Patent Publication Number: US-2022217339-A1

Title: Deblocking filter device, decoding device and program

Description:
RELATED APPLICATIONS 
     The present application is a continuation based on PCT Application No. PCT/JP2021/021814, filed on Jun. 8, 2021, which claims the benefit of Japanese Patent Application No. 2020-101293 filed on Jun. 10, 2020. The content of which is incorporated by reference herein in their entirety. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a deblocking filter device, decoding device and a program. 
     BACKGROUND ART 
     In HEVC (High Efficiency Video Coding), and VVC (Versatile Video Coding), which is a next-generation encoding scheme, a deblocking filter is adopted as an in-loop filter, to restrain distortion at a block boundary portion when an encoding process is performed on a block basis. In control of the deblocking filter, boundary filtering strength of the deblocking filter is controlled in accordance with whether or not a non-zero transform coefficient exists in at least one of two adjacent blocks. This is because, since energy of a prediction residual is distributed all over the block as a result of inverse transform of the non-zero transform coefficient, discontinuity is likely to occur at the boundary between the two blocks. 
     CITATION LIST 
     Patent Literature 
     
         
         Non Patent Literature 1: JVET-Q2001 “Versatile Video Coding (Draft 8)” 
       
    
     DISCLOSURE OF INVENTION 
     A deblocking filter device according to a first feature includes: a deblocking filter configured to perform a filter process on a boundary between a first reconstructed block and a second reconstructed block adjacent to the first reconstructed block; and a filter controller configured to control boundary filtering strength of the deblocking filter based on whether or not at least one of the first reconstructed block and the second reconstructed block is encoded using JCCR (Joint coding of chroma residual) in which one joint prediction residual is generated from prediction residuals of a Cb chrominance component and a Cr chrominance component. 
     A decoding device according to a second feature includes: the deblocking filter device according to the first feature; and an entropy decoder configured to acquire a flag indicating whether or not a block is encoded using the JCCR for each of the first reconstructed block and the second reconstructed block, wherein the filter controller controls the boundary filtering strength of the deblocking filter based on the flag for each of the first reconstructed block and the second reconstructed block. 
     A program according to a third feature causes a computer to function as the deblocking filter device according to the first feature. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a diagram illustrating the configuration of an encoding device according to an embodiment. 
         FIG. 2  is a diagram for explaining operation of a deblocking filter according to the embodiment. 
         FIG. 3  is a diagram illustrating a configuration of a decoding device according to the embodiment. 
         FIG. 4  is a diagram illustrating an operation example of a filter controller according to the embodiment. 
         FIG. 5  is a diagram illustrating a configuration of an encoding device according to modification 2. 
         FIG. 6  is a diagram illustrating a configuration of a decoding device according to modification 2. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     In a draft of VVC standards, a technique called adaptive color transform (ACT) is employed in which when a chroma format of an input image is 4:4:4, a color space (RGB space) of a prediction residual is transformed into a YCgCo space, and an encoding process such as a transform process and an entropy encoding process is performed on the prediction residual after color space transform (see Non Patent Literature 1). An encoding device can control whether or not to apply ACT for each encoding-target block and outputs a stream of an ACT application flag for each encoding-target block. A decoding device restores the prediction residual by performing an entropy decoding/inverse transform process, and the like, on the block encoded using ACT and inversely transforms the color space (YCgCo space) of the restored prediction residual into the RGB space. 
     In the above-described technique in related art, when a non-zero transform coefficient exists in at least one of two adjacent blocks, the decoding device applies a deblocking filter to a boundary of the two blocks. On the other hand, it is also possible to employ a configuration where, when a non-zero transform coefficient exists in neither of the two adjacent blocks, the decoding device does not apply the deblocking filter to the boundary of the two blocks. 
     However, for a block to which ACT is applied, after the prediction residual is restored from the transform coefficients through the inverse transform process, the color space of the prediction residual is inversely transformed from the YCgCo space into the RGB space through color space inverse transform. Thus, when a non-zero transform coefficient exists in a block of a certain color component, the non-zero transform coefficient affects blocks of other color components upon the color space inverse transform. 
     Thus, if deblocking filter control based on whether or not a non-zero transform coefficient exists is applied to a block encoded using ACT, boundary filtering strength of the deblocking filter cannot be appropriately controlled, which may cause deterioration of image quality. 
     A problem similar to the above can occur also when joint coding of chroma residual (JCCR) which is another encoding tool is applied. 
     The present disclosure is therefore directed to providing a deblocking filter device that prevents deterioration of image quality, a decoding device and a program. 
     An encoding device and a decoding device according to an embodiment are described with reference to the accompanying drawings. The encoding device and the decoding device according to the embodiment encode and decode videos such as MPEG videos. In the description of the drawings below, the same or similar reference signs are used for the same or similar parts. 
     First Embodiment 
     &lt;Configuration of Encoding Device&gt; 
     A configuration of an encoding device according to the present embodiment will be described first.  FIG. 1  is a diagram illustrating a configuration of an encoding device  1  according to the present embodiment. 
     As illustrated in  FIG. 1 , the encoding device  1  includes a block divider  100 , a residual generator  110 , a switcher  111 , a color space transformer  112 , a transformer/quantizer  120 , an entropy encoder  130 , an inverse quantizer/inverse transformer  140 , a switcher  143 , a color space inverse transformer  144 , a combiner  150 , a deblocking filter  160 , a memory  170 , and a predictor  180 . 
     The block divider  100  divides an original image which is an input image in frame (or picture) units that constitutes a video into a plurality of image blocks and outputs the image blocks obtained by division to the residual generator  110 . The size of the image blocks may be 32×32 pixels, 16×16 pixels, 8×8 pixels, or 4×4 pixels. The shape of the image blocks is not limited to square and may be rectangular (non-square). The image block is a unit (encoding-target block) in which the encoding device  1  performs encoding and is a unit (decoding-target block) in which a decoding device performs decoding. Such an image block is sometimes referred to as a CU (Coding Unit). 
     The input image is an RGB signal and may have a chroma format of 4:4:4. The RGB space is an example of a first color space. An “R” component corresponds to a first component, a “G” component corresponds to a second component, and a “B” component corresponds to a third component. The block divider  100  outputs a block by performing block division on each of the R component, the G component and the B component that constitute the image. In the following description of the encoding device, the block will be simply referred to as an encoding-target block when the respective color components are not distinguished from each other. 
     The residual generator  110  calculates prediction residuals that represent differences (errors) between an encoding-target block outputted by the block divider  100  and a prediction block obtained by the predictor  180  predicting the encoding-target block. More specifically, the residual generator  110  calculates the prediction residuals by subtracting each pixel value of the prediction block from each pixel value of the encoding-target block, and outputs the calculated prediction residuals to the switcher  111 . In the present embodiment, the residual generator  110  generates a prediction residual of each color component from a difference between the encoding-target block of each color component and a prediction block of each color component. 
     The switcher  111  outputs a prediction residual of each color component outputted from the residual generator  110  to one of the transformer/quantizer  120  and the color space transformer  112 . The switcher  111  outputs the prediction residual to the transformer/quantizer  120  when a color space transform process (ACT process) is not performed and outputs the prediction residual to the color space transformer  112  when the ACT process is performed. 
     The color space transformer  112  performs the ACT process on the prediction residual of each color component and outputs the prediction residual after the ACT process to the transformer/quantizer  120 . The color space transformer  112  generates a new prediction residual by performing transform calculation as follows for the R component, the G component and the B component of the prediction residual of the encoding-target block. 
     
