Patent Publication Number: US-11647233-B2

Title: Encoding device, decoding device and program

Description:
RELATED APPLICATIONS 
     The present application is a continuation based on PCT Application No. PCT/JP2020/024676, filed on Jun. 23, 2020, which claims the benefit of Japanese Patent Application No. 2019-117924 filed on Jun. 25, 2019. The content of which is incorporated by reference herein in their entirety. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to an encoding device, a 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 encoding in-loop filter, to restrain distortion at a block boundary portion when an encoding process is performed on a block basis (for example, see Non Patent Literature 1). 
     When intra prediction is applied to at least one of two blocks including an encoding-target block and a neighboring block, a filter controller, which controls the deblocking filter, applies the deblocking filter to the boundary between the two blocks. 
     When inter prediction is applied to each of the two blocks, and when a difference between motion vectors applied to the two blocks is equal to or larger than a predetermined threshold value, the filter controller performs control such as to apply the deblocking filter to the boundary between the two blocks. This is because discontinuity is likely to occur at a block boundary when motion vectors are different. 
     When a non-zero coefficient exists in at least one of the two blocks, the filter controller performs control such as to apply the deblocking filter to the boundary between the two blocks even if the difference between the motion vectors is smaller than the threshold value. This is because, since energy of a residual signal is distributed all over the block as a result of inverse transform of the non-zero coefficient, discontinuity is likely to occur at the boundary between the two blocks even if the motion vectors are identical. 
     Moreover, in HEVC and VVC, transform skip is introduced in which no transform is applied to a residual signal. In transform skip, the residual signal is scaled to obtain scaled transform coefficients, without applying a transform such as DCT (Discrete Cosine Transform) or DST (Discrete Sine Transform). Accordingly, an increase in entropy can be restrained by selecting transform skip for a residual signal including a sharp edge or a residual signal having high energy only in a local area. 
     CITATION LIST 
     Non Patent Literature 
     
         
         Non Patent Literature 1: Recommendation ITU-T H.265, (December 2016), “High efficiency video coding”, International Telecommunication Union 
       
