Patent Publication Number: US-11665355-B2

Title: Encoder, decoder, encoding method, and decoding method

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 17/031,087, filed Sep. 24, 2020, which is a continuation of U.S. application Ser. No. 16/684,012, filed Nov. 14, 2019, which is a U.S. continuation application of PCT International Patent Application Number PCT/JP2018/018443 filed on May 14, 2018, claiming the benefit of priority of U.S. Provisional Patent Application No. 62/508,515 filed on May 19, 2017, the entire contents of which are hereby incorporated by reference. 
    
    
     BACKGROUND 
     1. Technical Field 
     The present disclosure relates to an encoder etc. that encodes a video by inter picture prediction. 
     2. Description of the Related Art 
     There has been H.265 (H.265 (ISO/IEC 23008-2 HEVC (High Efficiency Video Coding))) as a standard for coding a video. H.265 is also referred to as high efficiency video coding (HEVC). 
     SUMMARY 
     An encoder according to one aspect of the present disclosure is an encoder that encodes a video using inter picture prediction, the encoder including: memory; and circuitry accessible to the memory. The circuitry  25  accessible to the memory: determines whether overlapped block motion compensation (OBMC) is applicable to generation of a prediction image of a current block, according to whether bi-directional optical flow (BIO) is to be applied to the generation of the prediction image of the current block, the current block being included in a current picture in the video; and when the BIO is to be applied to the generation of the prediction image of the current block, determines that the OBMC is not applicable to the generation of the prediction image of the current block, and applies the BIO to the generation of the prediction image of the current block without applying the OBMC. The BIO generates the prediction image of the current block by reference to a spatial gradient of luminance in an image obtained by performing motion compensation of the current block using a motion vector of the current block. The OBMC corrects the prediction image of the current block using an image obtained by performing motion compensation of the current block using a motion vector of a block surrounding the current block. 
     It should be noted that these general or specific aspects may be implemented by a system, a device, a method, an integrated circuit, a computer program, or a non-transitory computer-readable recording medium such as a compact disc read only memory (CD-ROM), or by any combination of systems, devices, methods, integrated circuits, computer programs, or recording media. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       These and other objects, advantages and features of the disclosure will become apparent from the following description thereof taken in conjunction with the accompanying drawings that illustrate a specific embodiment of the present disclosure. 
         FIG.  1    is a block diagram illustrating a functional configuration of an encoder according to Embodiment 1. 
         FIG.  2    illustrates one example of block splitting according to Embodiment 1. 
         FIG.  3    is a chart indicating transform basis functions for each transform type. 
         FIG.  4 A  illustrates one example of a filter shape used in ALF. 
         FIG.  4 B  illustrates another example of a filter shape used in ALF. 
         FIG.  4 C  illustrates another example of a filter shape used in ALF. 
         FIG.  5 A  illustrates  67  intra prediction modes used in intra prediction. 
         FIG.  5 B  is a flow chart for illustrating an outline of a prediction image correction process performed via OBMC processing. 
         FIG.  5 C  is a conceptual diagram for illustrating an outline of a prediction image correction process performed via OBMC processing. 
         FIG.  5 D  illustrates one example of FRUC. 
         FIG.  6    is for illustrating pattern matching (bilateral matching) between two blocks along a motion trajectory. 
         FIG.  7    is for illustrating pattern matching (template matching) between a template in the current picture and a block in a reference picture. 
         FIG.  8    is for illustrating a model assuming uniform linear motion. 
         FIG.  9 A  is for illustrating deriving a motion vector of each sub-block based on motion vectors of neighboring blocks. 
         FIG.  9 B  is for illustrating an outline of a process for deriving a motion vector via merge mode. 
         FIG.  9 C  is a conceptual diagram for illustrating an outline of DMVR processing. 
         FIG.  9 D  is for illustrating an outline of a prediction image generation method using a luminance correction process performed via LIC processing. 
         FIG.  10    is a block diagram illustrating a functional configuration of a decoder according to Embodiment 1. 
         FIG.  11    is a block diagram for illustrating processes pertain to inter frame prediction performed by an encoder according to Embodiment 1. 
         FIG.  12    is a block diagram for illustrating processes pertain to inter frame prediction performed by a decoder according to Embodiment 1. 
         FIG.  13    is a flow chart illustrating the first specific example of inter frame prediction according to Embodiment 1. 
         FIG.  14    is a flow chart illustrating a modification of the first specific example of inter frame prediction according to Embodiment 1. 
         FIG.  15    is a flow chart illustrating the second specific example of inter frame prediction according to Embodiment 1. 
         FIG.  16    is a flow chart illustrating a modification of the second specific example of inter frame prediction according to Embodiment 1. 
         FIG.  17    is a conceptual diagram illustrating OBMC according to Embodiment 1. 
         FIG.  18    is a flow chart illustrating OBMC according to Embodiment 1. 
         FIG.  19    is a conceptual diagram illustrating BIO according to Embodiment 1. 
         FIG.  20    is a flow chart illustrating BIO according to Embodiment 1. 
         FIG.  21    is a block diagram illustrating an implementation example of the encoder according to Embodiment 1. 
         FIG.  22    is a flow chart illustrating the first example of operations performed by the encoder according to Embodiment 1. 
         FIG.  23    is a flow chart illustrating a modification of the first example of the operations performed by the encoder according to Embodiment 1. 
         FIG.  24    is a flow chart illustrating an additional operation for the modification of the first example of the operations performed by the encoder according to Embodiment 1. 
         FIG.  25    is a flow chart illustrating the second example of operations performed by the encoder according to Embodiment 1. 
         FIG.  26    is a flow chart illustrating a modification of the second example of the operations performed by the encoder according to Embodiment 1. 
         FIG.  27    is a flow chart illustrating an additional operation for the modification of the second example of the operations performed by the encoder according to Embodiment 1. 
         FIG.  28    is a block diagram illustrating an implementation example of the decoder according to Embodiment 1. 
         FIG.  29    is a flow chart illustrating the first example of operations performed by the decoder according to Embodiment 1. 
         FIG.  30    is a flow chart illustrating a modification of the first example of the operations performed by the decoder according to Embodiment 1. 
         FIG.  31    is a flow chart illustrating an additional operation for the modification of the first example of the operations performed by the decoder according to Embodiment 1. 
         FIG.  32    is a flow chart illustrating the second example of operations performed by the decoder according to Embodiment 1. 
         FIG.  33    is a flow chart illustrating a modification of the second example of the operations performed by the decoder according to Embodiment 1. 
         FIG.  34    is a flow chart illustrating an additional operation for the modification of the second example of the operations performed by the decoder according to Embodiment 1. 
         FIG.  35    illustrates an overall configuration of a content providing system for implementing a content distribution service. 
         FIG.  36    illustrates one example of an encoding structure in scalable encoding. 
         FIG.  37    illustrates one example of an encoding structure in scalable encoding. 
         FIG.  38    illustrates an example of a display screen of a web page. 
         FIG.  39    illustrates an example of a display screen of a web page. 
         FIG.  40    illustrates an example of a smartphone. 
         FIG.  41    is a block diagram illustrating a configuration example of a smartphone. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     (Underlying Knowledge Forming Basis of the Present Disclosure) 
     There are cases in which inter picture prediction is performed when a video is encoded and decoded. Inter picture prediction is also referred to as inter frame prediction or inter prediction. In inter picture prediction, a motion vector is derived for a current block, and a prediction image corresponding to the current block is generated using the derived motion vector. Further, when a prediction image is generated using a motion vector, a prediction image generation mode for generating a prediction image can be selected from among prediction image generation modes. 
     Specifically, there are, as the prediction image generation mode, a mode that generates a prediction image from a processed picture by performing motion compensation based on a motion vector, a mode called bi-directional optical flow (BIO), etc. For example, in BIO, a prediction image is generated using a provisional prediction image generated from two processed pictures by performing motion compensation based on a motion vector, and a gradient image indicating a spatial gradient of luminance in the provisional prediction image. 
     Moreover, in inter picture prediction, a prediction image may be corrected by a mode called overlapped block motion compensation (OBMC). In OBMC, a motion vector of a block surrounding a current block is used. Specifically, in OBMC, a prediction image is corrected using an image generated by performing motion compensation based on the motion block of the surrounding block. 
     It may be determined whether OBMC is to be applied, based on the size of the current block. There are cases in which a difference in error between an input image corresponding to the current block and a corrected prediction image is reduced by OBMC being applied. Alternatively, there are cases in which an amount of data after transformation and quantization is reduced by OBMC being applied. In other words, it should produce advantageous effects of improving coding efficiency and reducing a coding amount. 
     In inter picture prediction, however, an amount of processing in OBMC increases due to OBMC being applied. 
     For example, an amount of processing in inter picture prediction varies depending on which prediction image generation mode is used, whether OBMC is to be applied, etc. To put it differently, the amount of processing in inter picture prediction differs depending on a path in the procedure of inter picture prediction. Further, a time set aside for inter picture prediction is determined according to an amount of processing in a path with the maximum amount of processing in the procedure of inter picture prediction. Accordingly, an amount of processing in the path with the maximum amount of processing in the procedure of inter picture prediction is desired to be small. 
     In view of the above, for example, an encoder according to one aspect of the present disclosure is an encoder that encodes a video using inter picture prediction, the encoder including: memory; and circuitry accessible to the memory. The circuitry accessible to the memory: determines whether overlapped block motion compensation (OBMC) is applicable to generation of a prediction image of a current block, according to whether bi-directional optical flow (BIO) is to be applied to the generation of the prediction image of the current block, the current block being included in a current picture in the video; and when the BIO is to be applied to the generation of the prediction image of the current block, determines that the OBMC is not applicable to the generation of the prediction image of the current block, and applies the BIO to the generation of the prediction image of the current block without applying the OBMC. The BIO generates the prediction image of the current block by reference to a spatial gradient of luminance in an image obtained by performing motion compensation of the current block using a motion vector of the current block. The OBMC corrects the prediction image of the current block using an image obtained by performing motion compensation of the current block using a motion vector of a block surrounding the current block. 
     With this, it is possible to remove a path that applies both BIO and OBMC in the procedure of inter picture prediction. Accordingly, it is possible to reduce an amount of processing in the path with a large amount of processing in the procedure of inter picture prediction. 
     Further, it is assumed that a degree of reduction in coding amount by applying OBMC is low when BIO is applied, compared to a case in which BIO is not applied. In other words, when it is assumed that the degree of reduction in coding amount by applying OBMC is low, the encoder can generate the prediction image efficiently without applying OBMC. 
     As a result, the encoder can perform prediction efficiently. In addition, a circuit size can be reduced. 
     Moreover, for example, the circuitry further: selects one mode from among (i) a first mode that applies neither the BIO nor the OBMC to the generation of the prediction image of the current block, (ii) a second mode that applies the OBMC to the generation of the prediction image of the current block without applying the BIO, and (iii) a third mode that applies the BIO to the generation of the prediction image of the current block without applying the OBMC; generates the prediction image of the current block in the one mode selected; and when selecting the third mode as the one mode, determines that the OBMC is not applicable to the generation of the prediction image of the current block, in accordance with application of the BIO to the generation of the prediction image of the current block, and generates the prediction image of the current block in the third mode that applies the BIO to the generation of the prediction image of the current block without applying the OBMC. 
     With this, the encoder can perform the generation of the prediction image in the one mode selected adaptively from among the three modes. In addition, it is possible to remove a path that applies both BIO and OBMC in the procedure of inter picture prediction. 
     Moreover, for example, the circuitry encodes a signal indicating, among a first value corresponding to the first mode, a second value corresponding to the second mode, and a third value corresponding to the third mode, one value corresponding to the one mode selected from among the first mode, the second mode, and the third mode. 
     With this, the encoder can encode the one mode selected adaptively from among the three modes. Accordingly, the encoder and a decoder can select the same mode adaptively from among the three modes. 
     A decoder according to one aspect of the present disclosure is, for example, a decoder that decodes a video using inter picture prediction, the decoder including: memory; and circuitry accessible to the memory. The circuitry accessible to the memory: determines whether overlapped block motion compensation (OBMC) is applicable to generation of a prediction image of a current block, according to whether bi-directional optical flow (BIO) is to be applied to the generation of the prediction image of the current block, the current block being included in a current picture in the video; and when the BIO is to be applied to the generation of the prediction image of the current block, determines that the OBMC is not applicable to the generation of the prediction image of the current block, and applies the BIO to the generation of the prediction image of the current block without applying the OBMC. The BIO generates the prediction image of the current block by reference to a spatial gradient of luminance in an image obtained by performing motion compensation of the current block using a motion vector of the current block. The OBMC corrects the prediction image of the current block using an image obtained by performing motion compensation of the current block using a motion vector of a block surrounding the current block. 
     With this, it is possible to remove a path that applies both BIO and OBMC in the procedure of inter picture prediction. Accordingly, it is possible to reduce an amount of processing in the path with a large amount of processing in the procedure of inter picture prediction. 
     Further, it is assumed that a degree of reduction in coding amount by applying OBMC is low when BIO is applied, compared to a case in which BIO is not applied. In other words, when it is assumed that the degree of reduction in coding amount by applying OBMC is low, the decoder can generate the prediction image efficiently without applying OBMC. 
     As a result, the decoder can perform prediction efficiently. In addition, a circuit size can be reduced. 
     Moreover, for example, the circuitry further: selects one mode from among (i) a first mode that applies neither the BIO nor the OBMC to the generation of the prediction image of the current block, (ii) a second mode that applies the OBMC to the generation of the prediction image of the current block without applying the BIO, and (iii) a third mode that applies the BIO to the generation of the prediction image of the current block without applying the OBMC; generates the prediction image of the current block in the one mode selected; and when selecting the third mode as the one mode, determines that the OBMC is not applicable to the generation of the prediction image of the current block, in accordance with application of the BIO to the generation of the prediction image of the current block, and generates the prediction image of the current block in the third mode that applies the BIO to the generation of the prediction image of the current block without applying the OBMC. 
     With this, the decoder can perform the generation of the prediction image in the one mode selected adaptively from among the three modes. In addition, it is possible to remove a path that applies both BIO and OBMC in the procedure of inter picture prediction. 
     Moreover, for example, the circuitry decodes a signal indicating, among a first value corresponding to the first mode, a second value corresponding to the second mode, and a third value corresponding to the third mode, one value corresponding to the one mode selected from among the first mode, the second mode, and the third mode. 
     With this, the decoder can decode the one mode selected adaptively from among the three modes. Accordingly, the encoder and the decoder can select the same mode adaptively from among the three modes. 
     An encoding method according to one aspect of the present disclosure is, for example, an encoding method of encoding a video using inter picture prediction, the encoding method including: determining whether overlapped block motion compensation (OBMC) is applicable to generation of a prediction image of a current block, according to whether bi-directional optical flow (BIO) is to be applied to the generation of the prediction image of the current block, the current block being included in a current picture in the video; and when the BIO is to be applied to the generation of the prediction image of the current block, determining that the OBMC is not applicable to the generation of the prediction image of the current block, and applying the BIO to the generation of the prediction image of the current block without applying the OBMC. The BIO generates the prediction image of the current block by reference to a spatial gradient of luminance in an image obtained by performing motion compensation of the current block using a motion vector of the current block. The OBMC corrects the prediction image of the current block using an image obtained by performing motion compensation of the current block using a motion vector of a block surrounding the current block. 