       
         
           
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     However, “&gt;&gt;” represents right shift operation. Further, the “Y” component corresponds to the first component, the “Cg” component corresponds to the second component, and the “Co” component corresponds to the third component. Such a YCgCo space is an example of a second color space. 
     The switcher  111  and the color space transformer  112  can control whether or not to perform a color transform process for each encoding-target block basis. The entropy encoder  130  signals a flag (ACT application flag) indicating whether or not the color transform process is performed on the encoded block in a bit stream. 
     Note that in the ACT process at the color space transformer  112 , it is only necessary to generate a prediction residual constituted with new color components by performing addition, subtraction, multiplication, division, a shift process, and the like, on each color component. Further, the ACT process does not have to be transform that affects all color components. For example, the color space transformer  112  may apply an ACT process of maintaining the first component without changing the first component, setting an average value of the second component and the third component as a new second component, and setting a difference between the second component and the third component as a new third component. 
     The transformer/quantizer  120  executes a transform process and a quantization process on each of blocks. The transformer/quantizer  120  includes a transformer  121  and a quantizer  122 . 
     The transformer  121  calculates transform coefficients by performing a transform process on the prediction residual (which will be referred to as a prediction residual regardless of whether or not the ACT process is applied) outputted from the switcher  111  or the color space transformer  112  and outputs the calculated transform coefficients to the quantizer  122 . More specifically, the transformer  121  generates transform coefficients of each color component by performing a transform process on the prediction residual of each color component on a block basis. The transform process only requires to be a frequency transform such as DCT, DST, and discrete wavelet transform. Further, the transformer  121  outputs information regarding the transform process to the entropy encoder  130 . 
     The transform process includes transform skip in which no transform process is performed, employed in the HEVC or a draft of the VVC standards. While in a transform skip mode in the HEVC, transform coefficients is obtained by performing scaling on the prediction residual without performing horizontal and vertical transform processes, in transform skip according to the present embodiment, the transform process includes transform in which only a horizontal transform process is applied and transform in which only a vertical transform process is applied. Further, the transformer  121  may perform a secondary transform process in which a transform process is further applied to the transform coefficients obtained through the transform process. Further, the secondary transform process may be applied to only part of the area of the transform coefficients. 
     The quantizer  122  quantizes the transform coefficients outputted from the transformer  121  using a quantization parameter and a scaling list and outputs the quantized transform coefficients to the entropy encoder  130  and the inverse quantizer/inverse transformer  140 . Further, the quantizer  122  outputs information regarding the quantization process (more specifically, information on the quantization parameter and the scaling list used in the quantization process) to the entropy encoder  130  and the inverse quantizer  141 . 
     The entropy encoder  130  performs entropy encoding on the quantized transform coefficients outputted by the quantizer  122 , generates a bit stream (encoded data) by performing data compression, and outputs the bit stream to the decoding side. For the entropy encoding, Huffman coding and/or CABAC (Context-based Adaptive Binary Arithmetic Coding) or the like can be used. Moreover, the entropy encoder  130  adds information regarding the transform process inputted from the transformer  121  into the bit stream and signals to the decoding side, and adds information regarding a prediction process inputted from the predictor  180  into the bit stream and signals to the decoding side. 
     Further, the entropy encoder  130  adds a color space transform flag indicating whether or not ACT is applied, for each encoding-target block, into the bit stream and signals to the decoding side. Such a color space transform flag is also referred to as an ACT application flag. When the ACT application flag is ON (“1”), it is indicated that ACT is applied to a corresponding encoding-target block. When the ACT application flag is OFF (“0”), it is indicated that the ACT is not applied to the corresponding encoding-target block. Note that an ACT non-application flag may be used instead of the ACT application flag. In that case, when the ACT non-application flag is ON (“1”), it is indicated that the ACT is not applied to the corresponding encoding-target block. When the ACT non-application flag is OFF (“0”), it is indicated that the ACT is applied to the corresponding encoding-target block. 
     The inverse quantizer/inverse transformer  140  executes an inverse quantization process and an inverse transform process on each of blocks. The inverse quantizer/inverse transformer  140  includes an inverse quantizer  141  and an inverse transformer  142 . 
     The inverse quantizer  141  performs the inverse quantization process corresponding to the quantization process performed by the quantizer  122 . More specifically, the inverse quantizer  141  inverse quantizes the quantized transform coefficients outputted by the quantizer  122  by using the quantization parameter (Qp) and the scaling list to restore the transform coefficients, and outputs the restored transform coefficients to the inverse transformer  142 . 
     The inverse transformer  142  performs an inverse transform process corresponding to the transform process performed by the transformer  121 . For example, when the transformer  121  performs discrete cosine transform, the inverse transformer  142  performs inverse discrete cosine transform. The inverse transformer  142  restores the prediction residual by performing the inverse transform process on the transform coefficients outputted from the inverse quantizer  141  and outputs a restoration prediction residual that is the restored prediction residual to the switcher  143 . 
     The switcher  143  outputs the restoration prediction residual of each color component outputted from the inverse transformer  142  to one of the combiner  150  and the color space inverse transformer  144 . The switcher  143  outputs the restoration prediction residual to the combiner  150  for a block to which the ACT is not applied and outputs the restoration prediction residual to the color space inverse transformer  144  for a block to which the ACT is applied. 
     The color space inverse transformer  144  performs a color space inverse transform process (inverse ACT process) that is an inverse process of the ACT process performed by the color space transformer  112  and outputs the prediction residual after the inverse ACT process to the combiner  150 . More specifically, the color space inverse transformer  144  inversely transforms the YCgCo space into the RGB space by performing inverse transform calculation as follows using the Y component, the Cg component and the Co component of the restored prediction residual. 
     