    
     DISCLOSURE OF INVENTION 
     An encoding device according to a first feature encodes a block obtained by dividing an image, the encoding device comprises: a transformer/quantizer configured to perform a transform process and a quantization process on a residual signal that represents a difference between the block and a prediction block obtained by predicting the block; an inverse quantizer/inverse transformer configured to restore the residual signal by performing an inverse quantization process and an inverse transform process on transform coefficients obtained by the transformer/quantizer; a combiner configured to reconstruct the block by combining the restored residual signal and the prediction block; a deblocking filter configured to perform a filter process on a boundary between two blocks including the reconstructed block and a block adjacent to the reconstructed block; and a filter controller configured to control the deblocking filter, based on a type of the transform process applied with respect to the two blocks. 
     A decoding device according to a second feature decodes a block obtained by dividing an image, the decoding device comprises: an entropy decoder configured to output quantized transform coefficients corresponding to the block by decoding an encoded stream; an inverse quantizer/inverse transformer configured to restore a residual signal by performing an inverse quantization process and an inverse transform process on the transform coefficients outputted by the entropy decoder; a combiner configured to reconstruct the block by combining the restored residual signal and a prediction block obtained by predicting the block; a deblocking filter configured to perform a filter process on a boundary between two blocks including the reconstructed block and a block adjacent to the reconstructed block; and a filter controller configured to control the deblocking filter, based on a type of the inverse transform process applied with respect to the two blocks. 
     A program according to a third feature causes a computer to function as the encoding device according to the first feature. 
     A program according to a fourth feature causes a computer to function as the decoding device according to the second 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 illustrating an example of operation of a deblocking filter according to the embodiment. 
         FIG.  3 A  is a diagram illustrating a specific example of operation of a filter controller according to the embodiment. 
         FIG.  3 B  is a diagram illustrating a specific example of operation of a filter controller according to the embodiment. 
         FIG.  3 C  is a diagram illustrating a specific example of operation of a filter controller according to the embodiment. 
         FIG.  3 D  is a diagram illustrating a specific example of operation of a filter controller according to the embodiment. 
         FIG.  4    is a diagram illustrating the configuration of a decoding device according to the embodiment. 
         FIG.  5    is a diagram illustrating an example of operation of the filter controller according to the embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Even if a non-zero coefficient exists in at least one of two blocks, the non-zero coefficient is outputted as it is after subjected to adjustment such as scaling when transform skip is applied to the block in which the non-zero coefficient exists. Accordingly, energy is not distributed all over the block, so that discontinuity between the blocks does not occur. 
     In conventional deblocking filter control, a deblocking filter is not controlled with transform types, including transform skip, taken into consideration. Accordingly, the deblocking filter can be applied even when discontinuity between blocks does not occur as described above. Consequently, a blur may appear in a decoded image, so that image quality may be degraded, and encoding efficiency may be lowered. 
     Accordingly, an object of the present disclosure is to enhance image quality and encoding efficiency by appropriately controlling the deblocking filter. 
     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. 
     &lt;Configuration of Encoding Device&gt; 
     First, a configuration of the encoding device according to the present embodiment is described.  FIG.  1    is a diagram illustrating the configuration of the encoding device  1  according to the present embodiment. The encoding device  1  is a device that encodes an encoding-target block obtained by dividing an image. 
     As illustrated in  FIG.  1   , the encoding device  1  includes a block divider  100 , a subtractor  110 , a transformer/quantizer  120 , an entropy encoder  130 , an inverse quantizer/inverse transformer  140 , a combiner  150 , a deblocking filter  160 , a filter controller  161 , a memory  170 , and a predictor  180 . 
     The block divider  100  divides an input image given in the form of a frame (or a picture) that constitutes a part of a video into a plurality of image blocks and outputs the resulting image blocks to the subtractor  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 block divider  100  performs block division on a luminance signal and a chrominance signal. Although a following description is given mainly of a case in which shapes made by the block division are identical for the luminance signal and the chrominance signal, the division may be controllable independently for the luminance signal and the chrominance signal. A luminance block and a chrominance block are simply referred to as an encoding-target block when the blocks are not particularly distinguished from each other. 
     The subtractor  110  calculates prediction residuals that represent differences (errors) between an encoding-target block outputted from the block divider  100  and a prediction block obtained by the predictor  180  predicting the encoding-target block. The subtractor  110  calculates a prediction residual by subtracting each pixel value in the prediction block from each pixel value in the block, and outputs the calculated prediction residuals to the transformer/quantizer  120 . Hereinafter, a signal including the prediction residuals in units of block is referred to as a residual signal. 
     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 for each frequency component by performing the transform process on the residual signal outputted from the subtractor  110 , and outputs the calculated transform coefficients to the quantizer  122 . The transform process (transformation) is a process of transforming a pixel-domain signal into a frequency-domain signal, and includes, for example, discrete cosine transform (DCT), discrete sine transform (DST), Karhunen Loeve transform (KLT), an integer transform based on any one of such transforms, or the like. 