     With this, it is possible to remove a path that applies both BIO and OBMC in the procedure of inter picture prediction. Accordingly, it is possible to reduce an amount of processing in the path with a large amount of processing in the procedure of inter picture prediction. 
     Further, it is assumed that a degree of reduction in coding amount by applying OBMC is low when BIO is applied, compared to a case in which BIO is not applied. In other words, when it is assumed that the degree of reduction in coding amount by applying OBMC is low, a device using the encoding method etc. can generate the prediction image efficiently without applying OBMC. 
     As a result, the device using the encoding method etc. can perform prediction efficiently. In addition, a circuit size can be reduced. 
     A decoding method according to one aspect of the present disclosure is, for example, a decoding method of decoding a video using inter picture prediction, the decoding method including: determining whether overlapped block motion compensation (OBMC) is applicable to generation of a prediction image of a current block, according to whether bi-directional optical flow (BIO) is to be applied to the generation of the prediction image of the current block, the current block being included in a current picture in the video; and when the BIO is to be applied to the generation of the prediction image of the current block, determining that the OBMC is not applicable to the generation of the prediction image of the current block, and applying the BIO to the generation of the prediction image of the current block without applying the OBMC. The BIO generates the prediction image of the current block by reference to a spatial gradient of luminance in an image obtained by performing motion compensation of the current block using a motion vector of the current block. The OBMC corrects the prediction image of the current block using an image obtained by performing motion compensation of the current block using a motion vector of a block surrounding the current block. 
     With this, it is possible to remove a path that applies both BIO and OBMC in the procedure of inter picture prediction. Accordingly, it is possible to reduce an amount of processing in the path with a large amount of processing in the procedure of inter picture prediction. 
     Further, it is assumed that a degree of reduction in coding amount by applying OBMC is low when BIO is applied, compared to a case in which BIO is not applied. In other words, when it is assumed that the degree of reduction in coding amount by applying OBMC is low, a device using the decoding method etc. can generate the prediction image efficiently without applying OBMC. 
     As a result, the device using the decoding method etc. can perform prediction efficiently. In addition, a circuit size can be reduced. 
     An encoder according to one aspect of the present disclosure is, for example, an encoder that encodes a video using inter picture prediction, the encoder including: memory; and circuitry accessible to the memory. The circuitry accessible to the memory: determines whether overlapped block motion compensation (OBMC) is applicable to generation of a prediction image of a current block, according to whether bi-prediction that refers to two processed picture is to be applied to the generation of the prediction image of the current block, the current block being included in a current picture in the video; and when the bi-prediction is to be applied to the generation of the prediction image of the current block, determines that the OBMC is not applicable to the generation of the prediction image of the current block, and applies the bi-prediction to the generation of the prediction image of the current block without applying the OBMC. The OBMC corrects the prediction image of the current block using an image obtained by performing motion compensation of the current block using a motion vector of a block surrounding the current block. 
     With this, it is possible to remove a path that applies both bi-prediction, which refers to the two processed pictures, and OBMC in the procedure of inter picture prediction. Accordingly, it is possible to reduce an amount of processing in the path with a large amount of processing in the procedure of inter picture prediction. 
     When bi-prediction, which refers to the two processed pictures, and OBMC are applied, an amount of processing is considerably increased by a prediction image obtained from each of the two processed pictures being corrected. The encoder can suppress such an increase in the amount of processing. 
     As a result, the encoder can perform prediction efficiently. In addition, a circuit size can be reduced. 
     Moreover, for example, the circuitry further: selects one mode from among (i) a first mode that applies neither the bi-prediction nor the OBMC to the generation of the prediction image of the current block, (ii) a second mode that applies the OBMC to the generation of the prediction image of the current block without applying the bi-prediction, and (iii) a third mode that applies the bi-prediction to the generation of the prediction image of the current block without applying the OBMC; generates the prediction image of the current block in the one mode selected; and when selecting the third mode as the one mode, determines that the OBMC is not applicable to the generation of the prediction image of the current block, in accordance with application of the bi-prediction to the generation of the prediction image of the current block, and generates the prediction image of the current block in the third mode that applies the bi-prediction to the generation of the prediction image of the current block without applying the OBMC. 
     With this, the encoder can perform the generation of the prediction image in the one mode selected adaptively from among the three modes. With this, it is possible to remove a path that applies both bi-prediction, which refers to the two processed pictures, and OBMC in the procedure of inter picture prediction. 
     Moreover, for example, the circuitry encodes a signal indicating, among a first value corresponding to the first mode, a second value corresponding to the second mode, and a third value corresponding to the third mode, one value corresponding to the one mode selected from among the first mode, the second mode, and the third mode. 
     With this, the encoder can encode the one mode selected adaptively from among the three modes. Accordingly, the encoder and a decoder can select the same mode adaptively from among the three modes. 
     A decoder according to one aspect of the present disclosure is, for example, a decoder that decodes a video using inter picture prediction, the decoder including: memory; and circuitry accessible to the memory. The circuitry accessible to the memory: determines whether overlapped block motion compensation (OBMC) is applicable to generation of a prediction image of a current block, according to whether bi-prediction that refers to two processed picture is to be applied to the generation of the prediction image of the current block, the current block being included in a current picture in the video; and when the bi-prediction is to be applied to the generation of the prediction image of the current block, determines that the OBMC is not applicable to the generation of the prediction image of the current block, and applies the bi-prediction to the generation of the prediction image of the current block without applying the OBMC. The OBMC corrects the prediction image of the current block using an image obtained by performing motion compensation of the current block using a motion vector of a block surrounding the current block. 
     With this, it is possible to remove a path that applies both bi-prediction, which refers to the two processed pictures, and OBMC in the procedure of inter picture prediction. Accordingly, it is possible to reduce an amount of processing in the path with a large amount of processing in the procedure of inter picture prediction. 
     When bi-prediction, which refers to the two processed pictures, and OBMC are applied, an amount of processing is considerably increased by a prediction image obtained from each of the two processed pictures being corrected. The decoder can suppress such an increase in the amount of processing. 
     As a result, the decoder can perform prediction efficiently. In addition, a circuit size can be reduced. 
     Moreover, for example, the circuitry further: selects one mode from among (i) a first mode that applies neither the bi-prediction nor the OBMC to the generation of the prediction image of the current block, (ii) a second mode that applies the OBMC to the generation of the prediction image of the current block without applying the bi-prediction, and (iii) a third mode that applies the bi-prediction to the generation of the prediction image of the current block without applying the OBMC; generates the prediction image of the current block in the one mode selected; and when selecting the third mode as the one mode, determines that the OBMC is not applicable to the generation of the prediction image of the current block, in accordance with application of the bi-prediction to the generation of the prediction image of the current block, and generates the prediction image of the current block in the third mode that applies the bi-prediction to the generation of the prediction image of the current block without applying the OBMC. 
     With this, the decoder can perform the generation of the prediction image in the one mode selected adaptively from among the three modes. With this, it is possible to remove a path that applies both bi-prediction, which refers to the two processed pictures, and OBMC in the procedure of inter picture prediction. 
     Moreover, for example, the circuitry decodes a signal indicating, among a first value corresponding to the first mode, a second value corresponding to the second mode, and a third value corresponding to the third mode, one value corresponding to the one mode selected from among the first mode, the second mode, and the third mode. 
     With this, the decoder can decode the one mode selected adaptively from among the three modes. Accordingly, the encoder and the decoder can select the same mode adaptively from among the three modes. 
     An encoding method according to one aspect of the present disclosure is, for example, an encoding method of encoding a video using inter picture prediction, the encoding method including: determining whether overlapped block motion compensation (OBMC) is applicable to generation of a prediction image of a current block, according to whether bi-prediction that refers to two processed pictures is to be applied to the generation of the prediction image of the current block, the current block being included in a current picture in the video; and when the bi-prediction is to be applied to the generation of the prediction image of the current block, determining that the OBMC is not applicable to the generation of the prediction image of the current block, and applying the bi-prediction to the generation of the prediction image of the current block without applying the OBMC. The OBMC corrects the prediction image of the current block using an image obtained by performing motion compensation of the current block using a motion vector of a block surrounding the current block. 
     With this, it is possible to remove a path that applies both bi-prediction, which refers to the two processed pictures, and OBMC in the procedure of inter picture prediction. Accordingly, it is possible to reduce an amount of processing in the path with a large amount of processing in the procedure of inter picture prediction. 
     When bi-prediction, which refers to the two processed pictures, and OBMC are applied, an amount of processing is considerably increased by a prediction image obtained from each of the two processed pictures being corrected. A device using the encoding method etc. can suppress such an increase in the amount of processing. 
     As a result, the device using the encoding method etc. can perform prediction efficiently. In addition, a circuit size can be reduced. 
     A decoding method according to one aspect of the present disclosure is, for example, a decoding method of decoding a video using inter picture prediction, the decoding method including: determining whether overlapped block motion compensation (OBMC) is applicable to generation of a prediction image of a current block, according to whether bi-prediction that refers to two processed pictures is to be applied to the generation of the prediction image of the current block, the current block being included in a current picture in the video; and when the bi-prediction is to be applied to the generation of the prediction image of the current block, determining that the OBMC is not applicable to the generation of the prediction image of the current block, and applying the bi-prediction to the generation of the prediction image of the current block without applying the OBMC. The OBMC corrects the prediction image of the current block using an image obtained by performing motion compensation of the current block using a motion vector of a block surrounding the current block. 
     With this, it is possible to remove a path that applies both bi-prediction, which refers to the two processed pictures, and OBMC in the procedure of inter picture prediction. Accordingly, it is possible to reduce an amount of processing in the path with a large amount of processing in the procedure of inter picture prediction. 
     When bi-prediction, which refers to the two processed pictures, and OBMC are applied, an amount of processing is considerably increased by a prediction image obtained from each of the two processed pictures being corrected. A device using the decoding method etc. can suppress such an increase in the amount of processing. 
     As a result, the device using the decoding method etc. can perform prediction efficiently. In addition, a circuit size can be reduced. 
     An encoder according to one aspect of the present disclosure is, for example, an encoder that encodes a video using inter picture prediction, the encoder including: memory; and circuitry accessible to the memory. The circuitry accessible to the memory: determines whether overlapped block motion compensation (OBMC) is applicable to generation of a prediction image of a current block, according to whether bi-prediction is to be applied to the generation of the prediction image of the current block, the current block being included in a current picture in the video, the bi-prediction referring to a processed picture preceding the current picture in display order, and a processed picture following the current picture in display order; and when the bi-prediction is to be applied to the generation of the prediction image of the current block, determines that the OBMC is not applicable to the generation of the prediction image of the current block, and applies the bi-prediction to the generation of the prediction image of the current block without applying the OBMC. The OBMC corrects the prediction image of the current block using an image obtained by performing motion compensation of the current block using a motion vector of a block surrounding the current block. 
     With this, it is possible to remove a path that applies both bi-prediction, which refers to the two processed pictures preceding and following in display order, and OBMC in the procedure of inter picture prediction. Accordingly, it is possible to reduce an amount of processing in the path with a large amount of processing in the procedure of inter picture prediction. 
     When bi-prediction, which refers to the two processed pictures, and OBMC are applied, an amount of processing is considerably increased by a prediction image obtained from each of the two processed pictures being corrected. The encoder can suppress such an increase in the amount of processing. 
     As a result, the encoder can perform prediction efficiently. In addition, a circuit size can be reduced. 
     Moreover, for example, the circuitry further: selects one mode from among (i) a first mode that applies neither the bi-prediction nor the OBMC to the generation of the prediction image of the current block, (ii) a second mode that applies the OBMC to the generation of the prediction image of the current block without applying the bi-prediction, and (iii) a third mode that applies the bi-prediction to the generation of the prediction image of the current block without applying the OBMC; generates the prediction image of the current block in the one mode selected; and when selecting the third mode as the one mode, determines that the OBMC is not applicable to the generation of the prediction image of the current block, in accordance with application of the bi-prediction to the generation of the prediction image of the current block, and generates the prediction image of the current block in the third mode that applies the bi-prediction to the generation of the prediction image of the current block without applying the OBMC. 
     With this, the encoder can perform the generation of the prediction image in the one mode selected adaptively from among the three modes. With this, it is possible to remove a path that applies both bi-prediction, which refers to the two processed pictures preceding and following in display order, and OBMC in the procedure of inter picture prediction. 
     Moreover, for example, the circuitry encodes a signal indicating, among a first value corresponding to the first mode, a second value corresponding to the second mode, and a third value corresponding to the third mode, one value corresponding to the one mode selected from among the first mode, the second mode, and the third mode. 
     With this, the encoder can encode the one mode selected adaptively from among the three modes. Accordingly, the encoder and a decoder can select the same mode adaptively from among the three modes. 
     A decoder according to one aspect of the present disclosure is, for example, a decoder that decodes a video using inter picture prediction, the decoder including: memory; and circuitry accessible to the memory. The circuitry accessible to the memory: determines whether overlapped block motion compensation (OBMC) is applicable to generation of a prediction image of a current block, according to whether bi-prediction is to be applied to the generation of the prediction image of the current block, the current block being included in a current picture in the video, the bi-prediction referring to a processed picture preceding the current picture in display order, and a processed picture following the current picture in display order; and when the bi-prediction is to be applied to the generation of the prediction image of the current block, determines that the OBMC is not applicable to the generation of the prediction image of the current block, and applies the bi-prediction to the generation of the prediction image of the current block without applying the OBMC. The OBMC corrects the prediction image of the current block using an image obtained by performing motion compensation of the current block using a motion vector of a block surrounding the current block. 
     With this, it is possible to remove a path that applies both bi-prediction, which refers to the two processed pictures preceding and following in display order, and OBMC in the procedure of inter picture prediction. Accordingly, it is possible to reduce an amount of processing in the path with a large amount of processing in the procedure of inter picture prediction. 
     When bi-prediction, which refers to the two processed pictures, and OBMC are applied, an amount of processing is considerably increased by a prediction image obtained from each of the two processed pictures being corrected. The decoder can suppress such an increase in the amount of processing. 
     As a result, the decoder can perform prediction efficiently. In addition, a circuit size can be reduced. 
     Moreover, for example, the circuitry further: selects one mode from among (i) a first mode that applies neither the bi-prediction nor the OBMC to the generation of the prediction image of the current block, (ii) a second mode that applies the OBMC to the generation of the prediction image of the current block without applying the bi-prediction, and (iii) a third mode that applies the bi-prediction to the generation of the prediction image of the current block without applying the OBMC; generates the prediction image of the current block in the one mode selected; and when selecting the third mode as the one mode, determines that the OBMC is not applicable to the generation of the prediction image of the current block, in accordance with application of the bi-prediction to the generation of the prediction image of the current block, and generates the prediction image of the current block in the third mode that applies the bi-prediction to the generation of the prediction image of the current block without applying the OBMC. 