       
         
           
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     The combiner  150  combines the restoration prediction residual outputted from the inverse transformer  142  or the color space inverse transformer  144  and a prediction block outputted from the predictor  180  on a pixel-by-pixel basis. The combiner  150  restores (reconstructs) the encoding-target block by adding individual pixel values of the restoration prediction residual to individual pixel values of the prediction block and outputs the restored block to the deblocking filter  160 . Note that the restored block may be also referred to as a reconstructed block. 
     The deblocking filter  160  performs a filter process on the restored block outputted from the combiner  150  and outputs the restored block after the filter process to the memory  170 . The filter controller  161  controls the deblocking filter  160 . The deblocking filter  160  and the filter controller  161  will be described in detail later. 
     The memory  170  stores the restored block after the filter process outputted from the deblocking filter  160  and accumulates the restored blocks as restored images in units of frames. The memory  170  outputs the stored restored blocks or the restored images to the predictor  180 . 
     The predictor  180  performs a prediction process in units of the block. The predictor  180  generates a prediction block for each color component by performing a prediction process such as intra prediction and inter prediction on the encoding-target block. The predictor  180  includes an inter predictor  181 , an intra predictor  182 , and a switcher  183 . 
     The inter predictor  181  performs inter prediction utilizing an inter-frame correlation. Specifically, the inter predictor  181  calculates a motion vector through a scheme such as block matching by using the restored image stored in the memory  170  as a reference image, generates an inter prediction block by predicting the encoding-target block, and outputs the generated inter prediction block to the switcher  183 . Here, the inter predictor  181  selects an optimal inter prediction method from inter prediction using a plurality of reference images (typically, bi-prediction), inter prediction using one reference image (uni-directional prediction), and performs the inter prediction by using the selected inter prediction method. The inter predictor  181  outputs information regarding the inter prediction (the motion vector and the like) to the entropy encoder  130 . 
     The intra predictor  182  performs intra prediction utilizing an intra-frame spatial correlation. Specifically, the intra predictor  182  generates an intra prediction block by referring to restored pixels present around the encoding-target block of the restored image stored in the memory  170 , and outputs the generated intra prediction block to the switcher  183 . The intra predictor  182  selects an intra prediction mode to be applied to the encoding-target block from among a plurality of intra prediction modes, and predicts the encoding-target block by using the selected intra prediction mode. 
     The switcher  183  switches the inter prediction block outputted from the inter predictor  181  and the intra prediction block outputted from the intra predictor  182  and outputs one of the prediction blocks to the residual generator  110  and the combiner  150 . 
     The deblocking filter  160  and the filter controller  161  according to the present embodiment will be described next. 
     The deblocking filter  160  performs a filter process on a block boundary between two blocks including a restored block (first block) and a restored block (second block) adjacent to the restored block and outputs the respective restored blocks after the filter process to the memory  170 . The filter process is a process for mitigating signal deterioration caused by the block-based processes and is a filter process of smoothing a signal gap at a block boundary between two adjacent blocks. 
     The filter controller  161  controls the deblocking filter  160 . More specifically, the filter controller  161  controls boundary filtering strength Bs (Boundary strength) indicating whether or not to perform a filter process on the block boundary of the block pair and filtering strength of the deblocking filter  160 . The boundary filtering strength Bs refers to a parameter for determining whether or not to apply a filter process and a type of the filter process. Note that control of whether or not a filter process is performed can be regarded as control of whether the boundary filtering strength Bs is set to one or more, or to zero. 
       FIG. 2  is a diagram for explaining operation of the deblocking filter  160  according to the present embodiment. In the example illustrated in  FIG. 2 , the deblocking filter  160  performs a filter process on a block boundary for each block of 8×8 pixels. Further, the deblocking filter  160  performs a filter process in units of four rows or four columns.  FIG. 2  illustrates an example where each of a block P (first reconstructed block) and a block Q (second reconstructed block) is one unit of the filter process of the deblocking filter  160 , and a block size is 4×4 pixels. Each of the blocks P and Q may be called a sub-block. The block Q is a restored block corresponding to the encoding-target block, and the block P is a restored block adjacent to the block Q. 
     The filter controller  161  determines the boundary filtering strength Bs on the basis of the following Table 1. It is assumed in the present embodiment that a value of the boundary filtering strength Bs is one of 0, 1 and 2. 
     