     In the present embodiment, the transformer  121  performs the transform process of a type selected from among a plurality of types of transform processes including transform skip in which no transform is performed on the residual signal. More specifically, the transformer  121  selects a type of transform process to be applied to the residual signal, for each of horizontal and vertical directions. For example, the transformer  121  selects which transform is applied, among candidates including DCT-2, DST-7, DCT-8, and other transforms (including integer transforms based on such transforms). The transformer  121  outputs transform type information indicating the selected type of transform process to the entropy encoder  130  and the filter controller  161 . 
     The transformer  121  can select transform skip. For example, the transformer  121  selects transform skip for a residual signal including a sharp edge or a residual signal having high energy only in a local area. When transform skip is selected, the transformer  121  outputs, as transform coefficients, the residual signal adjusted through a scaling process or the like, without applying a transform such as DCT or DST. The transformer  121  may be able to apply transform skip to only one of the horizontal and vertical directions, or may be able to apply transform skip to both directions. A following description is given mainly of a case in which transform skip is applied to both the horizontal and vertical directions. 
     Note that the transformer  121  may select a type of transform process, for example, by performing such optimization that minimizes a linear combination of an amount of information generated in each encoding-target block and a signal distortion (that is, RD optimization), or may select a type of transform process based on a block size or a shape after division of an encoding-target block, or on a type of prediction process. 
     The quantizer  122  quantizes the transform coefficients outputted from the transformer  121  by using a quantization parameter (Qp) and a quantization matrix, and outputs the quantized transform coefficients to the entropy encoder  130  and the inverse quantizer/inverse transformer  140 . The quantization parameter (Qp) is a parameter that is applied in common to each transform coefficient in a block, and is a parameter that determines quantization granularity. The quantization matrix is a matrix that has, as elements, quantization values used when each transform coefficient is quantized. 
     The entropy encoder  130  performs entropy encoding on the transform coefficients outputted from the quantizer  122 , generates an encoded stream (bit stream) by performing data compression, and outputs the encoded stream to an outside of the encoding device  1 . For the entropy encoding, Huffman coding, CABAC (Context-based Adaptive Binary Arithmetic Coding), or the like can be used. 
     The entropy encoder  130  acquires information on the size, the shape and the like of each encoding-target block from the block divider  100 , acquires the transform type information from the transformer  121 , acquires information related to prediction (for example, information on a prediction mode and a motion vector) from the predictor  180 , and performs encoding also on the information. 
     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 transform coefficients outputted from the quantizer  122  by using the quantization parameter (Qp) and the quantization matrix to restore the transform coefficients, and outputs the restored transform coefficients to the inverse transformer  142 . 
     The inverse transformer  142  performs the inverse transform process corresponding to the transform process performed by the transformer  121 . For example, when the transformer  121  performs DCT, the inverse transformer  142  performs inverse DCT. The inverse transformer  142  restores the residual signal by performing the inverse transform process on the transform coefficients outputted from the inverse quantizer  141 , and outputs a restoration residual signal that is the restored residual signal to the combiner  150 . However, when the transformer  121  applies transform skip, the inverse transformer  142  performs an inverse process corresponding to coefficient adjustment performed by the transformer  121 , without performing the inverse transform process. 
     The combiner  150  combines the restoration residual signal outputted from the inverse transformer  142  with a prediction block outputted from the predictor  180 , on a pixel-by-pixel basis. The combiner  150  reconstructs (decodes) an encoding-target block by adding individual pixel values of the restoration residual signal to individual pixel values of the prediction block, and outputs a decoded image (reconstructed block) on each of reconstructed blocks to the deblocking filter  160 . 
     The deblocking filter  160  performs a deblocking filter process that is a filter process for the boundary between two blocks including a reconstructed block and a block adjacent to the reconstructed block (hereinafter, referred to as “filtering-target block pair”), and outputs the reconstructed block after the deblocking filter process to the memory  170 . The deblocking 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 the boundary of a filtering-target block pair. The deblocking filter  160  is configured, in general, as a low-pass filter that makes signal changes more gradual. 
     The filter controller  161  controls the deblocking filter  160 . More specifically, the filter controller  161  controls whether or not the deblocking filter process is performed on a filtering-target block pair, and filter strength (BS: Boundary Strength) of the deblocking filter  160 . The filter strength BS refers to a parameter specifying to what extent correction of pixels is allowed in the deblocking filter process. Note that control of whether or not the deblocking filter process is performed can be regarded as control of whether the filter strength BS is set to one or more, or to zero. 
     Specifically, the filter controller  161  controls the deblocking filter  160 , based on variations of pixel values in an area near the boundary of the filtering-target block pair, the prediction mode, the quantization parameter, and values of motion vectors used in motion-compensated prediction (inter prediction). In the present embodiment, the filter controller  161  controls the deblocking filter  160 , further based on whether or not the type of transform process applied to the filtering-target block pair is transform skip. 
       FIG.  2    is a diagram illustrating an example of operation of the deblocking filter  160  according to the present embodiment. In  FIG.  2   , a block Q is a reconstructed block corresponding to an encoding-target block, and a block P is a reconstructed block adjacent to the block Q. 
     In the example illustrated in  FIG.  2   , the block size based on which the deblocking filter  160  performs the deblocking filter process is 8×8 pixels. The filter controller  161  obtains a filter strength BS, for example, based on Table 1 below. In the present embodiment, it is assumed that the value of the filter strength BS is any one of 0, 1, 2. 
     
