present invention is a divisional of pending U.S. patent application Ser. No. 14/368,264, filed on Jun. 23, 2014, which is a National Phase of PCT Application No. PCT/CN2012/085034, filed on Nov. 22, 2012, which claims priority to U.S. Provisional Patent Application Ser. No. 61/582,725, filed Jan. 3, 2012, entitled “Block-based Significance Map and Significance Group Flag Context Selection Method”. The priority applications are hereby incorporated by reference in their entireties. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to video coding or video processing. In particular, the present invention relates to significance map coding and significance group flag coding. 
       BACKGROUND AND RELATED ARTS 
       [0003]    The arithmetic coding is known as an efficient data compressing method and is widely used in coding standards, such as JBIG, JPEG2000, H.264/AVC, and High-Efficiency Video Coding (HEVC). In H.264/AVC JVT Test Model (JM) and HEVC Test Model (HM), Context-Based Adaptive Binary Arithmetic Coding (CABAC) is adopted as the entropy coding tool for various syntax elements in the video coding system. 
         [0004]      FIG. 1  illustrates an example of CABAC encoder  100  which includes three parts: Binarization  110 , Context Modeling  120 , and Binary Arithmetic Coding (BAC)  130 . In the binarization step, each syntax element is uniquely mapped into a binary string (also called bin or bins in this disclosure). In the context modeling step, a probability model is selected for each bin. The corresponding probability model may depend on previously encoded syntax elements, bin indexes, side information, or any combination of the above. After the binarization and the context model assignment, a bin value along with its associated context model is provided to the binary arithmetic coding engine, i.e., the BAC  130  block in  FIG. 1 . The bin value can be coded in two coding modes depending on the syntax element and bin indexes, where one is the regular coding mode, and the other is the bypass mode. The bins corresponding to regular coding mode are referred to as regular bins and the bins corresponding to bypass coding mode are referred to as bypass bins in this disclosure. In the regular coding mode, the probability of the Most Probable Symbol (MPS) and the probability of the Least Probable Symbol (LPS) for BAC are derived from the associated context model. In the bypass coding mode, the probability of the MPS and the LPS are equal. In CABAC, the bypass mode is introduced to speed up the encoding process. 
         [0005]    High-Efficiency Video Coding (HEVC) is a new international video coding standard that is being developed by the Joint Collaborative Team on Video Coding (JCT-VC). HEVC is based on the hybrid block-based motion-compensated DCT-like transform coding architecture. The basic unit for compression, termed Coding Unit (CU), is a 2N×2N square block, and each CU can be recursively split into four smaller CUs until a predefined minimum size is reached. Each CU contains one or several variable-block-sized Prediction Unit(s) (PUs) and Transform Unit(s) (TUs). For each PU, either intra-picture or inter-picture prediction is selected. Each TU is processed by a spatial block transformation and the transform coefficients for the TU are then quantized. The smallest TU size allowed for HEVC is 4×4. 
         [0006]    In HEVC Test Model Version 5.0 (HM-5.0), the transform coefficients are coded TU by TU. For each TU, syntax elements last_significant_coeff_x and last_significant_coeff_y are transmitted to indicate the last non-zero coefficient horizontal and vertical positions respectively according to a selected scanning order. A TU is divided into multiple subsets for the TUs having size larger than 4×4. For an 8×8 TU, the 64 coefficients are divided into 4 subsets according to the diagonal scanning order through the entire 8×8 TU as shown in  FIG. 2 . The scanning through the transform coefficients will convert the two-dimensional data into a one-dimensional data. Each subset contains 16 continuous coefficients of the diagonally scanned coefficients. For TUs having size larger than 8×8 (e.g. 16×16, 32×32) and non-square TUs (e.g. 16×4, 4×16, 32×8, 8×32), the TUs are divided into 4×4 sub-blocks. Each sub-block corresponds to a coefficient sub-set. For each sub-block (i.e. each subset), the significance map, which is represented by significant_coeff_flag[x,y], is coded first. Variable x is the horizontal position of the coefficient within the sub-block and the value of x is from 0 to (sub-block width−1). Variable y is the vertical position of the coefficient within the sub-block and the value of y is from 0 to (sub-block height−1). The flag, significant_coeff_flag[x,y] indicates whether the corresponding coefficient of the TU is zero or non-zero. For convenience, the index [x,y] is omitted from significant_coeff_flag[x,y]. For each non-zero coefficient as indicated by significant_coeff_flag, the level and sign of the non-zero coefficient is represented by coeff_abs_level_greater1_flag, coeff_abs_level_greater2_flag, coeff_abs_level_minus3, and coeff_sign_flag. 