     With this, the decoder can perform the generation of the prediction image in the one mode selected adaptively from among the three modes. In addition, it is possible to remove a path that applies both bi-prediction, which refers to the two processed pictures preceding and following in display order, and OBMC in the procedure of inter picture prediction. 
     Moreover, for example, the circuitry decodes a signal indicating, among a first value corresponding to the first mode, a second value corresponding to the second mode, and a third value corresponding to the third mode, one value corresponding to the one mode selected from among the first mode, the second mode, and the third mode. 
     With this, the decoder can decode the one mode selected adaptively from among the three modes. Accordingly, the encoder and the decoder can select the same mode adaptively from among the three modes. 
     An encoding method according to one aspect of the present disclosure is, for example, an encoding method of encoding a video using inter picture prediction, the encoding method including: determining whether overlapped block motion compensation (OBMC) is applicable to generation of a prediction image of a current block, according to whether bi-prediction is to be applied to the generation of the prediction image of the current block, the current block being included in a current picture in the video, the bi-prediction referring to a processed picture preceding the current picture in display order, and a processed picture following the current picture in display order; and when the bi-prediction is to be applied to the generation of the prediction image of the current block, determining that the OBMC is not applicable to the generation of the prediction image of the current block, and applying the bi-prediction to the generation of the prediction image of the current block without applying the OBMC. The OBMC corrects the prediction image of the current block using an image obtained by performing motion compensation of the current block using a motion vector of a block surrounding the current block. 
     With this, it is possible to remove a path that applies both bi-prediction, which refers to the two processed pictures preceding and following in display order, and OBMC in the procedure of inter picture prediction. Accordingly, it is possible to reduce an amount of processing in the path with a large amount of processing in the procedure of inter picture prediction. 
     When bi-prediction, which refers to the two processed pictures, and OBMC are applied, an amount of processing is considerably increased by a prediction image obtained from each of the two processed pictures being corrected. A device using the encoding method etc. can suppress such an increase in the amount of processing. 
     As a result, the device using the encoding method etc. can perform prediction efficiently. In addition, a circuit size can be reduced. 
     A decoding method according to one aspect of the present disclosure is, for example, a decoding method of decoding a video using inter picture prediction, the decoding method including: determining whether overlapped block motion compensation (OBMC) is applicable to generation of a prediction image of a current block, according to whether bi-prediction is to be applied to the generation of the prediction image of the current block, the current block being included in a current picture in the video, the bi-prediction referring to a processed picture preceding the current picture in display order, and a processed picture following the current picture in display order; and when the bi-prediction is to be applied to the generation of the prediction image of the current block, determining that the OBMC is not applicable to the generation of the prediction image of the current block, and applying the bi-prediction to the generation of the prediction image of the current block without applying the OBMC. The OBMC corrects the prediction image of the current block using an image obtained by performing motion compensation of the current block using a motion vector of a block surrounding the current block. 
     With this, it is possible to remove a path that applies both bi-prediction, which refers to the two processed pictures preceding and following in display order, and OBMC in the procedure of inter picture prediction. Accordingly, it is possible to reduce an amount of processing in the path with a large amount of processing in the procedure of inter picture prediction. 
     When bi-prediction, which refers to the two processed pictures, and OBMC are applied, an amount of processing is considerably increased by a prediction image obtained from each of the two processed pictures being corrected. A device using the decoding method etc. can suppress such an increase in the amount of processing. 
     As a result, the device using the decoding method etc. can perform prediction efficiently. In addition, a circuit size can be reduced. 
     Furthermore, these general or specific aspects may be implemented by a system, a device, a method, an integrated circuit, a computer program, or a non-transitory computer-readable recording medium such as a compact disc read only memory (CD-ROM), or by any combination of systems, devices, methods, integrated circuits, computer programs, or recording media. 
     Hereinafter, embodiments will be described with reference to the drawings. 
     Note that the embodiments described below each show a general or specific example. The numerical values, shapes, materials, components, the arrangement and connection of the components, steps, order of the steps, etc., indicated in the following embodiments are mere examples, and therefore are not intended to limit the scope of the claims. Therefore, among the components in the following embodiments, those not recited in any of the independent claims defining the broadest inventive concepts are described as optional components. 
     Embodiment 1 
     First, an outline of Embodiment 1 will be presented. Embodiment 1 is one example of an encoder and a decoder to which the processes and/or configurations presented in subsequent description of aspects of the present disclosure are applicable. Note that Embodiment 1 is merely one example of an encoder and a decoder to which the processes and/or configurations presented in the description of aspects of the present disclosure are applicable. The processes and/or configurations presented in the description of aspects of the present disclosure can also be implemented in an encoder and a decoder different from those according to Embodiment 1. 
     When the processes and/or configurations presented in the description of aspects of the present disclosure are applied to Embodiment 1, for example, any of the following may be performed. 
     (1) regarding the encoder or the decoder according to Embodiment 1, among components included in the encoder or the decoder according to Embodiment 1, substituting a component corresponding to a component presented in the description of aspects of the present disclosure with a component presented in the description of aspects of the present disclosure; 
     (2) regarding the encoder or the decoder according to Embodiment 1, implementing discretionary changes to functions or implemented processes performed by one or more components included in the encoder or the decoder according to Embodiment 1, such as addition, substitution, or removal, etc., of such functions or implemented processes, then substituting a component corresponding to a component presented in the description of aspects of the present disclosure with a component presented in the description of aspects of the present disclosure; 
     (3) regarding the method implemented by the encoder or the decoder according to Embodiment 1, implementing discretionary changes such as addition of processes and/or substitution, removal of one or more of the processes included in the method, and then substituting a processes corresponding to a process presented in the description of aspects of the present disclosure with a process presented in the description of aspects of the present disclosure; 
     (4) combining one or more components included in the encoder or the decoder according to Embodiment 1 with a component presented in the description of aspects of the present disclosure, a component including one or more functions included in a component presented in the description of aspects of the present disclosure, or a component that implements one or more processes implemented by a component presented in the description of aspects of the present disclosure; 
     (5) combining a component including one or more functions included in one or more components included in the encoder or the decoder according to Embodiment 1, or a component that implements one or more processes implemented by one or more components included in the encoder or the decoder according to Embodiment 1 with a component presented in the description of aspects of the present disclosure, a component including one or more functions included in a component presented in the description of aspects of the present disclosure, or a component that implements one or more processes implemented by a component presented in the description of aspects of the present disclosure; 
     (6) regarding the method implemented by the encoder or the decoder according to Embodiment 1, among processes included in the method, substituting a process corresponding to a process presented in the description of aspects of the present disclosure with a process presented in the description of aspects of the present disclosure; and 
     (7) combining one or more processes included in the method implemented by the encoder or the decoder according to Embodiment 1 with a process presented in the description of aspects of the present disclosure. 
     Note that the implementation of the processes and/or configurations presented in the description of aspects of the present disclosure is not limited to the above examples. For example, the processes and/or configurations presented in the description of aspects of the present disclosure may be implemented in a device used for a purpose different from the moving picture/picture encoder or the moving picture/picture decoder disclosed in Embodiment 1. Moreover, the processes and/or configurations presented in the description of aspects of the present disclosure may be independently implemented. Moreover, processes and/or configurations described in different aspects may be combined. 
     [Encoder Outline] 
     First, the encoder according to Embodiment 1 will be outlined.  FIG.  1    is a block diagram illustrating a functional configuration of encoder  100  according to Embodiment 1. Encoder  100  is a moving picture/picture encoder that encodes a moving picture/picture block by block. 
     As illustrated in  FIG.  1   , encoder  100  is a device that encodes a picture block by block, and includes splitter  102 , subtractor  104 , transformer  106 , quantizer  108 , entropy encoder  110 , inverse quantizer  112 , inverse transformer  114 , adder  116 , block memory  118 , loop filter  120 , frame memory  122 , intra predictor  124 , inter predictor  126 , and prediction controller  128 . 
     Encoder  100  is realized as, for example, a generic processor and memory. In this case, when a software program stored in the memory is executed by the processor, the processor functions as splitter  102 , subtractor  104 , transformer  106 , quantizer  108 , entropy encoder  110 , inverse quantizer  112 , inverse transformer  114 , adder  116 , loop filter  120 , intra predictor  124 , inter predictor  126 , and prediction controller  128 . Alternatively, encoder  100  may be realized as one or more dedicated electronic circuits corresponding to splitter  102 , subtractor  104 , transformer  106 , quantizer  108 , entropy encoder  110 , inverse quantizer  112 , inverse transformer  114 , adder  116 , loop filter  120 , intra predictor  124 , inter predictor  126 , and prediction controller  128 . 
     Hereinafter, each component included in encoder  100  will be described. 
     [Splitter] 
     Splitter  102  splits each picture included in an input moving picture into blocks, and outputs each block to subtractor  104 . For example, splitter  102  first splits a picture into blocks of a fixed size (for example, 128×128). The fixed size block is also referred to as coding tree unit (CTU). Splitter  102  then splits each fixed size block into blocks of variable sizes (for example, 64×64 or smaller), based on recursive quadtree and/or binary tree block splitting. The variable size block is also referred to as a coding unit (CU), a prediction unit (PU), or a transform unit (TU). Note that in this embodiment, there is no need to differentiate between CU, PU, and TU; all or some of the blocks in a picture may be processed per CU, PU, or TU. 
       FIG.  2    illustrates one example of block splitting according to Embodiment 1. In  FIG.  2   , the solid lines represent block boundaries of blocks split by quadtree block splitting, and the dashed lines represent block boundaries of blocks split by binary tree block splitting. 
     Here, block  10  is a square 128×128 pixel block (128×128 block). This 128×128 block  10  is first split into four square 64×64 blocks (quadtree block splitting). 
     The top left 64×64 block is further vertically split into two rectangle 32×64 blocks, and the left 32×64 block is further vertically split into two rectangle 16×64 blocks (binary tree block splitting). As a result, the top left 64×64 block is split into two 16×64 blocks  11  and  12  and one 32×64 block  13 . 
     The top right 64×64 block is horizontally split into two rectangle 64×32 blocks  14  and  15  (binary tree block splitting). 
     The bottom left 64×64 block is first split into four square 32×32 blocks (quadtree block splitting). The top left block and the bottom right block among the four 32×32 blocks are further split. The top left 32×32 block is vertically split into two rectangle 16×32 blocks, and the right 16×32 block is further horizontally split into two 16×16 blocks (binary tree block splitting). The bottom right 32×32 block is horizontally split into two 32×16 blocks (binary tree block splitting). As a result, the bottom left 64×64 block is split into 16×32 block  16 , two 16×16 blocks  17  and  18 , two 32×32 blocks  19  and  20 , and two 32×16 blocks  21  and  22 . 
     The bottom right 64×64 block  23  is not split. 
     As described above, in  FIG.  2   , block  10  is split into 13 variable size blocks  11  through  23  based on recursive quadtree and binary tree block splitting. This type of splitting is also referred to as quadtree plus binary tree (QTBT) splitting. 
     Note that in  FIG.  2   , one block is split into four or two blocks (quadtree or binary tree block splitting), but splitting is not limited to this example. For example, one block may be split into three blocks (ternary block splitting). Splitting including such ternary block splitting is also referred to as multi-type tree (MBT) splitting. 
     [Subtractor] 
     Subtractor  104  subtracts a prediction signal (prediction sample) from an original signal (original sample) per block split by splitter  102 . In other words, subtractor  104  calculates prediction errors (also referred to as residuals) of a block to be encoded (hereinafter referred to as a current block). Subtractor  104  then outputs the calculated prediction errors to transformer  106 . 
     The original signal is a signal input into encoder  100 , and is a signal representing an image for each picture included in a moving picture (for example, a luma signal and two chroma signals). Hereinafter, a signal representing an image is also referred to as a sample. 
     [Transformer] 
     Transformer  106  transforms spatial domain prediction errors into frequency domain transform coefficients, and outputs the transform coefficients to quantizer  108 . More specifically, transformer  106  applies, for example, a predefined discrete cosine transform (DCT) or discrete sine transform (DST) to spatial domain prediction errors. 
     Note that transformer  106  may adaptively select a transform type from among a plurality of transform types, and transform prediction errors into transform coefficients by using a transform basis function corresponding to the selected transform type. This sort of transform is also referred to as explicit multiple core transform (EMT) or adaptive multiple transform (AMT). 
     The transform types include, for example, DCT-II, DCT-V, DCT-VIII, DST-I, and DST-VII.  FIG.  3    is a chart indicating transform basis functions for each transform type. In  FIG.  3   , N indicates the number of input pixels. For example, selection of a transform type from among the plurality of transform types may depend on the prediction type (intra prediction and inter prediction), and may depend on intra prediction mode. 
     Information indicating whether to apply such EMT or AMT (referred to as, for example, an AMT flag) and information indicating the selected transform type is signalled at the CU level. Note that the signaling of such information need not be performed at the CU level, and may be performed at another level (for example, at the sequence level, picture level, slice level, tile level, or CTU level). 
     Moreover, transformer  106  may apply a secondary transform to the transform coefficients (transform result). Such a secondary transform is also referred to as adaptive secondary transform (AST) or non-separable secondary transform (NSST). For example, transformer  106  applies a secondary transform to each sub-block (for example, each 4×4 sub-block) included in the block of the transform coefficients corresponding to the intra prediction errors. Information indicating whether to apply NSST and information related to the transform matrix used in NSST are signalled at the CU level. Note that the signaling of such information need not be performed at the CU level, and may be performed at another level (for example, at the sequence level, picture level, slice level, tile level, or CTU level). 
     Here, a separable transform is a method in which a transform is performed a plurality of times by separately performing a transform for each direction according to the number of dimensions input. A non-separable transform is a method of performing a collective transform in which two or more dimensions in a multidimensional input are collectively regarded as a single dimension. 
     In one example of a non-separable transform, when the input is a 4×4 block, the 4×4 block is regarded as a single array including 16 components, and the transform applies a 16×16 transform matrix to the array. 
     Moreover, similar to above, after an input 4×4 block is regarded as a single array including 16 components, a transform that performs a plurality of Givens rotations on the array (i.e., a Hypercube-Givens Transform) is also one example of a non-separable transform. 
     [Quantizer] 
     Quantizer  108  quantizes the transform coefficients output from transformer  106 . More specifically, quantizer  108  scans, in a predetermined scanning order, the transform coefficients of the current block, and quantizes the scanned transform coefficients based on quantization parameters (QP) corresponding to the transform coefficients. Quantizer  108  then outputs the quantized transform coefficients (hereinafter referred to as quantized coefficients) of the current block to entropy encoder  110  and inverse quantizer  112 . 
     A predetermined order is an order for quantizing/inverse quantizing transform coefficients. For example, a predetermined scanning order is defined as ascending order of frequency (from low to high frequency) or descending order of frequency (from high to low frequency). 
     A quantization parameter is a parameter defining a quantization step size (quantization width). For example, if the value of the quantization parameter increases, the quantization step size also increases. In other words, if the value of the quantization parameter increases, the quantization error increases. 
     [Entropy Encoder] 
     Entropy encoder  110  generates an encoded signal (encoded bitstream) by variable length encoding quantized coefficients, which are inputs from quantizer  108 . More specifically, entropy encoder  110 , for example, binarizes quantized coefficients and arithmetic encodes the binary signal. 