       
         
           
               
               
             
               
                   
               
               
                 Bs value 
                 Conditions for determining Bs value 
               
               
                   
               
             
            
               
                 2 
                 Intra prediction is applied to at least one of blocks 
               
               
                 1 
                 Difference between motion vectors of both blocks is equal to  
               
               
                   
                 or larger than threshold value 
               
               
                 1 
                 The numbers of motion vectors of both blocks, or reference  
               
               
                   
                 images thereof are different 
               
               
                 1 
                 At least one of blocks includes a significant transform coefficient  
               
               
                   
                 (a non-zero transform coefficient) 
               
               
                 1 
                 ACT is applied to at least one of blocks 
               
               
                 0 
                 Other than above 
               
               
                   
               
            
           
         
       
     
     As illustrated in  FIG. 2  and Table 1, the filter controller  161  sets the Bs value to 2 when intra prediction is applied to at least one of the blocks P and Q. 
     The filter controller  161  sets the value of Bs to 1 when motion-compensated prediction (inter prediction) is applied to both of the blocks P and Q, and when at least one condition of the following (a) to (d) is satisfied, and otherwise sets the value of Bs to 0. 
     (a) The absolute value of a difference between motion vectors of the blocks P and Q is equal to or larger than a threshold value (for example, one pixel). 
     (b) The numbers of motion vectors of the blocks P and Q, or reference images thereof, are different. 
     (c) At least one of the blocks P and Q includes a significant transform coefficient (that is, a non-zero transform coefficient). 
     (d) ACT is applied to at least one of the blocks P and Q. 
     The filter controller  161  controls the deblocking filter  160  such that the deblocking filter process is not performed when the value of the boundary filtering strength Bs is 0. Hereinafter, a description is given, taking a vertical block boundary illustrated in  FIG. 2  as an example. 
     When the value of the boundary filtering strength Bs is 1 or 2, the filter controller  161  may control the deblocking filter  160  such that the deblocking filter process is performed only when a following expression (1) is satisfied. 
     
       
         
           
             
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     Note that when the deblocking filter process is performed, the filter controller  161  may apply a stronger filter when all of following conditional expressions (2) to (7) are satisfied, and otherwise apply a weaker filter. 
     
       
         
           
             
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                             3 
                             3 
                           
                         
                         - 
                         
                           p 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             0 
                             3 
                           
                         
                       
                        
                     
                     + 
                     
                        
                       
                         
                           q 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             0 
                             3 
                           
                         
                         - 
                         
                           q 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             3 
                             3 
                           
                         
                       
                        
                     
                   
                   &lt; 
                   
                     β 
                     ⁢ 
                     
                       / 
                     
                     ⁢ 
                     8 
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
             
               
                 
                   
                      
                     
                       
                         p 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         
                           0 
                           0 
                         
                       
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                         ⁢ 
                         
                             
                         
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                           0 
                         
                       
                     
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                   &lt; 
                   
                     
                       ( 
                       
                         
                           5 
                           ⁢ 
                           
                             t 
                             C 
                           
                         
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                         1 
                       
                       ) 
                     
                     ⁢ 
                     
                       / 
                     
                     ⁢ 
                     2 
                   
                 
               
               
                 
                   ( 
                   6 
                   ) 
                 
               
             
             
               
                 
                   
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                         p 
                         ⁢ 
                         
                             
                         
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                       ( 
                       
                         
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                           ⁢ 
                           
                             t 
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                       ) 
                     
                     ⁢ 
                     
                       / 
                     
                     ⁢ 
                     2 
                   
                 
               
               
                 
                   ( 
                   7 
                   ) 
                 
               
             
           
         
       
     