         [0007]    In HM-5.0, if the TU size is equal to 16×16, 32×32, 16×4, 4×16, 32×8, or 8×32, one significant_coeffgroup_flag is coded for each sub-block prior to the coding of level and sign of the sub-block (e.g. the significant_coeff_flag, coeff_abs_level_greater1_flag, coeff_abs_level_greater2_flag, coeff_abs_level_minus3, and coeff_sign_flag). If significant_coeffgroup_flag is equal to 0, it indicates that the entire 4×4 sub-block is zero. Therefore, there is no need for any additional information to represent this sub-block. Accordingly, the coding of level and sign of sub-block can be skipped. If significant_coeffgroup_flag is equal to 1, it indicates that at least one coefficient in the 4×4 sub-block is non-zero. The level and sign of each non-zero coefficient in the sub-block will be coded after the significant_coeffgroup_flag. The value of significant_coeff_flag is inferred as 1 for the sub-block containing the DC term (i.e., the transform coefficient with the lowest spatial frequency). 
         [0008]    In HM-5.0, significant_coeff_flag is coded in regular CABAC mode with context modeling. Different context selection methods are used for different TU sizes. For TUs with size of 4×4 or 8×8, the context selection is based on the position of the coefficient within the TU.  FIG. 3  shows the position-based context selection map for a 4×4 TU and  FIG. 4  shows the position-based context selection map for an 8×8 TU as adopted in HM-5.0. In  FIG. 3 , significance map  310  is used for the luma component and significance map  320  is used for the chroma component, where each number corresponds to a context selection. In  FIG. 4 , luma and chroma 8×8 TUs share the same significance map. 
         [0009]    For other TU sizes, the neighboring-information-dependent context selection is adopted.  FIGS. 5A and 5B  illustrate examples of the neighboring-information-dependent context selection for luma and chroma components respectively. One context is used for the DC coefficient. For non-DC coefficients (i.e., AC coefficients), the context selection depends on the neighboring coefficients. For example, a group of neighboring non-zero coefficients including I, H, F, E, and B around a current coefficient X are used for the context selection. If none of the neighboring pixels is non-zero, context # 0  is used for coefficient X. If one or two of the neighboring pixels are non-zero, context # 1  is used for X. Otherwise context # 2  is used for coefficient X. 
         [0010]    In the above neighboring-information-dependent context selection, the non-DC coefficients of the entire TU are divided into two regions (i.e., region- 1  and region- 2 ) for the luma component and one region (region- 2 ) for the chroma component. Different regions will use different context sets. Each context set includes three contexts (i.e., context # 0 , # 1 , and # 2 ). The area of region- 1  for the luma component can be mathematically specified by the x-position and y-position of a coefficient X within the TU. As shown in  FIG. 5A , if the sum of x-position and y-position of coefficient X is smaller than a threshold value and greater than 0, region- 1  context set is selected for coefficient X. Otherwise, region- 2  context set is selected. The threshold value can be determined based on the width and the height of the TU. For example, the threshold can be set to a quarter of the maximum value of the TU width and the TU height. Accordingly, in the case of TU sizes 32×32, 32×8 or 8×32, the threshold value can be set to 8. 