     [Inverse Quantizer] 
     Inverse quantizer  112  inverse quantizes quantized coefficients, which are inputs from quantizer  108 . More specifically, inverse quantizer  112  inverse quantizes, in a predetermined scanning order, quantized coefficients of the current block. Inverse quantizer  112  then outputs the inverse quantized transform coefficients of the current block to inverse transformer  114 . 
     [Inverse Transformer] 
     Inverse transformer  114  restores prediction errors by inverse transforming transform coefficients, which are inputs from inverse quantizer  112 . More specifically, inverse transformer  114  restores the prediction errors of the current block by applying an inverse transform corresponding to the transform applied by transformer  106  on the transform coefficients. Inverse transformer  114  then outputs the restored prediction errors to adder  116 . 
     Note that since information is lost in quantization, the restored prediction errors do not match the prediction errors calculated by subtractor  104 . In other words, the restored prediction errors include quantization errors. 
     [Adder] 
     Adder  116  reconstructs the current block by summing prediction errors, which are inputs from inverse transformer  114 , and prediction samples, which are inputs from prediction controller  128 . Adder  116  then outputs the reconstructed block to block memory  118  and loop filter  120 . A reconstructed block is also referred to as a local decoded block. 
     [Block Memory] 
     Block memory  118  is storage for storing blocks in a picture to be encoded (hereinafter referred to as a current picture) for reference in intra prediction. More specifically, block memory  118  stores reconstructed blocks output from adder  116 . 
     [Loop Filter] 
     Loop filter  120  applies a loop filter to blocks reconstructed by adder  116 , and outputs the filtered reconstructed blocks to frame memory  122 . A loop filter is a filter used in an encoding loop (in-loop filter), and includes, for example, a deblocking filter (DF), a sample adaptive offset (SAO), and an adaptive loop filter (ALF). 
     In ALF, a least square error filter for removing compression artifacts is applied. For example, one filter from among a plurality of filters is selected for each 2×2 sub-block in the current block based on direction and activity of local gradients, and is applied. 
     More specifically, first, each sub-block (for example, each 2×2 sub-block) is categorized into one out of a plurality of classes (for example, 15 or 25 classes). The classification of the sub-block is based on gradient directionality and activity. For example, classification index C is derived based on gradient directionality D (for example, 0 to 2 or 0 to 4) and gradient activity A (for example, 0 to 4) (for example, C=5D+A). Then, based on classification index C, each sub-block is categorized into one out of a plurality of classes (for example, 15 or 25 classes). 
     For example, gradient directionality D is calculated by comparing gradients of a plurality of directions (for example, the horizontal, vertical, and two diagonal directions). Moreover, for example, gradient activity A is calculated by summing gradients of a plurality of directions and quantizing the sum. 
     The filter to be used for each sub-block is determined from among the plurality of filters based on the result of such categorization. 
     The filter shape to be used in ALF is, for example, a circular symmetric filter shape.  FIG.  4 A  through  FIG.  4 C  illustrate examples of filter shapes used in ALF.  FIG.  4 A  illustrates a 5×5 diamond shape filter,  FIG.  4 B  illustrates a 7×7 diamond shape filter, and  FIG.  4 C  illustrates a 9×9 diamond shape filter. Information indicating the filter shape is signalled at the picture level. Note that the signaling of information indicating the filter shape need not be performed at the picture level, and may be performed at another level (for example, at the sequence level, slice level, tile level, CTU level, or CU level). 
     The enabling or disabling of ALF is determined at the picture level or CU level. For example, for luma, the decision to apply ALF or not is done at the CU level, and for chroma, the decision to apply ALF or not is done at the picture level. Information indicating whether ALF is enabled or disabled is signalled at the picture level or CU level. Note that the signaling of information indicating whether ALF is enabled or disabled need not be performed at the picture level or CU level, and may be performed at another level (for example, at the sequence level, slice level, tile level, or CTU level). 
     The coefficients set for the plurality of selectable filters (for example, 15 or 25 filters) is signalled at the picture level. Note that the signaling of the coefficients set need not be performed at the picture level, and may be performed at another level (for example, at the sequence level, slice level, tile level, CTU level, CU level, or sub-block level). 
     [Frame Memory] 
     Frame memory  122  is storage for storing reference pictures used in inter prediction, and is also referred to as a frame buffer. More specifically, frame memory  122  stores reconstructed blocks filtered by loop filter  120 . 
     [Intra Predictor] 
     Intra predictor  124  generates a prediction signal (intra prediction signal) by intra predicting the current block with reference to a block or blocks in the current picture and stored in block memory  118  (also referred to as intra frame prediction). More specifically, intra predictor  124  generates an intra prediction signal by intra prediction with reference to samples (for example, luma and/or chroma values) of a block or blocks neighboring the current block, and then outputs the intra prediction signal to prediction controller  128 . 
     For example, intra predictor  124  performs intra prediction by using one mode from among a plurality of predefined intra prediction modes. The intra prediction modes include one or more non-directional prediction modes and a plurality of directional prediction modes. 
     The one or more non-directional prediction modes include, for example, planar prediction mode and DC prediction mode defined in the H.265/high-efficiency video coding (HEVC) standard (see NPL 1). 
     The plurality of directional prediction modes include, for example, the 33 directional prediction modes defined in the H.265/HEVC standard. Note that the plurality of directional prediction modes may further include 32 directional prediction modes in addition to the 33 directional prediction modes (for a total of 65 directional prediction modes).  FIG.  5 A  illustrates  67  intra prediction modes used in intra prediction (two non-directional prediction modes and 65 directional prediction modes). The solid arrows represent the 33 directions defined in the H.265/HEVC standard, and the dashed arrows represent the additional 32 directions. 
     Note that a luma block may be referenced in chroma block intra prediction. In other words, a chroma component of the current block may be predicted based on a luma component of the current block. Such intra prediction is also referred to as cross-component linear model (CCLM) prediction. Such a chroma block intra prediction mode that references a luma block (referred to as, for example, CCLM mode) may be added as one of the chroma block intra prediction modes. 
     Intra predictor  124  may correct post-intra-prediction pixel values based on horizontal/vertical reference pixel gradients. Intra prediction accompanied by this sort of correcting is also referred to as position dependent intra prediction combination (PDPC). Information indicating whether to apply PDPC or not (referred to as, for example, a PDPC flag) is, for example, signalled at the CU level. Note that the signaling of this information need not be performed at the CU level, and may be performed at another level (for example, on the sequence level, picture level, slice level, tile level, or CTU level). 
     [Inter Predictor] 
     Inter predictor  126  generates a prediction signal (inter prediction signal) by inter predicting the current block with reference to a block or blocks in a reference picture, which is different from the current picture and is stored in frame memory  122  (also referred to as inter frame prediction). Inter prediction is performed per current block or per sub-block (for example, per 4×4 block) in the current block. For example, inter predictor  126  performs motion estimation in a reference picture for the current block or sub-block. Inter predictor  126  then generates an inter prediction signal of the current block or sub-block by motion compensation by using motion information (for example, a motion vector) obtained from motion estimation. Inter predictor  126  then outputs the generated inter prediction signal to prediction controller  128 . 
     The motion information used in motion compensation is signalled. A motion vector predictor may be used for the signaling of the motion vector. In other words, the difference between the motion vector and the motion vector predictor may be signalled. 
     Note that the inter prediction signal may be generated using motion information for a neighboring block in addition to motion information for the current block obtained from motion estimation. More specifically, the inter prediction signal may be generated per sub-block in the current block by calculating a weighted sum of a prediction signal based on motion information obtained from motion estimation and a prediction signal based on motion information for a neighboring block. Such inter prediction (motion compensation) is also referred to as overlapped block motion compensation (OBMC). 
     In such an OBMC mode, information indicating sub-block size for OBMC (referred to as, for example, OBMC block size) is signalled at the sequence level. Moreover, information indicating whether to apply the OBMC mode or not (referred to as, for example, an OBMC flag) is signalled at the CU level. Note that the signaling of such information need not be performed at the sequence level and CU level, and may be performed at another level (for example, at the picture level, slice level, tile level, CTU level, or sub-block level). 
     Hereinafter, the OBMC mode will be described in further detail.  FIG.  5 B  is a flowchart and  FIG.  5 C  is a conceptual diagram for illustrating an outline of a prediction image correction process performed via OBMC processing. 
     First, a prediction image (Pred) is obtained through typical motion compensation using a motion vector (MV) assigned to the current block. 
     Next, a prediction image (Pred_L) is obtained by applying a motion vector (MV_L) of the encoded neighboring left block to the current block, and a first pass of the correction of the prediction image is made by superimposing the prediction image and Pred_L. 
     Similarly, a prediction image (Pred_U) is obtained by applying a motion vector (MV_U) of the encoded neighboring upper block to the current block, and a second pass of the correction of the prediction image is made by superimposing the prediction image resulting from the first pass and Pred_U. The result of the second pass is the final prediction image. 
     Note that the above example is of a two-pass correction method using the neighboring left and upper blocks, but the method may be a three-pass or higher correction method that also uses the neighboring right and/or lower block. 
     Note that the region subject to superimposition may be the entire pixel region of the block, and, alternatively, may be a partial block boundary region. 
     Note that here, the prediction image correction process is described as being based on a single reference picture, but the same applies when a prediction image is corrected based on a plurality of reference pictures. In such a case, after corrected prediction images resulting from performing correction based on each of the reference pictures are obtained, the obtained corrected prediction images are further superimposed to obtain the final prediction image. 
     Note that the unit of the current block may be a prediction block and, alternatively, may be a sub-block obtained by further dividing the prediction block. 
     One example of a method for determining whether to implement OBMC processing is by using an obmc_flag, which is a signal that indicates whether to implement OBMC processing. As one specific example, the encoder determines whether the current block belongs to a region including complicated motion. The encoder sets the obmc_flag to a value of “1” when the block belongs to a region including complicated motion and implements OBMC processing when encoding, and sets the obmc_flag to a value of “0” when the block does not belong to a region including complication motion and encodes without implementing OBMC processing. The decoder switches between implementing OBMC processing or not by decoding the obmc_flag written in the stream and performing the decoding in accordance with the flag value. 
     Note that the motion information may be derived on the decoder side without being signalled. For example, a merge mode defined in the H.265/HEVC standard may be used. Moreover, for example, the motion information may be derived by performing motion estimation on the decoder side. In this case, motion estimation is performed without using the pixel values of the current block. 
     Here, a mode for performing motion estimation on the decoder side will be described. A mode for performing motion estimation on the decoder side is also referred to as pattern matched motion vector derivation (PMMVD) mode or frame rate up-conversion (FRUC) mode. 
     One example of FRUC processing is illustrated in  FIG.  5 D . First, a candidate list (a candidate list may be a merge list) of candidates each including a motion vector predictor is generated with reference to motion vectors of encoded blocks that spatially or temporally neighbor the current block. Next, the best candidate MV is selected from among a plurality of candidate MVs registered in the candidate list. For example, evaluation values for the candidates included in the candidate list are calculated and one candidate is selected based on the calculated evaluation values. 
     Next, a motion vector for the current block is derived from the motion vector of the selected candidate. More specifically, for example, the motion vector for the current block is calculated as the motion vector of the selected candidate (best candidate MV), as-is. Alternatively, the motion vector for the current block may be derived by pattern matching performed in the vicinity of a position in a reference picture corresponding to the motion vector of the selected candidate. In other words, when the vicinity of the best candidate MV is searched via the same method and an MV having a better evaluation value is found, the best candidate MV may be updated to the MV having the better evaluation value, and the MV having the better evaluation value may be used as the final MV for the current block. Note that a configuration in which this processing is not implemented is also acceptable. 
     The same processes may be performed in cases in which the processing is performed in units of sub-blocks. 
     Note that an evaluation value is calculated by calculating the difference in the reconstructed image by pattern matching performed between a region in a reference picture corresponding to a motion vector and a predetermined region. Note that the evaluation value may be calculated by using some other information in addition to the difference. 
     The pattern matching used is either first pattern matching or second pattern matching. First pattern matching and second pattern matching are also referred to as bilateral matching and template matching, respectively. 
     In the first pattern matching, pattern matching is performed between two blocks along the motion trajectory of the current block in two different reference pictures. Therefore, in the first pattern matching, a region in another reference picture conforming to the motion trajectory of the current block is used as the predetermined region for the above-described calculation of the candidate evaluation value. 
       FIG.  6    is for illustrating one example of pattern matching (bilateral matching) between two blocks along a motion trajectory. As illustrated in  FIG.  6   , in the first pattern matching, two motion vectors (MV0, MV1) are derived by finding the best match between two blocks along the motion trajectory of the current block (Cur block) in two different reference pictures (Ref0, Ref1). More specifically, a difference between (i) a reconstructed image in a specified position in a first encoded reference picture (Ref1) specified by a candidate MV and (ii) a reconstructed picture in a specified position in a second encoded reference picture (Ref1) specified by a symmetrical MV scaled at a display time interval of the candidate MV may be derived, and the evaluation value for the current block may be calculated by using the derived difference. The candidate MV having the best evaluation value among the plurality of candidate MVs may be selected as the final MV. 
     Under the assumption of continuous motion trajectory, the motion vectors (MV0. MV1) pointing to the two reference blocks shall be proportional to the temporal distances (TD0, TD1) between the current picture (Cur Pic) and the two reference pictures (Ref0, Ref1). For example, when the current picture is temporally between the two reference pictures and the temporal distance from the current picture to the two reference pictures is the same, the first pattern matching derives a mirror based bi-directional motion vector. 
     In the second pattern matching, pattern matching is performed between a template in the current picture (blocks neighboring the current block in the current picture (for example, the top and/or left neighboring blocks)) and a block in a reference picture. Therefore, in the second pattern matching, a block neighboring the current block in the current picture is used as the predetermined region for the above-described calculation of the candidate evaluation value. 
       FIG.  7    is for illustrating one example of pattern matching (template matching) between a template in the current picture and a block in a reference picture. As illustrated in  FIG.  7   , in the second pattern matching, a motion vector of the current block is derived by searching a reference picture (Ref0) to find the block that best matches neighboring blocks of the current block (Cur block) in the current picture (Cur Pic). More specifically, a difference between (i) a reconstructed image of an encoded region that is both or one of the neighboring left and neighboring upper region and (i) a reconstructed picture in the same position in an encoded reference picture (Ref0) specified by a candidate MV may be derived, and the evaluation value for the current block may be calculated by using the derived difference. The candidate MV having the best evaluation value among the plurality of candidate MVs may be selected as the best candidate MV. 
     Information indicating whether to apply the FRUC mode or not (referred to as, for example, a FRUC flag) is signalled at the CU level. Moreover, when the FRUC mode is applied (for example, when the FRUC flag is set to true), information indicating the pattern matching method (first pattern matching or second pattern matching) is signalled at the CU level. Note that the signaling of such information need not be performed at the CU level, and may be performed at another level (for example, at the sequence level, picture level, slice level, tile level, CTU level, or sub-block level). 
     Here, a mode for deriving a motion vector based on a model assuming uniform linear motion will be described. This mode is also referred to as a bi-directional optical flow (BIO) mode. 
       FIG.  8    is for illustrating a model assuming uniform linear motion. In  FIG.  8   , (v x , v y ) denotes a velocity vector, and τ 0  and τ 1  denote temporal distances between the current picture (Cur Pic) and two reference pictures (Ref 0 , Ref 1 ). (MVx 0 , MVy 0 ) denotes a motion vector corresponding to reference picture Ref 0 , and (MVx 1 , MVy 1 ) denotes a motion vector corresponding to reference picture Ref 1 . 
     Here, under the assumption of uniform linear motion exhibited by velocity vector (v x , v y ), (MVx 0 , MVy 0 ) and (MVx 1 , MVy 1 ) are represented as (v x τ 0 , v y τ 0 ) and (−v x τ 1 , −v y τ 1 ), respectively, and the following optical flow equation is given.
 