     However, the threshold value β and the value t C  change according to an average value Q av  of the quantization parameter between the adjacent blocks P and Q. 
     &lt;Configuration of Decoding Device&gt; 
     Next, a decoding device according to the present embodiment is described, focusing mainly on differences from the encoding device  1 .  FIG. 3  is a diagram illustrating a configuration of the decoding device  2  according to the present embodiment. 
     As illustrated in  FIG. 3 , the decoding device  2  includes an entropy decoder  200 , an inverse quantizer/inverse transformer  210 , a switcher  215 , a color space inverse transformer  216 , a combiner  220 , a deblocking filter  230 , a memory  240 , and a predictor  250 . 
     The entropy decoder  200  decodes the encoded data (bit stream), acquires quantized transform coefficients corresponding to a decoding-target block and outputs the acquired quantized transform coefficients to the inverse quantizer/inverse transformer  210 . Further, the entropy decoder  200  acquires information regarding the transform process and the quantization process and outputs the information regarding the transform process and the quantization process to the inverse quantizer/inverse transformer  210 . Further, the entropy decoder  200  acquires information regarding a prediction process and outputs the information regarding the prediction process to the predictor  250 . The entropy decoder  200  acquires a color space transform flag for each decoding-target block and outputs the acquired color space transform flag to the switcher  215  and a filter controller  231 . 
     The inverse quantizer/inverse transformer  210  executes an inverse quantization process and an inverse transform process on each of blocks. The inverse quantizer/inverse transformer  210  includes an inverse quantizer  211  and an inverse transformer  212 . 
     The inverse quantizer  211  performs an inverse quantization process corresponding to the quantization process performed by the quantizer  122  of the encoding device  1 . The inverse quantizer  211  restores the transform coefficients of the decoding-target block by inversely quantizing the quantized transform coefficients outputted from the entropy decoder  200  using the quantization parameter (Qp) and the scaling list and outputs the restored transform coefficients to the inverse transformer  212 . 
     The inverse transformer  212  performs the inverse transform process corresponding to the transform process performed by the transformer  121  of the encoding device  1 . The inverse transformer  212  restores prediction residuals by performing the inverse transform process on the transform coefficients outputted by the inverse quantizer  211 , and outputs the restored prediction residuals to the switcher  215 . 
     The switcher  215  outputs the prediction residual of each color component outputted from the inverse transformer  212  to one of the combiner  220  and the color space inverse transformer  216  on the basis of the color space transform flag. The switcher  215  outputs the prediction residual to the combiner  220  for a block to which a color space inverse transform process (ACT) is not applied and outputs the prediction residual to the color space inverse transformer  216  for a block to which ACT is applied. 
     The color space inverse transformer  216  performs a color space inverse transform process (inverse ACT process) which is an inverse process of the ACT process performed by the color space transformer  112  of the encoding device  1  and outputs the prediction residual after the inverse ACT process to the combiner  220 . More specifically, the color space inverse transformer  216  performs inverse transform calculation as follows using the Y component, the Cg component and the Co component of the restored prediction residual. 
     
       
         
           
             t 
             = 
             
               Y 
               - 
               
                 ( 
                 
                   Cg 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     &gt;&gt; 
                   
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   1 
                 
                 ) 
               
             
           
         
       
       
         
           
             G 
             = 
             
               Cg 
               + 
               t 
             
           
         
       
       
         
           
             B 
             = 
             
               t 
               - 
               
                 ( 
                 
                   Co 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     &gt;&gt; 
                   
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   1 
                 
                 ) 
               
             
           
         
       
       
         
           
             R 
             = 
             
               Co 
               + 
               B 
             
           
         
       
     