       
         
           
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 BS value 
                 Condition of determining BS value 
               
               
                   
               
             
            
               
                 2 
                 Intra prediction is applied to at least one of two blocks 
               
               
                 1 
                 Difference between motion vectors of two blocks is equal 
               
               
                   
                 to or larger than threshold value 
               
               
                 1 
                 At least one of two blocks includes non-zero coefficient, 
               
               
                   
                 and the block including the non-zero coefficient is not 
               
               
                   
                 transform skip block 
               
               
                 1 
                 Numbers of motion vectors of two blocks, or reference images 
               
               
                   
                 thereof, are different 
               
               
                 0 
                 Other than the above 
               
               
                   
               
            
           
         
       
     
     As illustrated in  FIG.  2    and Table 1, the filter controller  161  sets the value of BS 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), (B), (C) 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) At least one of the blocks P and Q includes a significant transform coefficient (that is, a non-zero coefficient), and the block including the non-zero coefficient is not a transform skip block. 
     (C) The numbers of motion vectors of the blocks P and Q, or reference images thereof, are different. 
     When the value of the filter strength BS is 0, the filter controller  161  controls the deblocking filter  160  such that the deblocking filter process is not performed. Hereinafter, a description is given, taking the boundary between vertical blocks illustrated in  FIG.  2    as an example. 
     When the value of the filter 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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     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. 
     The memory  170  accumulates reconstructed blocks outputted from the deblocking filter  160  as decoded images in units of frames. The memory  170  outputs the stored decoded images to the predictor  180 . 
     The predictor  180  generates a prediction block corresponding to an encoding-target block by performing a prediction process in units of the block, and outputs the generated prediction block to the subtractor  110  and the combiner  150 . The predictor  180  includes an inter predictor  181 , an intra predictor  182  and a switcher  183 . 
     The inter predictor  181  calculates a motion vector through a scheme such as block matching by using, for a reference image, a decoded image stored in the memory  170 , generates an inter prediction block by predicting an encoding-target block, and outputs the generated inter prediction block to the switcher  183 . The inter predictor  181  selects an optimal inter prediction method, from inter prediction using a plurality of reference images (typically, bi-prediction) and inter prediction using one reference image (uni-directional prediction), and performs inter prediction by using the selected inter prediction method. The inter predictor  181  outputs information related to inter prediction (the motion vector and the like) to the entropy encoder  130  and the filter controller  161 . 
     The intra predictor  182  selects an optimal intra prediction mode to be applied to an 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 intra predictor  182  generates an intra prediction block by referencing decoded pixel values adjacent to the encoding-target block of a decoded image stored in the memory  170 , and outputs the generated intra prediction block to the switcher  183 . The intra predictor  182  outputs information related to the selected intra prediction mode to the entropy encoder  130  and the filter controller  161 . 
     The switch  183  switches the prediction block between 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 subtractor  110  and the combiner  150 . 
     As described above, the encoding device  1  according to the present embodiment includes: the transformer/quantizer  120  configured to perform a transform process and a quantization process on a residual signal that represents a difference between an encoding-target block and a prediction block obtained by predicting the encoding-target block; the inverse quantizer/inverse transformer  140  configured to restore the residual signal by performing an inverse quantization process and an inverse transform process on transform coefficients obtained by the transformer/quantizer  120 ; the combiner  150  configured to reconstruct the encoding-target block by combining the restored residual signal and the prediction block; and the deblocking filter  160  configured to perform a filter process on a boundary between two blocks (filtering-target block pair) including the reconstructed block and a block adjacent to the reconstructed block. 