MATH. 1
 
∂ I   (k)   /∂t+v   x   ∂I   (k)   /∂x+v   y   ∂I   (k)   /∂y= 0  (1)
 
     Here, I (k)  denotes a luma value from reference picture k (k=0, 1) after motion compensation. This optical flow equation shows that the sum of (i) the time derivative of the luma value, (ii) the product of the horizontal velocity and the horizontal component of the spatial gradient of a reference picture, and (iii) the product of the vertical velocity and the vertical component of the spatial gradient of a reference picture is equal to zero. A motion vector of each block obtained from, for example, a merge list is corrected pixel by pixel based on a combination of the optical flow equation and Hermite interpolation. 
     Note that a motion vector may be derived on the decoder side using a method other than deriving a motion vector based on a model assuming uniform linear motion. For example, a motion vector may be derived for each sub-block based on motion vectors of neighboring blocks. 
     Here, a mode in which a motion vector is derived for each sub-block based on motion vectors of neighboring blocks will be described. This mode is also referred to as affine motion compensation prediction mode. 
       FIG.  9 A  is for illustrating deriving a motion vector of each sub-block based on motion vectors of neighboring blocks. In  FIG.  9 A , the current block includes 16 4×4 sub-blocks. Here, motion vector v 0  of the top left corner control point in the current block is derived based on motion vectors of neighboring sub-blocks, and motion vector vi of the top right corner control point in the current block is derived based on motion vectors of neighboring blocks. Then, using the two motion vectors v 0  and vi, the motion vector (v x , v y ) of each sub-block in the current block is derived using Equation 2 below. 
     