     The combiner  220  decodes (reconstructs) the original block by combining the prediction residual outputted from the switcher  215  or the color space inverse transformer  216  and the prediction block outputted from the predictor  250  on a pixel-by-pixel basis and outputs the restored block to the deblocking filter  230 . 
     The deblocking filter  230  performs a filter process on the restored block outputted from the combiner  220  and outputs the restored block after the filter process to the memory  240 . More specifically, the deblocking filter  230  performs a filter process on a block boundary between two blocks including the restored block (first block) and a restored block (second block) adjacent to the restored block and outputs the respective restored blocks after the filter process to the memory  240 . Functions of the deblocking filter  230  are similar to the functions of the deblocking filter  160  of the encoding device  1 . 
     The filter controller  231  controls the deblocking filter  230 . More specifically, the filter controller  231  controls boundary filtering strength Bs (Boundary strength) indicating whether or not to perform a filter process on the block boundary of the block pair and filtering strength of the deblocking filter  230 . Functions of the filter controller  231  are similar to the functions of the filter controller  161  of the encoding device  1 . The functions of the filter controller  231  determines the boundary filtering strength Bs on the basis of the above-described Table 1. 
     In other words, the filter controller  231  according to the present embodiment controls the boundary filtering strength Bs of the deblocking filter  230  on the basis of whether or not at least one of the adjacent blocks P and Q is encoded using adaptive color transform (ACT). 
     As described above, the entropy decoder  200  acquires a flag (color space transform flag) indicating whether or not the block is encoded using adaptive color transform for each of the blocks P and Q. The filter controller  231  controls the boundary filtering strength Bs of the deblocking filter  230  on the basis of the color space transform flag for each of the blocks P and Q. 
     When at least one of the blocks P and Q is encoded using the adaptive color transform, the filter controller  231  controls the boundary filtering strength Bs such that the filter process is performed by the deblocking filter  230  (that is, sets the boundary filtering strength Bs to 1). More specifically, even when a non-zero transform coefficient exists in neither of the blocks P and Q, when at least one of the blocks P and Q is encoded using the adaptive color transform, the filter controller  231  controls the boundary filtering strength Bs such that the filter process is performed by the deblocking filter  230 . 
     The memory  240  stores the restored block outputted from the deblocking filter  230  and accumulates the restored blocks as restored images in units of frames. The memory  240  outputs the restored blocks or the restored images to the predictor  250 . Further, the memory  240  outputs the restored images in units of frames to outside of the decoding device  2 . 
     The predictor  250  performs prediction for each color component in units of blocks. The predictor  250  includes an inter predictor  251 , an intra predictor  252 , and a switcher  253 . 
     The inter predictor  251  performs inter prediction that utilizes correlation between frames. Specifically, the inter predictor  251  generates an inter prediction block by predicting an encoding-target block by using a restored image stored in the memory  240  as a reference image, based on information regarding inter prediction (for example, motion vector information) outputted by the entropy decoder  200 , and outputs the generated inter prediction block to the switcher  253 . 
     The intra predictor  252  performs intra prediction that utilizes spatial correlation within a frame. Specifically, the intra predictor  252  generates an intra prediction block by referring to restored pixels around an encoding-target block in a restored image stored in the memory  240 , by using an intra prediction mode corresponding to information regarding intra prediction (for example, intra prediction mode information) outputted by the entropy decoder  200 , and outputs the generated intra prediction block to the switcher  253 . 
     The switcher  253  switches the prediction block between the inter prediction block outputted from the inter predictor  251  and the intra prediction block outputted from the intra predictor  252  and outputs one of the prediction blocks to the combiner  220 . 
     As described above, the decoding device  2  according to the present embodiment includes the entropy decoder  200  configured to output transform coefficients corresponding to a block P by decoding an encoded stream, the inverse quantizer/inverse transformer  210  configured to restore a prediction residual corresponding to the block P (first block) by performing an inverse quantization process and an inverse transform process on the transform coefficients outputted from the entropy decoder  200 , the combiner  220  configured to reconstruct the block P by combining the restored prediction residual and a prediction block obtained by predicting the block P, the deblocking filter  230  configured to perform a filter process on a boundary between the restored block P and a restored block Q (second block) adjacent to the block P, and the filter controller  231  configured to control boundary filtering strength Bs of the deblocking filter  230  on the basis of whether or not at least one of the blocks P and Q is encoded using adaptive color transform (ACT). 
     For a block to which the ACT is applied, after the prediction residual is restored from the transform coefficients by the inverse transform process, a color space of the prediction residual is inversely transformed from a YCgCo space to an RGB space by color space inverse transform (inverse ACT). Thus, when a non-zero transform coefficient exists in a block of a certain color component, the non-zero transform coefficient affects blocks of other color components upon the color space inverse transform. 
     Thus, the filter controller  231  according to the present embodiment controls the boundary filtering strength Bs of the deblocking filter  230  in consideration of whether or not the ACT is applied as well as controlling the boundary filtering strength Bs of the deblocking filter  230  on the basis of whether or not a non-zero transform coefficient exists. This enables appropriate control of the boundary filtering strength Bs of the deblocking filter  230 , so that it is possible to prevent deterioration of image quality even when the ACT is applied. 
     &lt;Operation of Filter Controller&gt; 
     Operation of the filter controller  161  and the filter controller  231  according to the present embodiment will be described next. The filter controller  161  on an encoding side and the filter controller  231  on a decoding side perform the same operation, and thus, a description is given, taking the filter controller  231  on the decoding side as an example.  FIG. 4  is a diagram illustrating an operation example of the filter controller  231  according to the present embodiment. Note that order of determination illustrated in  FIG. 4  is an example, and the order of determination may be changed. 
     As illustrated in  FIG. 4 , in step S 1 , the filter controller  231  determines whether or not intra prediction is applied to at least one of a block pair including the blocks P and Q. When intra prediction is applied to at least one of the block pair (step S 1 : YES), in step S 2 , the filter controller  231  controls the deblocking filter  230  such that the deblocking filter process is performed. More specifically, the filter controller  231  sets the boundary filtering strength Bs to 2. 
     When intra prediction is applied to neither of the block pair (step S 1 : NO), in step S 3 , the filter controller  231  determines whether or not a difference between motion vectors of a target block pair is equal to or larger than a threshold value. When the difference between the motion vectors of the target block pair is equal to or larger than the threshold value (step S 3 : YES), in step S 4 , the filter controller  231  controls the deblocking filter  230  such that the deblocking filter process is performed. More specifically, the filter controller  231  sets the boundary filtering strength Bs to 1. 
     When the difference between the motion vectors of the block pair is not equal to or larger than the threshold value (step S 3 : NO), in step S 5 , the filter controller  231  determines whether or not the numbers of the motion vectors of the block pair or reference images thereof are different. When the numbers of the motion vectors of the block pairs or the reference images thereof are different (step S 5 : YES), in step S 4 , the filter controller  231  controls the deblocking filter  230  such that the deblocking filter process is performed. More specifically, the filter controller  231  sets the boundary filtering strength Bs to 1. 
     When the numbers of the motion vectors of the block pair or the reference images thereof are the same (step S 5 : NO), in step S 6 , the filter controller  231  determines whether or not a non-zero transform coefficient is included in at least one of the block pair. When a non-zero transform coefficient is included in at least one of the block pair (step S 6 : YES), in step S 4 , the filter controller  231  controls the deblocking filter  230  such that the deblocking filter process is performed. More specifically, the filter controller  231  sets the boundary filtering strength Bs to 1. 
     When a non-zero transform coefficient is included in neither of the blocks of the block pair (step S 6 : No), in step S 7 , the filter controller  231  determines whether or not at least one of the block pair is encoded using the ACT on the basis of the color space transform flag of each block outputted from the entropy decoder  200 . When at least one of the block pair is encoded using the ACT (step S 7 : Yes), in step S 4 , the filter controller  231  controls the deblocking filter  230  such that the deblocking filter process is performed. More specifically, the filter controller  231  sets the boundary filtering strength Bs to 1. On the other hand, when none of the blocks of the block pair is encoded using the ACT (step S 7 : NO), in step S 8 , the filter controller  231  controls the deblocking filter  230  such that the deblocking filter process is not performed. More specifically, the filter controller  231  sets the boundary filtering strength Bs to 0. 
     &lt;Modification 1&gt; 
     The filter controller  231  on the decoding side may determine whether or not at least one of the block pair (the block P and the block Q) includes a non-zero transform coefficient on the basis of a flag signaled from the encoding device  1 . 
     More specifically, the entropy encoder  130  of the encoding device  1  includes a flag (tu_coded_flag) indicating whether or not the block includes a non-zero transform coefficient in an encoded stream for each block. For example, the entropy encoder  130  sets the flag (tu_coded_flag) to “1” for a block including a non-zero transform coefficient and sets the flag (tu_coded_flag) to “0” for a block not including a non-zero transform coefficient. 
     The entropy decoder  200  of the decoding device  2  acquires the flag (tu_coded_flag) for each block and outputs the acquired flag (tu_coded_flag) to the filter controller  231 . The filter controller  231  interprets a block for which the flag (tu_coded_flag) is “1” as a block including a non-zero transform coefficient. Then, the filter controller  231  sets the boundary filtering strength Bs of the deblocking filter  230  as indicated in Table 2 below. 
     