     Next, details of operation of the filter controller  161  according to the present embodiment are described. 
     As mentioned above, when intra prediction is applied to at least one block of a filtering-target block pair including a reconstructed block corresponding to an encoding-target block and a block adjacent to the reconstructed block, the filter controller  161  applies the deblocking filter to the boundary of the filtering-target block pair. This is because discontinuity is likely to occur at a block boundary when intra prediction is applied. 
     Moreover, the filter controller  161  applies the deblocking filter to the boundary of the filtering-target block pair when inter prediction is applied to each block of the filtering-target block pair, and when a difference between motion vectors applied to the filtering-target block pair is equal to or larger than a predetermined threshold value. This is because discontinuity is likely to occur at a block boundary when motion vectors are different. 
     Further, the filter controller  161  applies the deblocking filter to the boundary of the filtering-target block pair, even if the difference between the motion vectors is smaller than the threshold value, when a non-zero coefficient exists in at least one block of the filtering-target block pair. This is because, since energy of the residual signal is distributed all over the block as a result of inverse transform of the non-zero coefficient, discontinuity is likely to occur at the boundary of the two blocks even if the motion vectors are identical. 
     However, even if a non-zero coefficient exists in at least one block of the filtering-target block pair, the non-zero coefficient is outputted as it is after subjected to adjustment such as scaling when transform skip is applied to the block in which the non-zero coefficient exists. Accordingly, energy is not distributed all over the block, so that discontinuity between the blocks does not occur. 
     Accordingly, in the present embodiment, when a non-zero coefficient exists in at least one block of a filtering-target block pair, and when transform skip is applied to the block, of the filtering-target block pair, in which the non-zero coefficient exists, the filter controller  161  controls the deblocking filter  160  such that the deblocking filter process is not applied. Thus, the deblocking filter process is eliminated when discontinuity between blocks does not occur, whereby it is possible to prevent a blur from appearing in a decoded image, and to prevent image quality from being degraded and encoding efficiency from being lowered. 
       FIG.  3 A to  3 D  are diagrams illustrating a specific example of the operation of the filter controller  161  according to the present embodiment. In  FIGS.  3 A to  3 D , blocks P and Q constitute a filtering-target block pair, and it is assumed that inter prediction is applied to each of the blocks. Moreover, an arrow in each block in  FIG.  3 A to  3 D  represents a motion vector, and it is assumed that the motion vector of the block P and the motion vector of the block Q are identical. 
     As illustrated in  FIG.  3 A , when no non-zero coefficient exists in either block of the filtering-target block pair, the filter controller  161  does not apply the deblocking filter process to the boundary of the filtering-target block pair. 
     As illustrated in  FIG.  3 B , when a non-zero coefficient exists in at least one block of the filtering-target block pair, and when transform skip is not applied to the block in which the non-zero coefficient exists, the filter controller  161  applies the deblocking filter process to the boundary of the filtering-target block pair. More specifically, since a non-zero coefficient exists in the block P, and transform skip is not applied to the block P, the filter controller  161  applies the deblocking filter process to the boundary of the filtering-target block pair. 
     As illustrated in  FIG.  3 C , when a non-zero coefficient exists in at least one block of the filtering-target block pair, and when transform skip is applied to the block in which the non-zero coefficient exists, the filter controller  161  does not apply the deblocking filter process to the boundary of the filtering-target block pair. More specifically, since a non-zero coefficient exists in the block P, and transform skip (TranSkip) is not applied to the block P, the filter controller  161  applies the deblocking filter process to the boundary of the filtering-target block pair. 
     As illustrated in  FIG.  3 D , when a non-zero coefficient exists in both blocks of the filtering-target block pair, and when transform skip is not applied to the blocks in which the non-zero coefficients exist, the filter controller  161  applies the deblocking filter process to the boundary of the filtering-target block pair. More specifically, since a non-zero coefficient exists in both of the blocks P and Q, transform skip (TranSkip) is applied to the block P, and transform skip (TranSkip) is not applied to the block Q, the filter controller  161  applies the deblocking filter process to the boundary of the filtering-target block pair. 