       
         
           
             
               
                 
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     Here, x and y are the horizontal and vertical positions of the sub-block, respectively, and w is a predetermined weighted coefficient. 
     Such an affine motion compensation prediction mode may include a number of modes of different methods of deriving the motion vectors of the top left and top right corner control points. Information indicating such an affine motion compensation prediction mode (referred to as, for example, an affine flag) is signalled at the CU level. Note that the signaling of information indicating the affine motion compensation prediction mode need not be performed at the CU level, and may be performed at another level (for example, at the sequence level, picture level, slice level, tile level, CTU level, or sub-block level). 
     [Prediction Controller] 
     Prediction controller  128  selects either the intra prediction signal or the inter prediction signal, and outputs the selected prediction signal to subtractor  104  and adder  116 . 
     Here, an example of deriving a motion vector via merge mode in a current picture will be given.  FIG.  9 B  is for illustrating an outline of a process for deriving a motion vector via merge mode. 
     First, an MV predictor list in which candidate MV predictors are registered is generated. Examples of candidate MV predictors include: spatially neighboring MV predictors, which are MVs of encoded blocks positioned in the spatial vicinity of the current block; a temporally neighboring MV predictor, which is an MV of a block in an encoded reference picture that neighbors a block in the same location as the current block; a combined MV predictor, which is an MV generated by combining the MV values of the spatially neighboring MV predictor and the temporally neighboring MV predictor; and a zero MV predictor, which is an MV whose value is zero. 
     Next, the MV of the current block is determined by selecting one MV predictor from among the plurality of MV predictors registered in the MV predictor list. 
     Furthermore, in the variable-length encoder, a merge_idx, which is a signal indicating which MV predictor is selected, is written and encoded into the stream. 
     Note that the MV predictors registered in the MV predictor list illustrated in  FIG.  9 B  constitute one example. The number of MV predictors registered in the MV predictor list may be different from the number illustrated in  FIG.  9 B , the MV predictors registered in the MV predictor list may omit one or more of the types of MV predictors given in the example in  FIG.  9 B , and the MV predictors registered in the MV predictor list may include one or more types of MV predictors in addition to and different from the types given in the example in  FIG.  9 B . 
     Note that the final MV may be determined by performing DMVR processing (to be described later) by using the MV of the current block derived via merge mode. 
     Here, an example of determining an MV by using DMVR processing will be given. 
       FIG.  9 C  is a conceptual diagram for illustrating an outline of DMVR processing. 
     First, the most appropriate MVP set for the current block is considered to be the candidate MV, reference pixels are obtained from a first reference picture, which is a picture processed in the L0 direction in accordance with the candidate MV, and a second reference picture, which is a picture processed in the L1 direction in accordance with the candidate MV, and a template is generated by calculating the average of the reference pixels. 
     Next, using the template, the surrounding regions of the candidate MVs of the first and second reference pictures are searched, and the MV with the lowest cost is determined to be the final MV. Note that the cost value is calculated using, for example, the difference between each pixel value in the template and each pixel value in the regions searched, as well as the MV value. 
     Note that the outlines of the processes described here are fundamentally the same in both the encoder and the decoder. 
     Note that processing other than the processing exactly as described above may be used, so long as the processing is capable of deriving the final MV by searching the surroundings of the candidate MV. 
     Here, an example of a mode that generates a prediction image by using LIC processing will be given. 
       FIG.  9 D  is for illustrating an outline of a prediction image generation method using a luminance correction process performed via LIC processing. 
     First, an MV is extracted for obtaining, from an encoded reference picture, a reference image corresponding to the current block. 
     Next, information indicating how the luminance value changed between the reference picture and the current picture is extracted and a luminance correction parameter is calculated by using the luminance pixel values for the encoded left neighboring reference region and the encoded upper neighboring reference region, and the luminance pixel value in the same location in the reference picture specified by the MV. 
     The prediction image for the current block is generated by performing a luminance correction process by using the luminance correction parameter on the reference image in the reference picture specified by the MV. 
     Note that the shape of the surrounding reference region illustrated in  FIG.  9 D  is just one example; the surrounding reference region may have a different shape. 
     Moreover, although a prediction image is generated from a single reference picture in this example, in cases in which a prediction image is generated from a plurality of reference pictures as well, the prediction image is generated after performing a luminance correction process, via the same method, on the reference images obtained from the reference pictures. 
     One example of a method for determining whether to implement LIC processing is by using an lic_flag, which is a signal that indicates whether to implement LIC processing. As one specific example, the encoder determines whether the current block belongs to a region of luminance change. The encoder sets the lic_flag to a value of “1” when the block belongs to a region of luminance change and implements LIC processing when encoding, and sets the lic_flag to a value of “0” when the block does not belong to a region of luminance change and encodes without implementing LIC processing. The decoder switches between implementing LIC processing or not by decoding the lic_flag written in the stream and performing the decoding in accordance with the flag value. 
     One example of a different method of determining whether to implement LIC processing is determining so in accordance with whether LIC processing was determined to be implemented for a surrounding block. In one specific example, when merge mode is used on the current block, whether LIC processing was applied in the encoding of the surrounding encoded block selected upon deriving the MV in the merge mode processing may be determined, and whether to implement LIC processing or not can be switched based on the result of the determination. Note that in this example, the same applies to the processing performed on the decoder side. 
     [Decoder Outline] 
     Next, a decoder capable of decoding an encoded signal (encoded bitstream) output from encoder  100  will be described.  FIG.  10    is a block diagram illustrating a functional configuration of decoder  200  according to Embodiment 1. Decoder  200  is a moving picture/picture decoder that decodes a moving picture/picture block by block. 
     As illustrated in  FIG.  10   , decoder  200  includes entropy decoder  202 , inverse quantizer  204 , inverse transformer  206 , adder  208 , block memory  210 , loop filter  212 , frame memory  214 , intra predictor  216 , inter predictor  218 , and prediction controller  220 . 
     Decoder  200  is realized as, for example, a generic processor and memory. In this case, when a software program stored in the memory is executed by the processor, the processor functions as entropy decoder  202 , inverse quantizer  204 , inverse transformer  206 , adder  208 , loop filter  212 , intra predictor  216 , inter predictor  218 , and prediction controller  220 . Alternatively, decoder  200  may be realized as one or more dedicated electronic circuits corresponding to entropy decoder  202 , inverse quantizer  204 , inverse transformer  206 , adder  208 , loop filter  212 , intra predictor  216 , inter predictor  218 , and prediction controller  220 . 
     Hereinafter, each component included in decoder  200  will be described. 
     [Entropy Decoder] 
     Entropy decoder  202  entropy decodes an encoded bitstream. More specifically, for example, entropy decoder  202  arithmetic decodes an encoded bitstream into a binary signal. Entropy decoder  202  then debinarizes the binary signal. With this, entropy decoder  202  outputs quantized coefficients of each block to inverse quantizer  204 . 
     [Inverse Quantizer] 
     Inverse quantizer  204  inverse quantizes quantized coefficients of a block to be decoded (hereinafter referred to as a current block), which are inputs from entropy decoder  202 . More specifically, inverse quantizer  204  inverse quantizes quantized coefficients of the current block based on quantization parameters corresponding to the quantized coefficients. Inverse quantizer  204  then outputs the inverse quantized coefficients (i.e., transform coefficients) of the current block to inverse transformer  206 . 
     [Inverse Transformer] 
     Inverse transformer  206  restores prediction errors by inverse transforming transform coefficients, which are inputs from inverse quantizer  204 . 
     For example, when information parsed from an encoded bitstream indicates application of EMT or AMT (for example, when the AMT flag is set to true), inverse transformer  206  inverse transforms the transform coefficients of the current block based on information indicating the parsed transform type. 
     Moreover, for example, when information parsed from an encoded bitstream indicates application of NSST, inverse transformer  206  applies a secondary inverse transform to the transform coefficients. 
     [Adder] 
     Adder  208  reconstructs the current block by summing prediction errors, which are inputs from inverse transformer  206 , and prediction samples, which is an input from prediction controller  220 . Adder  208  then outputs the reconstructed block to block memory  210  and loop filter  212 . 
     [Block Memory] 
     Block memory  210  is storage for storing blocks in a picture to be decoded (hereinafter referred to as a current picture) for reference in intra prediction. More specifically, block memory  210  stores reconstructed blocks output from adder  208 . 
     [Loop Filter] 
     Loop filter  212  applies a loop filter to blocks reconstructed by adder  208 , and outputs the filtered reconstructed blocks to frame memory  214  and, for example, a display device. 
     When information indicating the enabling or disabling of ALF parsed from an encoded bitstream indicates enabled, one filter from among a plurality of filters is selected based on direction and activity of local gradients, and the selected filter is applied to the reconstructed block. 
     [Frame Memory] 
     Frame memory  214  is storage for storing reference pictures used in inter prediction, and is also referred to as a frame buffer. More specifically, frame memory  214  stores reconstructed blocks filtered by loop filter  212 . 
     [Intra Predictor] 
     Intra predictor  216  generates a prediction signal (intra prediction signal) by intra prediction with reference to a block or blocks in the current picture and stored in block memory  210 . More specifically, intra predictor  216  generates an intra prediction signal by intra prediction with reference to samples (for example, luma and/or chroma values) of a block or blocks neighboring the current block, and then outputs the intra prediction signal to prediction controller  220 . 
     Note that when an intra prediction mode in which a chroma block is intra predicted from a luma block is selected, intra predictor  216  may predict the chroma component of the current block based on the luma component of the current block. 
     Moreover, when information indicating the application of PDPC is parsed from an encoded bitstream, intra predictor  216  corrects post-intra-prediction pixel values based on horizontal/vertical reference pixel gradients. 
     [Inter Predictor] 
     Inter predictor  218  predicts the current block with reference to a reference picture stored in frame memory  214 . Inter prediction is performed per current block or per sub-block (for example, per 4×4 block) in the current block. For example, inter predictor  218  generates an inter prediction signal of the current block or sub-block by motion compensation by using motion information (for example, a motion vector) parsed from an encoded bitstream, and outputs the inter prediction signal to prediction controller  220 . 
     Note that when the information parsed from the encoded bitstream indicates application of OBMC mode, inter predictor  218  generates the inter prediction signal using motion information for a neighboring block in addition to motion information for the current block obtained from motion estimation. 
     Moreover, when the information parsed from the encoded bitstream indicates application of FRUC mode, inter predictor  218  derives motion information by performing motion estimation in accordance with the pattern matching method (bilateral matching or template matching) parsed from the encoded bitstream. Inter predictor  218  then performs motion compensation using the derived motion information. 
     Moreover, when BIO mode is to be applied, inter predictor  218  derives a motion vector based on a model assuming uniform linear motion. Moreover, when the information parsed from the encoded bitstream indicates that affine motion compensation prediction mode is to be applied, inter predictor  218  derives a motion vector of each sub-block based on motion vectors of neighboring blocks. 
     [Prediction Controller] 
     Prediction controller  220  selects either the intra prediction signal or the inter prediction signal, and outputs the selected prediction signal to adder  208 . 
     [Inter Frame Prediction Performed by Encoder and Decoder] 
       FIG.  11    is a block diagram for illustrating processes pertaining to inter frame prediction performed by encoder  100  illustrated in  FIG.  1   . For example, splitter  102  illustrated in  FIG.  1    splits an input image in a unit of a block. Then, processing is performed for each block. 
     Subtractor  104  generates a difference image by obtaining a difference between the input image in the unit of the block and a prediction image generated by intra frame prediction or inter frame prediction. Then, transformer  106  and quantizer  108  generate a coefficient signal by respectively converting and quantizing the difference image. Entropy encoder  110  generates an encoded stream (encoded bitstream) by performing entropy encoding on the generated coefficient signal and other encoded signals. 
     Inverse quantizer  112  and inverse transformer  114  restore the difference image by respectively inverse quantizing and inverse transforming the generated coefficient signal. Intra frame predictor (intra predictor)  124  generates a prediction image by intra frame prediction, and inter frame predictor (inter predictor)  126  generates a prediction image by inter frame prediction. Adder  116  generates a reconstructed image by adding one of the prediction image generated by intra frame prediction and the prediction image generated by inter frame prediction, and the restored difference image. 
     Intra frame predictor  124  uses the reconstructed image of the processed block for intra frame prediction of another block to be processed. Loop filter  120  applies a loop filter to the reconstructed image of the processed block, and stores the filtered reconstructed image into frame memory  122 . Then, inter frame predictor  126  uses the reconstructed image stored in frame memory  122  for inter frame prediction of another block to be processed in another picture. 
       FIG.  12    is a block diagram for illustrating processes pertaining to inter frame prediction performed by decoder  200  illustrated in  FIG.  10   . Entropy decoder  202  obtains information in a unit of a block by performing entropy decoding on an input stream that is an encoded stream. Then, processing is performed for each block. 
     Inverse quantizer  204  and inverse transformer  206  restore a difference image by respectively inverse transforming and inverse quantizing a coefficient signal decoded for each block. 
     Intra frame predictor (intra predictor)  216  generates a prediction image by intra frame prediction, and inter frame predictor (inter predictor)  218  generates a prediction image by inter frame prediction. Adder  208  generates a reconstructed image by adding one of the prediction image generated by intra frame prediction and the prediction image generated by inter frame prediction, and the restored difference image. 
     Intra frame predictor  216  uses the reconstructed image of a processed block for intra frame prediction of another block to be processed. Loop filter  212  applies a loop filter to the reconstructed image of the processed block, and stores the filtered reconstructed image into frame memory  214 . Then, inter frame predictor  218  uses the reconstructed image stored in frame memory  214  for inter frame prediction of another block to be processed in another picture. 
     It should be noted that inter frame prediction is also referred to as inter picture prediction or inter prediction. Intra frame prediction is also referred to as intra picture prediction or intra prediction. 
     [Specific Example of Inter Frame Prediction] 
     Hereinafter, specific examples of inter frame prediction will be described. For instance, one of the specific examples is used. Moreover, although the following mainly describes how encoder  100  operates, decoder  200  basically operates in the same manner as encoder  100 . In particular, inter frame predictor  218  of decoder  200  operates in the same manner as inter frame predictor  126  of encoder  100 . 
     However, there are cases in which encoder  100  encodes information about inter frame prediction, and decoder  200  decodes information about inter frame prediction. For example, encoder  100  may encode information of a motion vector for inter frame prediction. Decoder  200  may decode information of a motion vector for inter frame prediction. 
     The above-described information of the motion vector relates to the motion vector and indicates the motion vector directly or indirectly. For example, the information of the motion vector may indicate the motion vector itself, and may indicate a motion vector difference that is a difference between the motion vector and a motion vector predictor, and the identifier of the motion vector predictor. 
     Hereinafter, processing will be described that is performed for each block. The block is also referred to as a prediction block. The block may be a unit of image data to be encoded and decoded, and may be a unit of image data to be reconstructed. Alternatively, the block in the following description may be a unit of finer image data, and may be a unit of image data called a sub-block. 
     Moreover, OBMC in the following description corrects a prediction image of a current block using an image obtained by performing motion compensation of the current block using a motion vector of a block surrounding the current block. A method to be described with reference to  FIG.  17    and  FIG.  18    may be used as OBMC, and another method may be used as OBMC. 
     Moreover, BIO in the following description generates a prediction image of a current block by reference to a spatial gradient of luminance in an image obtained by performing motion compensation of the current block using a motion vector of the current block. A method to be described with reference to  FIG.  19    and  FIG.  20    may be used as BIO, and another method may be used as BIO. 
     Moreover, a process for a current block, a processed picture, etc. in the following description is, for example, encoding or decoding, and may include prediction and reconstruction. 
       FIG.  13    is a flow chart illustrating the first specific example of inter frame prediction performed by inter frame predictor  126  of encoder  100 . As previously described, inter frame predictor  218  of decoder  200  operates in the same manner as inter frame predictor  126  of encoder  100 . 
     As illustrated in  FIG.  13   , inter frame predictor  126  performs processing for each block. Inter frame predictor  126  derives a motion vector (MV) of a current block (S 101 ). Any derivation method can be used as a motion vector derivation method. 
     For example, encoder  100  and decoder  200  may use a derivation method that performs the same motion vector search process. In addition, encoder  100  and decoder  200  may use a derivation method that transforms a motion vector based on affine transformation. 
     Further, encoder  100  may use a derivation method usable by only encoder  100  out of encoder  100  and decoder  200 . Then, encoder  100  may encode information of a motion vector into a stream, and decoder  200  may decode the information of the motion vector from the stream. With this, encoder  100  and decoder  200  may derive the same motion vector. 
     Next, inter frame predictor  126  determines whether BIO is to be applied to generation of a prediction image of the current block (S 102 ). For example, inter frame predictor  126  may determine that BIO is to be applied when a prediction image of the current block is generated by simultaneous reference to two processed pictures, and may determine that BIO is not to be applied in other cases. 
     Moreover, inter frame predictor  126  may determine whether BIO is to be applied when a prediction image of the current block is generated by simultaneous reference to a processed picture preceding a current picture in display order, and a processed picture following the current picture in display order. Inter frame predictor  126  may determine that BIO is not to be applied in other cases. 
     When inter frame predictor  126  determines that BIO is to be applied to the generation of the prediction image of the current block (APPLY in S 102 ), inter frame predictor  126  generates the prediction image of the current block by BIO (S 106 ). In this case, inter frame predictor  126  does not correct the prediction image of the current block by OBMC. The prediction image generated by BIO without OBMC being applied is used as a final prediction image of the current block. 
     In contrast, when inter frame predictor  126  determines that BIO is not to be applied to the generation of the prediction image of the current block (NOT APPLY in S 102 ), inter frame predictor  126  generates the prediction image of the current block by a normal method (S 103 ). In this example, generating the prediction image of the current block by the normal method means generating the prediction image of the current block without applying BIO. For example, inter frame predictor  126  generates a prediction image by performing normal motion compensation without applying BIO. 
     Next, inter frame predictor  126  determines whether OBMC is to be applied to the generation of the prediction image of the current block (S 104 ). In other words, inter frame predictor  126  determines whether the prediction image of the current block is to be corrected by OBMC. For example, when a derivation method that transforms a motion vector based on affine transformation is used in deriving a motion vector, inter frame predictor  126  may determine that OBMC is not to be applied. Inter frame predictor  126  may determine that OBMC is to be applied in other cases. 
     When inter frame predictor  126  determines that OBMC is to be applied to the generation of the prediction image of the current block (APPLY in S 104 ), inter frame predictor  126  corrects the prediction image of the current block by OBMC (S 105 ). The prediction image corrected by OBMC is used as a final prediction image of the current block. 
     In contrast, when inter frame predictor  126  determines that OBMC is not to be applied to the generation of the prediction image of the current block (NOT APPLY in S 104 ), inter frame predictor  126  does not correct the prediction image of the current block by OBMC. The prediction image generated by the normal method without OBMC being applied is used as a final prediction image of the current block. 
     The above-described operation makes it possible to remove a path that applies both BIO and OBMC to the generation of the prediction image of the current block. With this, it is possible to reduce an amount of processing in the path with a large amount of processing. Accordingly, it is possible to reduce a circuit size. 
     Moreover, in both BIO and OBMC, spatial continuity of a pixel value between blocks is considered. Consequently, when both BIO and OBMC are applied, it is assumed that similar functions are doubly applied. In the above-described operation, such an adverse effect is eliminated. 
     Moreover, it is assumed that a degree of reduction in coding amount by applying OBMC is low when BIO is applied, compared to a case in which BIO is not applied. In other words, when it is assumed that the degree of reduction in coding amount by applying OBMC is low, inter frame predictor  126  can generate the prediction image efficiently without applying OBMC. 
     It should be noted that determining whether BIO is to be applied (S 102 ) can be considered as determining whether OBMC is applicable. In other words, in determining whether BIO is to be applied (S 102 ), inter frame predictor  126  determines whether OBMC is applicable, according to whether BIO is to be applied. For example, when BIO is to be applied, inter frame predictor  126  determines that OBMC is not applicable, and applies BIO without applying OBMC. 
     Moreover, concerning determining whether BIO is to be applied (S 102 ), encoder  100  may cause entropy encoder  110  to encode a signal indicating whether BIO is applied into a stream. Decoder  200  may cause entropy decoder  202  to decode the signal indicating whether BIO is applied from the stream. The signal indicating whether BIO is applied may be encoded and decoded for each block, may be encoded and decoded for each slice, may be encoded and decoded for each picture, and may be encoded and decoded for each sequence. 
     Likewise, concerning determining whether OBMC is to be applied (S 104 ), encoder  100  may cause entropy encoder  110  to encode a signal indicating whether OBMC is applied into a stream. Decoder  200  may cause entropy decoder  202  to decode the signal indicating whether OBMC is applied from the stream. The signal indicating whether OBMC is applied may be encoded and decoded for each block, may be encoded and decoded for each slice, may be encoded and decoded for each picture, and may be encoded and decoded for each sequence. 
       FIG.  14    is a flow chart illustrating a modification of the first specific example illustrated in  FIG.  13   . Inter frame predictor  126  of encoder  100  may perform the operations illustrated in  FIG.  14   . Similarly, inter frame predictor  218  of decoder  200  may perform the operations illustrated in  FIG.  14   . 
     As illustrated in  FIG.  14   , inter frame predictor  126  performs processing for each block. As with the example illustrated in  FIG.  13   , inter frame predictor  126  derives a motion vector of a current block (Sill). Inter frame predictor  126  performs branch processing according to a prediction image generation mode (S 112 ). 