       
         
           
               
               
             
               
                   
               
               
                 Bs value 
                 Conditions for determining Bs value 
               
               
                   
               
             
            
               
                 2 
                 Intra prediction is applied to at least one of blocks 
               
               
                 1 
                 Difference between motion vectors of both blocks is equal to  
               
               
                   
                 or larger than threshold value 
               
               
                 1 
                 The numbers of motion vectors of both blocks, or reference  
               
               
                   
                 images thereof are different 
               
               
                 1 
                 tu_coded_flag of at least one of blocks is “1” 
               
               
                 1 
                 ACT is applied to at least one of blocks 
               
               
                 0 
                 Other than above 
               
               
                   
               
            
           
         
       
     
     As indicated in Table 2, when tu_coded_flag of at least one of the blocks of the block pair is “1”, the filter controller  231  controls the deblocking filter  230  such that the deblocking filter process is performed. More specifically, the filter controller  231  sets the boundary filtering strength Bs to 1. 
     &lt;Modification 2&gt; 
     As described above, for a block to which the ACT is applied, after the prediction residual is restored from the transform coefficients by the inverse transform process, the color space of the prediction residual is inversely transformed from the YCgCo space to the RGB space by inverse ACT. Thus, when a non-zero transform coefficient exists in a block of a certain color component, the non-zero transform coefficient affects blocks of other color components upon the inverse ACT. 
     Thus, when the ACT is applied, control of the boundary filtering strength Bs of the deblocking filter  230  based on whether or not a non-zero transform coefficient exists does not appropriately function. Thus, in the present modification, the boundary filtering strength Bs of the deblocking filter  230  is controlled not on the basis of the transform coefficients but on the basis of the prediction residual after the inverse transform process. By this means, even if the non-zero transform coefficient affects blocks of other color components upon the inverse ACT, the boundary filtering strength Bs of the deblocking filter  230  can be appropriately controlled on the basis of the prediction residual after the blocks are affected. 
     In other words, the filter controller  161  and the filter controller  231  according to the present modification set the boundary filtering strength Bs of the deblocking filter  230  on the basis of Table 3 below. 
     
       
         
           
               
               
             
               
                   
               
               
                 Bs value 
                 Conditions for determining Bs value 
               
               
                   
               
             
            
               
                 2 
                 Intra prediction is applied to at least one of blocks 
               
               
                 1 
                 Difference between motion vectors of both blocks is equal to  
               
               
                   
                 or larger than threshold value 
               
               
                 1 
                 The numbers of motion vectors of both blocks, or reference  
               
               
                   
                 images thereof are different 
               
               
                 1 
                 Prediction residual of at least one of blocks includes non-zero value 
               
               
                 0 
                 Other than above 
               
               
                   
               
            
           
         
       
     
       FIG. 5  is a diagram illustrating a configuration of the encoding device  1  according to the present modification. 
     As illustrated in  FIG. 5 , at the encoding device  1 , a prediction residual that is the same as the prediction residual (restoration prediction residual) to be inputted to the combiner  150  is inputted to the filter controller  161 . The prediction residual after the inverse ACT is inputted to the filter controller  161  for a block to which the ACT is applied. When the restoration prediction residual of at least one of the blocks of the block pair (the block P and the block Q) includes a non-zero value, the filter controller  161  controls the deblocking filter  160  such that the deblocking filter process is performed. More specifically, the filter controller  161  sets the boundary filtering strength Bs to 1. 
       FIG. 6  is a diagram illustrating a configuration of the decoding device  2  according to the present modification. 
     As illustrated in  FIG. 6 , at the decoding device  2 , a prediction residual that is the same as the prediction residual (restoration prediction residual) to be inputted to the combiner  220  is inputted to the filter controller  231 . The prediction residual after the inverse ACT is inputted to the filter controller  231  for a block to which the ACT is applied. When the restoration prediction residual of at least one of the blocks of the block pair (the block P and the block Q) includes a non-zero value, the filter controller  231  controls the deblocking filter  230  such that the deblocking filter process is performed. More specifically, the filter controller  231  sets the boundary filtering strength Bs to 1. 
     Second Embodiment 
     In the above-described embodiment and the modifications thereof, an example has been described where the ACT which is one of encoding tools is applied. However, a problem similar to the above can occur also when joint coding of chroma residual (JCCR) which is another encoding tool is applied. Thus, the above-described embodiment and the modifications thereof may be applied to the JCCR, and the ACT in the above-described embodiment and the modifications thereof may read as the JCCR as appropriate. For example, it is possible to control the boundary filtering strength Bs as indicated in Table 4 below. 
     