         [0011]    In HM-5.0, for TUs with sizes other than 4×4 and 8×8, the TUs will be divided into 4×4 sub-blocks for coefficient map coding. However, the criterion of region- 1 /region- 2  context selection depends on the x-position and y-position of the transform coefficient. Therefore, some sub-blocks may cross the boundary between region- 1  and region- 2  and two context sets will be required for these sub-blocks.  FIG. 6A  illustrates an example where one 4×4 sub-block  610  (the center of the sub-block is indicated by a dot) for 16×16 TU  621 , 16×4  622 , and 4×16 TU  623  will use two context sets for significant_coeff_flag coding.  FIG. 6B  illustrates an example where three 4×4 sub-blocks  631  to  633  for 32×32 TU  641 , 32×8 TU  642 , and 8×32 TU  643  will use two context sets for significant_coeff_flag coding. For sub-blocks  632  and  633 , the sum of x-potion and y-position of coefficient X has to be calculated in order to determine whether the coefficient X is in region- 1  or region- 2 . For the sub-block containing the DC term, i.e., sub-block  631 , the position of the DC term is known and all other coefficients in the sub-block belong to region- 1 . Therefore, significant_coeffgroup_flag can be inferred and there is no need to calculate the sum of x-position and y-position. For other sub-blocks, there is no need to calculate the sum of x-position and y-position of coefficient X since all coefficients of other sub-blocks are in region- 2  and one context set for significant_coeff_flag coding is used. 
         [0012]    Therefore, it is desirable to simplify the context selection process, such as to eliminate the requirement of calculating the sum of x-position and y-position of coefficient or to eliminate other operations. 
       BRIEF SUMMARY OF THE INVENTION 
       [0013]    A method and apparatus for significance group flag coding are disclosed. According to one embodiment of the present invention, the TUs are divided into one or more sub-blocks and the significance group flags are coded based on context set selection. The context set selection is associated with significance map coding of the sub-block, and the context set selection depends on horizontal sub-block index, vertical sub-block index, or both the horizontal sub-block index and the vertical sub-block index. 
         [0014]    In an embodiment, two sub-blocks use same second context selection, second context set selection, or second context formation selection for significance group flag coding if these two sub-blocks use same context selection, context set selection, or context formation selection for the significance map coding. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1  illustrates exemplary architecture of CABAC encoding system with a bypass mode. 
           [0016]      FIG. 2  illustrates an exemplary diagonal scanning order for the transform coefficients of an 8×8 TU. 
           [0017]      FIG. 3  illustrates an example of context selection maps for the 4×4 TU of luma and chroma components used by HEVC Test Model Version 5.0. 
           [0018]      FIG. 4  illustrates an example of context selection map for the 8×8 TU of luma and chroma components used by HEVC Test Model Version 5.0. 
           [0019]      FIG. 5A  illustrates an example of neighboring-information-dependent context selection for the 16×16 TU of luma component used by HEVC Test Model Version 5.0. 
           [0020]      FIG. 5B  illustrates an example of neighboring-information-dependent context selection for the 16×16 TU of chroma component used by HEVC Test Model Version 5.0. 
           [0021]      FIG. 6A  illustrates an example of context selection for the 16×16 TU of luma component used by HEVC Test Model Version 5.0. 
           [0022]      FIG. 6B  illustrates an example of context selection for the 32×32 TU of luma component used by HEVC Test Model Version 5.0. 
           [0023]      FIG. 7A  illustrates an example of block-based context selection for the 16×16 TU of luma component according to an embodiment of the present invention. 
           [0024]      FIG. 7B  illustrates an example of block-based context selection for the 32×32 TU of luma component according to an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0025]    In order to eliminate the need to calculate the sum of x-position and y-position of a coefficient, embodiments of the present invention use block-based context selection to simplify and unify the context set, context selection and context formation for significant_coeff_flag coding. 
         [0026]    For TU sizes other than 4×4 and 8×8, the region- 1 /region- 2  context selection according to one embodiment of the present invention depends on the x-block-index and y-block-index of the sub-block instead of the x-position and y-position of the coefficient X. The x-block-index and y-block-index refer to the horizontal sub-block index and the vertical sub-block index respectively. The value of the x-block-index is from 0 to (number of horizontal sub-blocks−1). The value of the y-block-index is from 0 to (number of vertical sub-blocks−1). In a system incorporating an embodiment of the present invention, none of the sub-blocks will cross the boundary between region- 1  and region- 2 . There is no need to use two context sets for significant_coeff_flag coding or to calculate the sum of x-position and y-position for each coefficient. The region- 1 /region- 2  determination can be based on the sum of the x-block-index and y-block-index of each sub-block. The sum can be compared with a threshold. The threshold value can either depend on the TU width and/or height or can be a fixed value. 