     For example, when the prediction image generation mode indicates 0 (0 in S 112 ), inter frame predictor  126  generates the prediction image of the current block by a normal method (S 113 ). In this case, inter frame predictor  126  does not correct the prediction image of the current block by OBMC. The prediction image generated by the normal method without OBMC being applied is used as the final prediction image of the current block. 
     In this example, generating the prediction image of the current block by the normal method means generating the prediction image of the current block without applying BIO. For example, inter frame predictor  126  generates a prediction image by performing normal motion compensation without applying BIO. 
     Moreover, for example, when the prediction image generation mode indicates 1 (1 in S 112 ), inter frame predictor  126  generates the prediction image of the current block by the normal method (S 114 ) in the same manner as when the prediction image generation mode indicates 0. When the prediction image generation mode indicates 1 (1 in S 112 ), inter frame predictor  126  further corrects the prediction image of the current block by OBMC (S 115 ). The prediction image corrected by OBMC is used as the final prediction image of the current block. 
     Moreover, for example, when the prediction image generation mode indicates  2  ( 2  in S 112 ), inter frame predictor  126  generates the prediction image of the current block by BIO (S 116 ). In this case, inter frame predictor  126  does not correct the prediction image of the current block by OBMC. The prediction image generated by BIO without OBMC being applied is used as the final prediction image of the current block. 
     As with the example illustrated in  FIG.  13   , inter frame predictor  126  can generate the prediction image efficiently by the above-described operations. In other words, it is possible to produce the same effects as the example illustrated in  FIG.  13   . 
     It should be noted that branch processing (S 112 ) can be considered as a combination of determining whether BIO is to be applied (S 102 ) and determining whether OBMC is to be applied (S 104 ) in the example illustrated in  FIG.  13   . For example, because BIO is to be applied when the prediction image generation mode indicates  2 , it is determined that OBMC is not applicable. In this case, OBMC is not applied, and BIO is applied. 
     Moreover, concerning branch processing (S 112 ), encoder  100  may cause entropy encoder  110  to encode a signal indicating a prediction image generation mode into a stream. For example, encoder  100  may cause inter frame predictor  126  to select a prediction image generation mode for each block, and entropy encoder  110  to encode a signal indicating the selected prediction image generation mode into a stream. 
     Decoder  200  may cause entropy decoder  202  to decode the signal indicating the prediction image generation mode from the stream. For example, decoder  200  may cause entropy decoder  202  to decode a signal indicating a prediction image generation mode for each block, and inter frame predictor  218  to select the prediction image generation mode indicated by the decoded signal. 
     Alternatively, a prediction image generation mode may be selected for each slice, may be selected for each picture, or may be selected for each sequence, and a signal indicating the selected prediction image generation mode may be encoded or decoded. 
     The three prediction image generation modes are illustrated in the above example. These prediction image generation modes are one example, and a prediction image generation mode other than those may be used. Further, the three numbers corresponding to the three prediction image generation modes are one example, and a number other than those may be used. An additional number may be assigned to a prediction image generation mode other than those. 
       FIG.  15    is a flow chart illustrating the second specific example of inter frame prediction performed by inter frame predictor  126  of encoder  100 . As previously described, inter frame predictor  218  of decoder  200  operates in the same manner as inter frame predictor  126  of encoder  100 . 
     As illustrated in  FIG.  15   , inter frame predictor  126  performs processing for each block. As with the example illustrated in  FIG.  13   , inter frame predictor  126  derives a motion vector of a current block (S 201 ). 
     Next, inter frame predictor  126  determines whether bi-prediction that refers to two processed pictures is to be applied to generation of a prediction image of the current block (S 202 ). 
     When inter frame predictor  126  determines that bi-prediction is to be applied to the generation of the prediction image of the current block (APPLY in S 202 ), inter frame predictor  126  generates the prediction image of the current block by bi-prediction (S 206 ). In this case, inter frame predictor  126  does not correct the prediction image of the current block by OBMC. The prediction image generated by bi-prediction without OBMC being applied is used as the final prediction image of the current block. 
     In contrast, when inter frame predictor  126  determines that bi-prediction is not to be applied to the generation of the prediction image of the current block (NOT APPLY in S 202 ), inter frame predictor  126  generates the prediction image of the current block by a normal method (S 203 ). In this example, generating the prediction image of the current block by the normal method means generating the prediction image of the current block without applying bi-prediction. For example, inter frame predictor  126  generates a prediction image from one processed picture by performing normal motion compensation. 
     Next, as with the example illustrated in  FIG.  13   , inter frame predictor  126  determines whether OBMC is to be applied to the generation of the prediction image of the current block (S 204 ). 
     When inter frame predictor  126  determines that OBMC is to be applied to the generation of the prediction image of the current block (APPLY in S 204 ), inter frame predictor  126  corrects the prediction image of the current block by OBMC (S 205 ). The prediction image corrected by OBMC is used as the final prediction image of the current block. 
     In contrast, when inter frame predictor  126  determines that OBMC is not to be applied to the generation of the prediction image of the current block (NOT APPLY in S 204 ), inter frame predictor  126  does not correct the prediction image of the current block by OBMC. The prediction image generated by the normal method without OBMC being applied is used as the final prediction image of the current block. 
     The above-described operation makes it possible to remove a path that applies both bi-prediction and OBMC to the generation of the prediction image of the current block. In other words, by OBMC, inter frame predictor  126  corrects a prediction image generated from one processed picture, and does not correct prediction images generated from two processed pictures. With this, it is possible to reduce an amount of processing in a path in which OBMC is applied. Accordingly, it is possible to reduce a circuit size. 
     For example, there are cases in which it is stipulated that BIO be applied in bi-prediction that refers to two processed pictures. In such a case, it is possible to avoid applying both BIO and OBMC at the same time by determining that OBMC is not applicable in bi-prediction that refers to two processed pictures. 
     It should be noted that determining whether bi-prediction is to be applied (S 202 ) can be considered as determining whether OBMC is applicable. In other words, in determining whether bi-prediction is to be applied (S 202 ), inter frame predictor  126  determines whether OBMC is applicable, according to whether bi-prediction is to be applied. For example, when bi-prediction is to be applied, inter frame predictor  126  determines that OBMC is not applicable, and applies bi-prediction without applying OBMC. 
     Moreover, concerning determining whether bi-prediction is to be applied (S 202 ), encoder  100  may cause entropy encoder  110  to encode a signal indicating whether bi-prediction is applied into a stream. Decoder  200  may cause entropy decoder  202  to decode the signal indicating whether bi-prediction is applied from the stream. The signal indicating whether bi-prediction is applied may be encoded and decoded for each block, may be encoded and decoded for each slice, may be encoded and decoded for each picture, and may be encoded and decoded for each sequence. 
     Likewise, concerning determining whether OBMC is to be applied (S 204 ), encoder  100  may cause entropy encoder  110  to encode a signal indicating whether OBMC is applied into a stream. Decoder  200  may cause entropy decoder  202  to decode the signal indicating whether OBMC is applied from the stream. The signal indicating whether OBMC is applied may be encoded and decoded for each block, may be encoded and decoded for each slice, may be encoded and decoded for each picture, and may be encoded and decoded for each sequence. 
       FIG.  16    is a flow chart illustrating a modification of the second specific example illustrated in  FIG.  15   . Inter frame predictor  126  of encoder  100  may perform the operations illustrated in  FIG.  16   . Likewise, inter frame predictor  218  of decoder  200  may perform the operations illustrated in  FIG.  16   . 
     As illustrated in  FIG.  16   , inter frame predictor  126  performs processing for each block. As with the example illustrated in  FIG.  15   , inter frame predictor  126  derives a motion vector of a current block (S 211 ). Inter frame predictor  126  performs branch processing according to a prediction image generation mode (S 212 ). 
     For example, when the prediction image generation mode indicates 0 (0 in S 212 ), inter frame predictor  126  generates the prediction image of the current block by a normal method (S 213 ). In this case, inter frame predictor  126  does not correct the prediction image of the current block by OBMC. The prediction image generated by the normal method without OBMC being applied is used as the final prediction image of the current block. 
     In this example, generating the prediction image of the current block by the normal method means generating the prediction image of the current block without applying bi-prediction that refers to two processed pictures. For example, inter frame predictor  126  generates a prediction image from one processed picture by performing normal motion compensation. 
     Moreover, for example, when the prediction image generation mode indicates 1 (1 in S 212 ), inter frame predictor  126  generates the prediction image of the current block by the normal method (S 214 ) in the same manner as when the prediction image generation mode indicates 0. When the prediction image generation mode indicates 1 (1 in S 212 ), inter frame predictor  126  further corrects the prediction image of the current block by OBMC (S 215 ). The prediction image corrected by OBMC is used as the final prediction image of the current block. 
     Moreover, for example, when the prediction image generation mode indicates  2  ( 2  in S 212 ), inter frame predictor  126  generates the prediction image of the current block by bi-prediction (S 216 ). In this case, inter frame predictor  126  does not correct the prediction image of the current block by OBMC. The prediction image generated by bi-prediction without OBMC being applied is used as the final prediction image of the current block. 
     As with the example illustrated in  FIG.  15   , inter frame predictor  126  can generate the prediction image efficiently by the above-described operations. In other words, it is possible to produce the same effects as the example illustrated in  FIG.  15   . 
     It should be noted that branch processing (S 212 ) can be considered as a combination of determining whether bi-prediction is to be applied (S 202 ) and determining whether OBMC is to be applied (S 204 ) in the example illustrated in  FIG.  15   . For example, because bi-prediction is to be applied when the prediction image generation mode indicates  2 , it is determined that OBMC is not applicable. In this case, OBMC is not applied, and bi-prediction is applied. 
     Moreover, concerning branch processing (S 212 ), encoder  100  may cause entropy encoder  110  to encode a signal indicating a prediction image generation mode into a stream. For example, encoder  100  may cause inter frame predictor  126  to select a prediction image generation mode for each block, and entropy encoder  110  to encode a signal indicating the selected prediction image generation mode into a stream. 
     Decoder  200  may cause entropy decoder  202  to decode the signal indicating the prediction image generation mode from the stream. For example, decoder  200  may cause entropy decoder  202  to decode the signal indicating the prediction image generation mode for each block, and inter frame predictor  218  to select the prediction image generation mode indicated by the decoded signal. 
     Alternatively, a prediction image generation mode may be selected for each slice, may be selected for each picture, or may be selected for each sequence, and a signal indicating the selected prediction image generation mode may be encoded or decoded. 
     The three prediction image generation modes are illustrated in the above example. These prediction image generation modes are one example, and a prediction image generation mode other than those may be used. Further, the three numbers corresponding to the three prediction image generation modes are one example, and a number other than those may be used. An additional number may be assigned to a prediction image generation mode other than those. 
     In the description of  FIG.  15    and  FIG.  16   , bi-prediction is defined as prediction that refers to two processed pictures. However, bi-prediction may be defined as prediction that refers to a processed picture preceding a current picture in display order, and a processed picture following the current picture in display order. 
     Normal prediction may include prediction that refers to two current pictures preceding a current picture in display order, and prediction that refers to two current pictures following a current picture in display order. 
     For example, there are cases in which it is stipulated that BIO be applied in bi-prediction that refers to a processed picture preceding in display order, and a processed picture following in display order. In such a case, it is possible to avoid applying both BIO and OBMC by determining that OBMC is not applicable in bi-prediction that refers to the processed picture preceding in display order, and the processed picture following in display order. 
     [OBMC] 
       FIG.  17    is a conceptual diagram illustrating OBMC in encoder  100  and decoder  200 . In OBMC, a prediction image of a current block is corrected using an image obtained by performing motion compensation of the current block using a motion vector of a block surrounding the current block. 
     For example, before OBMC is applied, a prediction image (Pred) based on a motion vector (MV) of a current block is generated by performing motion compensation of the current block using the motion vector (MV) of the current block. Subsequently, OBMC is applied. 
     Specifically, an image (Pred_L) based on a motion vector (MV_L) of a neighboring left block, which is a processed block neighboring to the left of the current block, is generated by performing motion compensation of the current block using the motion vector (MV_L) of the neighboring left block. Further, an image (Pred_U) based on a motion vector (MV_U) of a neighboring upper block, which is a processed block neighboring above the current block, is generated by performing motion compensation of the current block using the motion vector (MV_U) of the neighboring upper block. 
     Then, the prediction image (Pred) based on the motion vector (MV) of the current block is corrected with the image (Pred_L) based on the motion vector (MV_L) of the neighboring left block and the image (Pred_U) based on the motion vector (MV_U) of the neighboring upper block. With this, a prediction image to which OBMC is applied is derived. 
       FIG.  18    is a flow chart illustrating operations performed as OBMC by inter frame predictor  126  of encoder  100 . Inter frame predictor  218  of decoder  200  operates in the same manner as inter frame predictor  126  of encoder  100 . 
     For example, inter frame predictor  126  obtains a prediction image (Pred) based on a motion vector (MV) of a current block by performing motion compensation of the current block using the motion vector (MV) of the current block prior to OBMC. In OBMC, inter frame predictor  126  obtains a motion vector (MV_-L) of a neighboring left block (S 301 ). 
     Then, inter frame predictor  126  performs motion compensation of the current block using the motion vector (MV_L) of the neighboring left block. With this, inter frame predictor  126  obtains an image (Pred_L) based on the motion vector (MV_L) of the neighboring left block (S 302 ). For example, inter frame predictor  126  obtains the image (Pred_L) based on the motion vector (MV_L) of the neighboring left block by reference to a reference block specified from the current block by the motion vector (MV_L) of the neighboring left block. 
     Then, inter frame predictor  126  assigns weights to the prediction image (Pred) based on the motion vector (MV) of the current block and the image (Pred_L) based on the motion vector (MV_L) of the neighboring left block, and superimposes these images. With this, inter frame predictor  126  performs the first correction of the prediction image (S 303 ). 
     Next, inter frame predictor  126  performs the same processing on a neighboring upper block as the neighboring left block. Specifically, inter frame predictor  126  obtains a motion vector (MV_U) of the neighboring upper block (S 304 ). 
     Then, inter frame predictor  126  performs motion compensation of the current block using the motion vector (MV_U) of the neighboring upper block. With this, inter frame predictor  126  obtains an image (Pred_U) based on the motion vector (MV_U) of the neighboring upper block (S 305 ). For example, inter frame predictor  126  obtains the image (Pred_U) based on the motion vector (MV_U) of the neighboring upper block by reference to a reference block specified from the current block by the motion vector (MV_U) of the neighboring upper block. 
     Then, inter frame predictor  126  assigns weights to the prediction image of which the first correction is performed and the image (Pred_U) based on the motion vector (MV_U) of the neighboring upper block, and superimposes these images. With this, inter frame predictor  126  performs the second correction of the prediction image (S 306 ). Accordingly, a prediction image to which OBMC is applied is derived. 
     It should be noted that the two-stage correction is performed using the neighboring left block and the neighboring upper block in the above description. However, a neighboring right block or a neighboring lower block may be used. In addition, a three or more-stage correction may be performed. Alternatively, a one-stage correction may be performed using any of those blocks. Alternatively, a one-stage correction may be performed using blocks simultaneously without going through stages. 
     For example, the prediction image (Pred) based on the motion vector (MV) of the current block, the image (Pred_L) based on the motion vector (MV_L) of the neighboring left block, and the image (Pred_U) based on the motion vector (MV_U) of the neighboring upper block may be superimposed. At that time, weights may be assigned to these images. 
     Although OBMC is applied in prediction that refers to one processed picture in the above description, OBMC can be also similarly applied in prediction that refers to two processed pictures. For example, a prediction image obtained from two processed pictures is corrected by OBMC being applied in prediction that refers to the two processed pictures. 
     The current block in the above description may be a block referred to as a prediction block, may be a unit of image data to be encoded and decoded, and may be a unit of image data to be reconstructed. Alternatively, the block in the above description may be a unit of finer image data, and may be a unit of image data called a sub-block. 
     Encoder  100  and decoder  200  can apply common OBMC. In other words, encoder  100  and decoder  200  can apply OBMC in the same manner. 
     [BIO] 
       FIG.  19    is a conceptual diagram illustrating BIO in encoder  100  and decoder  200 . In BIO, a prediction image of a current block is generated by reference to a spatial gradient of luminance in an image obtained by performing motion compensation of the current block using a motion vector of the current block. 
     An L0 motion vector (MV_L0) and an L1 motion vector (MV_L1) that are two motion vectors of the current block are derived prior to BIO. The L0 motion vector (MV_L0) is a motion vector for referring to an L0 reference picture that is a processed picture, and the L1 motion vector (MV_L1) is a motion vector for referring to an L1 reference picture that is a processed picture. The L0 reference picture and the L1 reference picture are two reference pictures simultaneously referred to in bi-prediction of the current block. 
     A normal inter frame prediction mode, a merge mode, a FRUC mode, etc. may be used as a method for deriving the L0 motion vector (MV_L0) and the L1 motion vector (MV_L1). For example, in the normal inter frame prediction mode, encoder  100  derives a motion vector by performing motion estimation using an image of a current block, and encodes information of the motion vector. Further, in the normal inter frame prediction mode, decoder  200  derives the motion vector by decoding the information of the motion vector. 
     Then, in BIO, an L0 prediction image is obtained by referring to the L0 reference picture and performing motion compensation of the current block using the L0 motion vector (MV_L0). For example, the L0 prediction image may be obtained by applying a motion compensation filter to an image of an L0 reference pixel range including a block specified from the current block in the L0 reference picture by the L0 motion vector (MV_L0), and the surroundings of the block. 
     Moreover, an L0 gradient image is obtained that indicates a spatial gradient of luminance in each pixel of the L0 prediction image. For example, the L0 gradient image is obtained by referring to the luminance of each pixel in the L0 reference pixel range including the block specified from the current block in the L) reference picture by the L) motion vector (MV_L0), and the surroundings of the block. 
     Moreover, an L1 prediction image is obtained by referring to the L1 reference picture and performing motion compensation of the current block using the L1 motion vector (MV_L1). For example, the L1 prediction image may be obtained by applying a motion compensation filter to an image of an L1 reference pixel range including a block specified from the current block in the L1 reference picture by the L1 motion vector (MV_L1), and the surroundings of the block. 
     Moreover, an L1 gradient image is obtained that indicates a spatial gradient of luminance in each pixel of the L1 prediction image. For example, the L1 gradient image is obtained by referring to the luminance of each pixel in the L1 reference pixel range including the block specified from the current block in the L1 reference picture by the L1 motion vector (MV_L1), and the surroundings of the block. 
     Then, a local motion estimation value is derived for each pixel of the current block. Specifically, at that time, a pixel value of a corresponding pixel position in the L0 prediction image, a gradient value of a corresponding pixel position in the L0 gradient image, a pixel value of a corresponding pixel position in the L1 prediction image, and a gradient value of a corresponding pixel position in the L1 gradient value are used. The local motion estimation value can be also referred to as a correction motion vector (correction MV). 
     Then, a pixel correction value is derived for each pixel of the current block using the gradient value of the corresponding pixel position in the L0 gradient image, the gradient value of the corresponding pixel position in the L1 gradient image, and the local motion estimation value. After that, a prediction pixel value is derived for each pixel of the current block using the pixel value of the corresponding pixel position in the L0 prediction image, the pixel value of the corresponding pixel position in the L1 prediction image, and the pixel correction value. With this, a prediction image to which BIO is applied is derived. 
     In other words, the prediction pixel value obtained using the pixel value of the corresponding pixel position in the L0 prediction image and the pixel value of the corresponding pixel position in the L1 prediction image is corrected with the pixel correction value. To further put it differently, the prediction image obtained using the L0 prediction image and the L1 prediction image is corrected using the spatial gradients of the luminance in the L0 prediction image and the L1 prediction image. 
       FIG.  20    is a flow chart illustrating operations performed as BIO by inter frame predictor  126  of encoder  100 . Inter frame predictor  218  of decoder  200  operates in the same manner as inter frame predictor  126  of encoder  100 . 
     First, inter frame predictor  126  obtains an L0 prediction image by reference to an L0 reference picture using an L0 motion vector (MV_L0) (S 401 ). Then, inter frame predictor  126  obtains an L0 gradient image by reference to the L0 reference picture using the L0 motion vector (S 402 ). 
     Similarly, inter frame predictor  126  obtains an L1 prediction image by reference to an L1 reference picture using an L1 motion vector (MV_L1) (S 401 ). Then, inter frame predictor  126  obtains an L1 gradient image by reference to the L1 reference picture using the L1 motion vector (S 402 ). 
     Next, inter frame predictor  126  derives a local motion estimation value for each pixel of a current block (S 411 ). At that time, a pixel value of a corresponding pixel position in the L0 prediction image, a gradient value of a corresponding pixel position in the L0 gradient image, a pixel value of a corresponding pixel position in the L1 prediction image, and a gradient value of a corresponding pixel position in the L1 gradient value are used. 
     Then, inter frame predictor  126  derives a pixel correction value for each pixel of the current block using the gradient value of the corresponding pixel position in the L0 gradient image, the gradient value of the corresponding pixel position in the L1 gradient image, and the local motion estimation value. After that, inter frame predictor  126  derives a prediction pixel value for each pixel of the current block using the pixel value of the corresponding pixel position in the L0 prediction image, the pixel value of the corresponding pixel position in the L1 prediction image, and the pixel correction value (S 412 ). 
     Inter frame predictor  126  generates, by the above operations, a prediction image to which BIO is applied 
     It should be noted that specifically, the following equation (3) may be used in deriving a local motion estimation value and a pixel correction value. 
     