     &lt;Configuration of Decoding Device&gt; 
     Next, a configuration of the decoding device according to the present embodiment is described, focusing mainly on differences from the configuration of the encoding device described above.  FIG.  4    is a diagram illustrating the configuration of the decoding device  2  according to the present embodiment. The decoding device  2  is a device that decodes a decoding-target block from an encoded stream. 
     As illustrated in  FIG.  4   , the decoding device  2  includes an entropy decoder  200 , an inverse quantizer/inverse transformer  210 , a combiner  220 , a deblocking filter  230 , a filter controller  231 , a memory  240 , and a predictor  250 . 
     The entropy decoder  200  decodes various signaling information by decoding an encoded stream generated by the encoding device  1 . More specifically, the entropy decoder  200  acquires information related to a transform process applied to a decoding-target block (for example, transform type information), and outputs the acquired information to an inverse transformer  212  and the filter controller  231 . Moreover, the entropy decoder  200  acquires information related to prediction applied to the decoding-target block (for example, prediction type information, motion vector information), and outputs the acquired information to the predictor  250  and the filter controller  231 . 
     The entropy decoder  200  decodes the encoded stream, acquires quantized transform coefficients, and outputs the acquired transform coefficients to the inverse quantizer/inverse transformer  210  (inverse quantizer  211 ). 
     The inverse quantizer/inverse transformer  210  that 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 the inverse quantization process corresponding to the quantization process performed by the quantizer  122  of the encoding device  1 . The inverse quantizer  211  inverse-quantizes the quantized transform coefficients outputted from the entropy decoder  200  by using the quantization parameter (Qp) and the quantization matrix to restore transform coefficients in the decoding-target block, 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 a residual signal by performing the inverse transform process on the transform coefficients outputted from the inverse quantizer  211 , and outputs the restored residual signal (restoration residual signal) to the combiner  220 . In the present embodiment, the inverse transformer  212  performs the inverse transform process of a type selected, based on the transform type information outputted from the entropy decoder  200 , from among a plurality of types of inverse transform processes including transform skip in which no inverse transform is performed. In a case of transform skip, the inverse transformer  212  performs an inverse process corresponding to coefficient adjustment performed by the transformer  121  of the encoding device  1 , without performing the inverse transform process. 
     The combiner  220  reconstructs (decodes) the decoding-target block by combining the residual signal outputted from the inverse transformer  212  and a prediction block outputted from the predictor  250  on a pixel-by-pixel basis, and outputs a reconstructed block to the deblocking filter  230 . 
     The deblocking filter  230  performs operation similar to the operation of the deblocking filter  160  of the encoding device  1 . The deblocking filter  230  performs a deblocking filter process on the boundary of a filtering-target block pair including the reconstructed block outputted from the combiner  220  and a block adjacent to the reconstructed block, and outputs the reconstructed block after the deblocking filter process to the memory  240 . 
     The filter controller  231  performs operation similar to the operation of the filter controller  161  of the encoding device  1 , based on the information outputted from the entropy decoder  200 . The filter controller  231  selects a filter strength BS, for example, through the method illustrated in Table 1, and controls the deblocking filter  230 , according to the selected filter strength BS. 
     The memory  240  stores the reconstructed blocks outputted from the deblocking filter  230  as decoded images in units of frames. The memory  240  outputs the decoded images in units of frames to an outside of the decoding device  2 . 
     The predictor  250  performs prediction in units of blocks. The predictor  250  includes an inter predictor  251 , an intra predictor  252  and a switch  253 . 
     The inter predictor  251  predicts a decoding-target block through inter prediction by using, for a reference image, a decoded image stored in the memory  240 . The inter predictor  251  generates an inter prediction block by performing inter prediction, by using the motion vector information outputted from the entropy decoder  200 , and outputs the generated inter prediction block to the switcher  253 . 