       
         
           
               
               
             
               
                   
               
               
                 Bs value 
                 Conditions for determining Bs value 
               
               
                   
               
             
            
               
                 2 
                 Intra prediction is applied to at least one of blocks 
               
               
                 1 
                 Difference between motion vectors of both blocks is equal to  
               
               
                   
                 or larger than threshold value 
               
               
                 1 
                 The numbers of motion vectors of both blocks, or reference  
               
               
                   
                 images thereof are different 
               
               
                 1 
                 At least one of blocks includes a significant transform coefficient  
               
               
                   
                 (a non-zero transform coefficient) 
               
               
                 1 
                 JCCR is applied to at least one of blocks 
               
               
                 1 
                 ACT is applied to at least one of blocks 
               
               
                 0 
                 Other than above 
               
               
                   
               
            
           
         
       
     
     The JCCR is a coefficient encoding mode of a chrominance component (see Non Patent Literature 1). In the JCCR, the encoding device  1  generates one joint prediction residual from a prediction residual of a first chrominance component and a prediction residual of a second chrominance component by utilizing correlation between the prediction residual of the first chrominance component (Cb component) and the prediction residual of the second chrominance component (Cr component). For example, the encoding device  1  generates a joint prediction residual by combining a prediction residual obtained by inverting the polarity of the prediction residual of the second chrominance component with the prediction residual of the first chrominance component. Then, the encoding device  1  performs a transform process, a quantization process and an entropy encoding process on the generated joint prediction residual and transmits the joint prediction residual. 
     The decoding device  2  reconstructs the prediction residual of the first chrominance component and the prediction residual of the second chrominance component from the transmitted joint prediction residual. In this manner, by transmitting only one joint prediction residual for two chrominance components, encoding efficiency is improved. 
     As described above, the deblocking filter device according to the present embodiment includes a deblocking filter ( 160 ,  230 ) configured to perform a filter process on a boundary between a first reconstructed block (block P) and a second reconstructed block (block Q), and a filter controller ( 161 ,  231 ) configured to control boundary filtering strength Bs of the deblocking filter ( 160 ,  230 ) on the basis of whether or not at least one of the first reconstructed block (block P) and the second reconstructed block (block Q) is encoded using JCCR (Joint coding of chroma residual) in which one joint prediction residual is generated from prediction residuals of a Cb chrominance component and a Cr chrominance component. 
     The filter controller ( 161 ,  231 ) may control the boundary filtering strength Bs such that the filter process is performed by the deblocking filter ( 160 ,  230 ) when at least one of the first reconstructed block (block P) and the second reconstructed block (block Q) is encoded using the JCCR. 
     Even when a non-zero transform coefficient exists in neither of the first reconstructed block (block P) and the second reconstructed block (block Q), when at least one of the first reconstructed block (block P) and the second reconstructed block (block Q) is encoded using the JCCR, the filter controller ( 161 ,  231 ) may control the boundary filtering strength Bs such that the filter process is performed by the deblocking filter ( 160 ,  230 ). 
     In the present embodiment, the entropy decoder  200  of the decoding device  2  may acquire a flag indicating whether or not the block is encoded using the JCCR for each of the first reconstructed block (block P) and the second reconstructed block (block Q). The filter controller  231  may control the boundary filtering strength Bs of the deblocking filter  230  on the basis of the flag of each of the first reconstructed block (block P) and the second reconstructed block (block Q). For example, when the flag of at least one of the first reconstructed block (block P) and the second reconstructed block (block Q) is “1”, the filter controller  231  may control the boundary filtering strength Bs such that the filter process is performed by the deblocking filter  230 . 
     In the present embodiment, the entropy decoder  200  of the decoding device  2  may further acquire tu_coded_flag indicating whether or not the block includes a non-zero transform coefficient for each of the first reconstructed block (block P) and the second reconstructed block (block Q). The filter controller  231  may control the boundary filtering strength Bs of the deblocking filter  230  further on the basis of tu_coded_flag for each of the first reconstructed block (block P) and the second reconstructed block (block Q). For example, when tu_coded_flag of at least one of the first reconstructed block (block P) and the second reconstructed block (block Q) is “1”, the filter controller  231  may control the boundary filtering strength Bs such that the filter process is performed by the deblocking filter  230 . 
     For each block of the first chrominance component (Cb component) and the second chrominance component (Cr component) to which the JCCR is applied, while tu_coded_flag described above is set to “1”, actually, transform coefficients do not exist in one of the color components. Thus, in a similar manner to modification 2 described above, it is preferable to use determination as to whether or not the reconstructed prediction residual includes a non-zero value instead of determination as to whether or not a non-zero transform coefficient exists for a certain color component. 
     Other Embodiments 
     In the above-described embodiments, an example where the boundary filtering strength Bs is controlled as control of the deblocking filter ( 160 ,  230 ) has been mainly described. However, the control is not limited to the control of the boundary filtering strength Bs, and a filter length or switching of a plurality of filters may be controlled. 
     A program may be provided to cause a computer to execute the operations of the above-described encoding device  1 . Further, a program may be provided to cause a computer to execute the operations of the decoding device  2 . The program may be stored in a computer-readable medium. The program can be installed on a computer from a computer-readable medium. Here, the computer-readable medium having the program stored thereon may be a non-transitory recording medium. The non-transitory recording medium may include, but is not limited to, a CD-ROM and a DVD-ROM for example. 
     The encoding device  1  may be embodied as a semiconductor integrated circuit (chipset, SoC, etc.) by integrating the circuits that execute the respective operations of the encoding device  1 . The decoding device  2  may be embodied as a semiconductor integrated circuit (chipset, SoC, etc.) by integrating the circuits that execute the respective operations of the decoding device  2 . 
     The embodiments have been described in detail above with reference to the drawings. Specific configurations are not limited to the above-described configurations, and various design changes, and the like are possible within the scope not deviating from the gist.