         [0027]      FIG. 7  illustrates an example of block-based context selection according to an embodiment of the present invention. In this example, the threshold value is set to the maximum value of TU width and TU height divided by 16. Therefore, the threshold value is 1 for 16×16 TU  721 , 16×4 TU  722 , and 4×16 TU  723  and the threshold value is 2 for 32×32 TU  741 , 32×8 TU  742  and 8×32 TU  743 . For the luma component, if the sum of x-block-index and y-block-index of the sub-block is smaller than the threshold value, region- 1  context set is used for the sub-block. Otherwise region- 2  context set is used for the sub-block. Accordingly, one sub-block  710  in  FIG. 7A  and three sub-blocks  731  through  733  in  FIG. 7B  use region- 1  context and other sub-blocks use region- 2  context. Furthermore, the value of significant_coeffgroup_flag can be inferred as 1 for region- 1  sub-blocks for unification. 
         [0028]    While the 4×4 sub-block is used as an example of the block-based context selection, other sub-block sizes may also be used. For example, instead of the 4×4 sub-blocks, other sub-blocks such as 4×8, 8×4, 8×8, 16×16 and 32×32 may also be used. While the above block-based significance map coding is used for context selection, the block-based significance map coding may also be used for context set selection or context formation selection. While the examples of block-based significance map coding shown above select context, context set or context formation based on sub-block index in scan order, horizontal sub-block index (i.e., x-block-index) and/or vertical sub-block index (i.e., y-block-index), the selection may also be based on the video component type and/or the TU width/height. The video component type may correspond to the luma component (Y) or the chroma component (Cr or Cb). The video component type may correspond to other video formats. Furthermore, the selection may depend on a combination of sub-block index in scan order, horizontal sub-block index, vertical sub-block index, video component type, and TU width/height. 
         [0029]    The block-based significance group flag coding may be based on sub-block index in scan order, horizontal sub-block index (i.e., x-block-index) and/or vertical sub-block index (i.e., y-block-index). The block-based significance group flag coding may also be based on the video component type and/or the TU width/height. Furthermore, the block-based significance group flag coding may also be based on the context, context set, or context formation selection associated with the significance map coding. The block-based significance group flag coding may also depend on a combination of sub-block index in scan order, horizontal sub-block index, vertical sub-block index, video component type, TU width/height, context, context set, and context formation selection associated with the significance map coding. 
         [0030]    The above description is presented to enable a person of ordinary skill in the art to practice the present invention as provided in the context of a particular application and its requirement. Various modifications to the described embodiments will be apparent to those with skill in the art, and the general principles defined herein may be applied to other embodiments. Therefore, the present invention is not intended to be limited to the particular embodiments shown and described, but is to be accorded the widest scope consistent with the principles and novel features herein disclosed. In the above detailed description, various specific details are illustrated in order to provide a thorough understanding of the present invention. Nevertheless, it will be understood by those skilled in the art that the present invention may be practiced. 
         [0031]    Embodiment of the present invention as described above may be implemented in various hardware, software codes, or a combination of both. For example, an embodiment of the present invention can be a circuit integrated into a video compression chip or program code integrated into video compression software to perform the processing described herein. An embodiment of the present invention may also be program code to be executed on a Digital Signal Processor (DSP) to perform the processing described herein. The invention may also involve a number of functions to be performed by a computer processor, a digital signal processor, a microprocessor, or field programmable gate array (FPGA). These processors can be configured to perform particular tasks according to the invention, by executing machine-readable software code or firmware code that defines the particular methods embodied by the invention. The software code or firmware code may be developed in different programming languages and different formats or styles. The software code may also be compiled for different target platforms. However, different code formats, styles and languages of software codes and other means of configuring code to perform the tasks in accordance with the invention will not depart from the spirit and scope of the invention. 
         [0032]    The invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described examples are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.