     The intra predictor  252  references reference pixels adjacent to a decoding-target block of a decoded image stored in the memory  240 , and predicts the decoding-target block through intra prediction, based on the information outputted from the entropy decoder  200 . The intra predictor  252  generates an intra-prediction block, and outputs the generated intra prediction block to the switcher  253 . 
     The switch  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 quantized transform coefficients corresponding to a decoding-target block by decoding an encoded stream; the inverse quantizer/inverse transformer  210  configured to restore a residual signal by performing an inverse quantization process and an inverse transform process on the transform coefficients outputted by the entropy decoder  200 ; the combiner  220  configured to reconstruct a block by combining the restored residual signal and a prediction block obtained by predicting the decoding-target block; the deblocking filter  230  configured to perform a filter process on a boundary of a filtering-target block pair including the reconstructed block and a block adjacent to the reconstructed block; and the filter controller  231  configured to control the deblocking filter  230 , based on a type of the inverse transform process applied with respect to the filtering-target block pair. 
     Here, the filter controller  231  performs operation as illustrated in  FIG.  3 A to  3 D . Specifically, when a non-zero coefficient exists in at least one block of a filtering-target block pair, and when transform skip is not applied to the block, of the filtering-target block pair, in which the non-zero coefficient exists, the filter controller  231  controls the deblocking filter  230  such that the deblocking filter process is performed. 
     When a non-zero coefficient exists in at least one block of the filtering-target block pair, and when transform skip is applied to the block, of the filtering-target block pair, in which the non-zero coefficient exists, the filter controller  231  controls the deblocking filter  230  such that the deblocking filter process is not performed. Thus, the deblocking filter process is eliminated when discontinuity between blocks does not occur, whereby it is possible to prevent a blur from appearing in a decoded image, and to prevent image quality from being degraded and encoding efficiency from being lowered. 
     &lt;Example of Operation of Filter Controller&gt; 
     Next, an example of the operation of the filter controllers  161  and  231  according to the present embodiment is described. Since the filter controllers  161  and  231  perform the same operation, a description is given by taking the filter controller  231  as an example.  FIG.  5    is a diagram illustrating the example of the operation of the filter controller  231  according to the present embodiment. 
     As illustrated in  FIG.  5   , in step S 1 , the filter controller  231  determines whether or not intra prediction is applied to at least one block of a filtering-target block pair including blocks P and Q. When intra prediction is applied to at least one block of the filtering-target 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  selects a filter strength BS=2. 
     When intra prediction is applied to neither of the filtering-target block pair (step S 1 : NO), in step S 3 , the filter controller  231  determines whether or not a difference between motion vectors of the 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  selects a filter strength BS=1. 
     When the difference between the motion vectors of the target 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 a non-zero coefficient is included in at least one block of the target block pair. When a non-zero coefficient is not included in at least one block of the target block pair (step S 5 : NO), in step S 7 , the filter controller  231  controls the deblocking filter  230  such that the deblocking filter process is not performed. More specifically, the filter controller  231  selects a filter strength BS=0. 
     When a non-zero coefficient is included in at least one block of the target block pair (step S 5 : YES), in step S 6 , the filter controller  231  determines whether or not transform skip is applied to the block in which the non-zero coefficient is included. When transform skip is not applied to the block in which the non-zero coefficient is included (step S 6 : NO), 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  selects a filter strength BS=1. When transform skip is applied to the block in which the non-zero coefficient is included (step S 6 : YES), in step S 7 , the filter controller  231  controls the deblocking filter  230  such that the deblocking filter process is not performed. More specifically, the filter controller  231  selects a filter strength BS=0. 
     A program may be provided to cause a computer to execute the operations of the image encoding device  1 . A program may be provided to cause a computer to execute the operations of the image 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 having the program stored thereon. 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.