       
         
           
             
               
                 
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                                 ] 
                               
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                           = 
                           
                             
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                             p 
                             = 
                             
                               ( 
                               
                                 
                                   
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                           &gt;&gt; 
                           1 
                         
                       
                     
                   
                   } 
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
     In equation (3), I x   0 [x, y] denotes a horizontal gradient value of a pixel position [x, y] in the L0 gradient image. I x   1 [x, y] denotes a horizontal gradient value of a pixel position [x, y] in the L1 gradient image. I y   0 [x, y] denotes a vertical gradient value of the pixel position [x, y] in the L0 gradient image. I y   1 [x, y] denotes a vertical gradient value of the pixel position [x, y] in the L1 gradient image. 
     Moreover, in equation (3), I x   0 [x, y] denotes a pixel value of a pixel position [x, y] in the L0 prediction image. I 1 [x, y] denotes a pixel value of a pixel position [x, y] in the L1 prediction image. ΔI[x, y] denotes a different between the pixel value of the pixel position [x, y] in the L0 prediction image and the pixel value of the pixel position [x, y] in the L1 prediction image. 
     Moreover, in equation (3), Ω denotes, for example, a set of pixel positions included in a region having the pixel position [x, y] at the center. w[i, j] denotes a weight coefficient for a pixel position [i, j]. The same value may be used for w[i, j]. G x [x, y], G y [x, y], G x G y [x, y], sG x G y [x, y], sG x   2 [x, y], sG y   2 [x, y], sG x dI[x, y], sG y dI[x, y], etc. are supplemental calculated values. 
     Moreover, in equation (3), u[x, y] denotes a horizontal value included in a local motion estimation value of a pixel position [x, y]. v[x, y] denotes a vertical value included in the local motion estimation value of the pixel position [x, y]. b[x, y] denotes a pixel correction value of the pixel position [x, y]. p[x, y] denotes a prediction pixel value of the pixel position [x, y]. 
     Although inter frame predictor  126  derives the local motion estimation value for each pixel in the above description, inter frame predictor  126  may derive a local motion estimation value for each sub-block that is a unit of data more coarse than the pixel and finer than the current block. 
     For example, in above equation (3), Ω may indicate a set of pixel positions included in a sub-block. In addition, sG x G y [x, y], sG x   2 [x, y], sG y   2 [x, y], sG x dI[x, y], sG y dI[x, y], u[x, y], and v[x, y] may be calculated not for each pixel but for each sub-block. 
     Encoder  100  and decoder  200  can apply common BIO. In other words, encoder  100  and decoder  200  can apply BIO in the same manner. 
     [Implementation Example of Encoder] 
       FIG.  21    is a block diagram illustrating an implementation example of encoder  100  according to Embodiment 1. Encoder  100  includes circuitry  160  and memory  162 . For example, the plurality of constituent elements of encoder  100  illustrated in  FIG.  1    and  FIG.  11    are implemented by circuitry  160  and memory  162  illustrated in  FIG.  21   . 
     Circuitry  160  performs information processing and is accessible to memory  162 . For example, circuitry  160  is exclusive or general electronic circuitry that encodes a video. Circuitry  160  may be a processor such as a central processing unit (CPU). Circuitry  160  may be an aggregate of a plurality of electronic circuits. Moreover, for example, circuitry  160  may perform the functions of two or more constituent elements among the plurality of constituent elements of encoder  100  illustrated in  FIG.  1    etc., except the constituent elements that store information. 
     Memory  162  is an exclusive or general memory for storing information used by circuitry  160  to encode a video. Memory  162  may be an electronic circuit, may be connected to circuitry  160 , or may be included in circuitry  160 . Memory  162  may be an aggregate of a plurality of electronic circuits. Memory  162  may be a magnetic disc, an optical disc, or the like, or may be expressed as storage, a recording medium, or the like. Memory  162  may be a non-volatile memory or a volatile memory. 
     For example, memory  162  may store a video to be encoded, or may store a bitstream corresponding to an encoded video. Memory  162  may store a program for causing circuitry  160  to encode a video. 
     Moreover, for example, memory  162  may perform the functions of, among the plurality of constituent elements of encoder  100  illustrated in  FIG.  1    etc., the constituent elements that store information. Specifically, memory  162  may perform the functions of block memory  118  and frame memory  122  illustrated in  FIG.  1   . More specifically, memory  162  may store a reconstructed block (processed block), a reconstructed picture (processed picture), etc. 
     It should be noted that not all of the plurality of constituent elements illustrated in  FIG.  1    etc. need to be implemented by encoder  100 , and not all of the processes described above need to be performed by encoder  100 . Some of the constituent elements illustrated in  FIG.  1    etc. may be included in another device, and some of the processes described above may be performed by another device. Encoder  100  performs prediction efficiently by implementing some of the constituent elements illustrated in  FIG.  1    etc. and performing some of the processes described above. 
       FIG.  22    is a flow chart illustrating the first example of operations performed by encoder  100  illustrated in  FIG.  21   . For example, encoder  100  illustrated in  FIG.  21    may perform the operations illustrated in  FIG.  22    when encoding a video using inter picture prediction. 
     Specifically, circuitry  160  of encoder  100  determines whether OBMC is applicable to generation of a prediction image of a current block, according to whether BIO is to be applied to the generation of the prediction image of the current block (S 501 ). Here, the current block is a block included in a current picture in a video. 
     BIO generates a prediction image of a current block by reference to a spatial gradient of luminance in an image obtained by performing motion compensation of the current block using a motion vector of the current block. Further, OBMC corrects a prediction image of a current block using an image obtained by performing motion compensation of the current block using a motion vector of a block neighboring the current block. 
     When BIO is to be applied to the generation of the prediction image of the current block, circuitry  160  determines that OBMC is not applicable to the generation of the prediction image of the current block. Next, circuitry  160  applies BIO to the generation of the prediction image of the current block without applying OBMC (S 502 ). 
     With this, it is possible to remove a path that applies both BIO and OBMC in the procedure of inter picture prediction. Accordingly, it is possible to reduce an amount of processing in the path with a large amount of processing in the procedure of inter picture prediction. 
     Moreover, it is assumed that a degree of reduction in coding amount by applying OBMC is low when BIO is applied, compared to a case in which BIO is not applied. In other words, when it is assumed that the degree of reduction in coding amount by applying OBMC is low, encoder  100  can generate the prediction image efficiently without applying OBMC. 
     As a result, encoder  100  can perform prediction efficiently. In addition, a circuit size can be reduced. 
     It should be noted that when affine transformation is to be applied to prediction of a current block, circuitry  160  may further determine that OBMC is not applicable to the prediction of the current block. 
       FIG.  23    is a flow chart illustrating a modification of the first example of the operations illustrated in  FIG.  22   . For example, encoder  100  illustrated in  FIG.  21    may perform the operations illustrated in  FIG.  23    when encoding a video using inter picture prediction. 
     Specifically, circuitry  160  of encoder  100  selects one mode from among a first mode, a second mode, and a third mode (S 511 ). Here, the first mode applies neither BIO nor OBMC to the generation of the prediction image of the current block. The second mode applies OBMC to the generation of the prediction image of the current block without applying BIO. The third mode applies BIO to the generation of the prediction image of the current block without applying OBMC. 
     Next, circuitry  160  generates the prediction image of the current block in the one mode selected (S 512 ). 
     The selecting (S 511 ) in  FIG.  23    can be considered as including the determining (S 501 ) in  FIG.  22   . For example, in the third mode, circuitry  160  determines that OBMC is not applicable to the generation of the prediction image of the current block, in accordance with application of BIO to the generation of the prediction image of the current block. Then, circuitry  160  applies BIO to the generation of the prediction image of the current block without applying OBMC. 
     In other words, as with the example illustrated in  FIG.  22   , in the example illustrated in  FIG.  23   , when BIO is to be applied to the generation of the prediction image of the current block, circuitry  160  determines that OBMC is not applicable to the generation of the prediction image of the current block. Then, circuitry  160  applies BIO to the generation of the prediction image of the current block without applying OBMC. To put it differently, the example illustrated in  FIG.  23    includes the example illustrated in  FIG.  22   . 
     The example illustrated in  FIG.  23    produces the same effects as the example illustrated in  FIG.  22   . Furthermore, encoder  100  can generate, by the operations illustrated in  FIG.  23   , the prediction image in the one mode selected adaptively from among the three modes. 
       FIG.  24    is a flow chart illustrating an additional operation for the modification of the first example of the operations illustrated in  FIG.  23   . For example, encoder  100  illustrated in  FIG.  21    may perform the operation illustrated in  FIG.  24    in addition to the operations illustrated in  FIG.  23   . 
     Specifically, circuitry  160  of encoder  100  encodes a signal indicating, among a first value, a second value, and a third value respectively corresponding to the first mode, the second mode, and the third mode, one value corresponding to one mode (S 521 ). The one mode is selected from among the first mode, the second mode, and the third mode. 
     With this, encoder  100  can encode the one mode selected adaptively from among the three modes. Accordingly, encoder  100  and decoder  200  can select the same mode adaptively from among the three modes. 
     It should be noted that the encoding (S 521 ) may be performed before the selecting (S 511 ), may be performed between the selecting (S 511 ) and the generating (S 512 ), and may be performed after the generating (S 512 ). In addition, the encoding (S 521 ) may be performed concurrently with the selecting (S 511 ) or the generating (S 512 ). 
     The selecting and encoding of a mode may be performed for each block, may be performed for each slice, may be performed for each picture, and may be performed for each sequence. At least one of the first mode, the second mode, and the third mode may include sub-modes. 
       FIG.  25    is a flow chart illustrating the second example of operations performed by encoder  100  illustrated in  FIG.  21   . For example, encoder  100  illustrated in  FIG.  21    may perform the operations illustrated in  FIG.  25    when encoding a video using inter picture prediction. 
     Specifically, circuitry  160  of encoder  100  determines whether OBMC is applicable to generation of a prediction image of a current block, according to whether bi-prediction that refers to two processed pictures is to be applied to the generation of the prediction image of the current block (S 601 ). 
     Here, the current block is a block included in a current picture in a video. Further, OBMC corrects a prediction image of a current block using an image obtained by performing motion compensation of the current block using a motion vector of a block surrounding the current block. 
     When bi-prediction is to be applied to the generation of the prediction image of the current block, circuitry  160  determines that OBMC is not applicable to the generation of the prediction image of the current block. Next, circuitry  160  applies bi-prediction to the generation of the prediction image of the current block without applying OBMC (S 602 ). 
     With this, it is possible to remove a path that applies both bi-prediction, which refers to the two processed pictures, and OBMC in the procedure of inter picture prediction. Accordingly, it is possible to reduce an amount of processing in the path with a large amount of processing in the procedure of inter picture prediction. 
     When bi-prediction, which refers to the two processed pictures, and OBMC are applied, an amount of processing is considerably increased by a prediction image obtained from each of the two processed pictures being corrected. Encoder  100  can suppress such an increase in the amount of processing. 
     As a result, encoder  100  can perform prediction efficiently. In addition, a circuit size can be reduced. 
     It should be noted that when affine transformation is to be applied to prediction of a current block, circuitry  160  may further determine that OBMC is not applicable to the prediction of the current block. 
       FIG.  26    is a flow chart illustrating a modification of the second example of the operations illustrated in  FIG.  25   . For example, encoder  100  illustrated in  FIG.  21    may perform the operations illustrated in  FIG.  26    when encoding a video using inter picture prediction. 
     Specifically, circuitry  160  of encoder  100  selects one mode from among a first mode, a second mode, and a third mode (S 611 ). Here, the first mode applies neither bi-prediction nor OBMC to the generation of the prediction image of the current block. The second mode applies OBMC to the generation of the prediction image of the current block without applying bi-prediction. The third mode applies bi-prediction to the generation of the prediction image of the current block without applying OBMC. 
     Next, circuitry  160  generates the prediction image of the current block in the one mode selected (S 612 ). 
     The selecting (S 611 ) in  FIG.  26    can be considered as including the determining (S 601 ) in  FIG.  25   . For example, in the third mode, circuitry  160  determines that OBMC is not applicable to the generation of the prediction image of the current block, in accordance with application of bi-prediction to the generation of the prediction image of the current block. Then, circuitry  160  applies bi-prediction to the generation of the prediction image of the current block without applying OBMC. 
     In other words, as with the example illustrated in  FIG.  25   , in the example illustrated in  FIG.  26   , when bi-prediction is to be applied to the generation of the prediction image of the current block, circuitry  160  determines that OBMC is not applicable to the generation of the prediction image of the current block. Then, circuitry  160  applies bi-prediction to the generation of the prediction image of the current block without applying OBMC. To put it differently, the example illustrated in  FIG.  26    includes the example illustrated in  FIG.  25   . 
     The example illustrated in  FIG.  26    produces the same effects as the example illustrated in  FIG.  25   . Furthermore, encoder  100  can generate, by the operations illustrated in  FIG.  26   , the prediction image in the one mode selected adaptively from among the three modes. 
       FIG.  27    is a flow chart illustrating an additional operation for the modification of the second example of the operations illustrated in  FIG.  26   . For example, encoder  100  illustrated in  FIG.  21    may perform the operation illustrated in  FIG.  27    in addition to the operations illustrated in  FIG.  26   . 
     Specifically, circuitry  160  of encoder  100  encodes a signal indicating, among a first value, a second value, and a third value respectively corresponding to the first mode, the second mode, and the third mode, one value corresponding to one mode (S 621 ). The one mode is selected from among the first mode, the second mode, and the third mode. 
     With this, encoder  100  can encode the one mode selected adaptively from among the three modes. Accordingly, encoder  100  and decoder  200  can select the same mode adaptively from among the three modes. 
     It should be noted that the encoding (S 621 ) may be performed before the selecting (S 611 ), may be performed between the selecting (S 611 ) and the generating (S 612 ), and may be performed after the generating (S 612 ). In addition, the encoding (S 621 ) may be performed concurrently with the selecting (S 611 ) or the generating (S 612 ). 
     The selecting and encoding of a mode may be performed for each block, may be performed for each slice, may be performed for each picture, and may be performed for each sequence. At least one of the first mode, the second mode, and the third mode may include sub-modes. 
     In the examples illustrated in  FIG.  25   ,  FIG.  26   , and  FIG.  27   , bi-prediction refers to two processed pictures. In these examples, however, bi-prediction may be limited to refer to a processed picture preceding a current picture in display order, and a processed picture following the current picture in display order. 
     [Implementation Example of Decoder] 
       FIG.  28    is a block diagram illustrating an implementation example of decoder  200  according to Embodiment 1. Decoder  200  includes circuitry  260  and memory  262 . For example, the plurality of constituent elements of decoder  200  illustrated in  FIG.  10    and  FIG.  12    are implemented by circuitry  260  and memory  262  illustrated in  FIG.  28   . 
     Circuitry  260  performs information processing and is accessible to memory  262 . For example, circuitry  260  is exclusive or general electronic circuitry that decodes a video. Circuitry  260  may be a processor such as a central processing unit (CPU). Circuitry  260  may be an aggregate of a plurality of electronic circuits. Moreover, for example, circuitry  260  may perform the functions of two or more constituent elements among the plurality of constituent elements of decoder  200  illustrated in  FIG.  10    etc., except the constituent elements that store information. 
     Memory  262  is an exclusive or general memory for storing information used by circuitry  260  to decode a video. Memory  262  may be an electronic circuit, may be connected to circuitry  260 , or may be included in circuitry  260 . Memory  262  may be an aggregate of a plurality of electronic circuits. Memory  262  may be a magnetic disc, an optical disc, or the like, or may be expressed as storage, a recording medium, or the like. Memory  262  may be a non-volatile memory or a volatile memory. 
     For example, memory  262  may store a bitstream corresponding to an encoded video, or may store a video corresponding to a decoded bitstream. Memory  262  may store a program for causing circuitry  260  to decode a video. 
     Moreover, for example, circuitry  262  may perform the functions of, among the plurality of constituent elements of decoder  200  illustrated in  FIG.  10    etc., the constituent elements that store information. Specifically, memory  262  may perform the functions of block memory  210  and frame memory  214  illustrated in  FIG.  10   . More specifically, memory  262  may store a reconstructed block (processed block), a reconstructed picture (processed picture), etc. 
     It should be noted that not all of the plurality of constituent elements illustrated in  FIG.  10    etc. need to be implemented by decoder  200 , and not all of the processes described above need to be performed by decoder  200 . Some of the constituent elements illustrated in  FIG.  10    etc. may be included in another device, and some of the processes described above may be performed by another device. Decoder  200  performs prediction efficiently by implementing some of the constituent elements illustrated in  FIG.  10    etc. and performing some of the processes described above. 
       FIG.  29    is a flow chart illustrating the first example of operations performed by decoder  200  illustrated in  FIG.  28   . For example, decoder  200  illustrated in  FIG.  28    may perform the operations illustrated in  FIG.  29    when decoding a video using inter picture prediction. 
     Specifically, circuitry  260  of decoder  200  determines whether OBMC is applicable to generation of a prediction image of a current block, according to whether BIO is to be applied to the generation of the prediction image of the current block (S 701 ). Here, the current block is a block included in a current picture in a video. 
     BIO generates a prediction image of a current block by reference to a spatial gradient of luminance in an image obtained by performing motion compensation of the current block using a motion vector of the current block. Further, OBMC corrects a prediction image of a current block using an image obtained by performing motion compensation of the current block using a motion vector of a block neighboring the current block. 
     When BIO is to be applied to the generation of the prediction image of the current block, circuitry  260  determines that OBMC is not applicable to the generation of the prediction image of the current block. Next, circuitry  260  applies BIO to the generation of the prediction image of the current block without applying OBMC (S 702 ). 
     With this, it is possible to remove a path that applies both BIO and OBMC in the procedure of inter picture prediction. Accordingly, it is possible to reduce an amount of processing in the path with a large amount of processing in the procedure of inter picture prediction. 
     Moreover, it is assumed that a degree of reduction in coding amount by applying OBMC is low when BIO is applied, compared to a case in which BIO is not applied. In other words, when it is assumed that the degree of reduction in coding amount by applying OBMC is low, decoder  200  can generate the prediction image efficiently without applying OBMC. 
     As a result, decoder  200  can perform prediction efficiently. In addition, a circuit size can be reduced. 
     It should be noted that when affine transformation is to be applied to prediction of a current block, circuitry  260  may further determine that OBMC is not applicable to the prediction of the current block. 
       FIG.  30    is a flow chart illustrating a modification of the first example of the operations illustrated in  FIG.  29   . For example, decoder  200  illustrated in  FIG.  28    may perform the operations illustrated in  FIG.  30    when decoding a video using inter picture prediction. 
     Specifically, circuitry  260  of decoder  200  selects one mode from among a first mode, a second mode, and a third mode (S 711 ). Here, the first mode applies neither BIO nor OBMC to the generation of the prediction image of the current block. The second mode applies OBMC to the generation of the prediction image of the current block without applying BIO. The third mode applies BIO to the generation of the prediction image of the current block without applying OBMC. 
     Next, circuitry  260  generates the prediction image of the current block in the one mode selected (S 712 ). 
     The selecting (S 711 ) in  FIG.  30    can be considered as including the determining (S 701 ) in  FIG.  29   . For example, in the third mode, circuitry  260  determines that OBMC is not applicable to the generation of the prediction image of the current block, in accordance with application of BIO to the generation of the prediction image of the current block. Then, circuitry  260  applies BIO to the generation of the prediction image of the current block without applying OBMC. 
     In other words, as with the example illustrated in  FIG.  29   , in the example illustrated in  FIG.  30   , when BIO is to be applied to the generation of the prediction image of the current block, circuitry  260  determines that OBMC is not applicable to the generation of the prediction image of the current block. Then, circuitry  260  applies BIO to the generation of the prediction image of the current block without applying OBMC. To put it differently, the example illustrated in  FIG.  30    includes the example illustrated in  FIG.  29   . 
     The example illustrated in  FIG.  30    produces the same effects as the example illustrated in  FIG.  29   . Furthermore, decoder  200  can generate, by the operations illustrated in  FIG.  30   , the prediction image in the one mode selected adaptively from among the three modes. 
       FIG.  31    is a flow chart illustrating an additional operation for the modification of the first example of the operations illustrated in  FIG.  30   . For example, decoder  200  illustrated in  FIG.  28    may perform the operation illustrated in  FIG.  31    in addition to the operations illustrated in  FIG.  30   . 
     Specifically, circuitry  260  of decoder  200  decodes a signal indicating, among a first value, a second value, and a third value respectively corresponding to the first mode, the second mode, and the third mode, one value corresponding to one mode (S 721 ). The one mode is selected from among the first mode, the second mode, and the third mode. 
     With this, decoder  200  can decode the one mode selected adaptively from among the three modes. Accordingly, encoder  100  and decoder  200  can select the same mode adaptively from among the three modes. 
     It should be noted that the decoding (S 721 ) is basically performed before the selecting (S 711 ). Specifically, circuitry  260  decodes a signal indicating one value corresponding to one mode, and selects the one mode corresponding to the one value indicated by the decoded signal. 
     The decoding and selecting of a mode may be performed for each block, may be performed for each slice, may be performed for each picture, and may be performed for each sequence. At least one of the first mode, the second mode, and the third mode may include sub-modes. 
       FIG.  32    is a flow chart illustrating the second example of operations performed by decoder  200  illustrated in  FIG.  28   . For example, decoder  200  illustrated in  FIG.  28    may perform the operations illustrated in  FIG.  32    when decoding a video using inter picture prediction. 
     Specifically, circuitry  260  of decoder  200  determines whether OBMC is applicable to generation of a prediction image of a current block, according to whether bi-prediction that refers to two processed pictures is to be applied to the generation of the prediction image of the current block (S 801 ). 
     Here, the current block is a block included in a current picture in a video. Further, OBMC corrects a prediction image of a current block using an image obtained by performing motion compensation of the current block using a motion vector of a block neighboring the current block. 
     When bi-prediction is to be applied to the generation of the prediction image of the current block, circuitry  260  determines that OBMC is not applicable to the generation of the prediction image of the current block. Next, circuitry  260  applies bi-prediction to the generation of the prediction image of the current block without applying OBMC (S 802 ). 
     With this, it is possible to remove a path that applies both bi-prediction, which refers to the two processed pictures, and OBMC in the procedure of inter picture prediction. Accordingly, it is possible to reduce an amount of processing in the path with a large amount of processing in the procedure of inter picture prediction. 
     When bi-prediction, which refers to the two processed pictures, and OBMC are applied, an amount of processing is considerably increased by a prediction image obtained from each of the two processed pictures being corrected. Decoder  200  can suppress such an increase in the amount of processing. 
     As a result, decoder  200  can perform prediction efficiently. In addition, a circuit size can be reduced. 
     It should be noted that when affine transformation is to be applied to prediction of a current block, circuitry  260  may further determine that OBMC is not applicable to the prediction of the current block. 
       FIG.  33    is a flow chart illustrating a modification of the second example of the operations illustrated in  FIG.  32   . For example, decoder  200  illustrated in  FIG.  28    may perform the operations illustrated in  FIG.  33    when decoding a video using inter picture prediction. 
     Specifically, circuitry  260  of decoder  200  selects one mode from among a first mode, a second mode, and a third mode (S 811 ). Here, the first mode applies neither bi-prediction nor OBMC to the generation of the prediction image of the current block. The second mode applies OBMC to the generation of the prediction image of the current block without applying bi-prediction. The third mode applies bi-prediction to the generation of the prediction image of the current block without applying OBMC. 
     Next, circuitry  260  generates the prediction image of the current block in the one mode selected (S 812 ). 
     The selecting (S 811 ) in  FIG.  33    can be considered as including the determining (S 801 ) in  FIG.  32   . For example, in the third mode, circuitry  260  determines that OBMC is not applicable to the generation of the prediction image of the current block, in accordance with application of bi-prediction to the generation of the prediction image of the current block. Then, circuitry  260  applies bi-prediction to the generation of the prediction image of the current block without applying OBMC. 
     In other words, as with the example illustrated in  FIG.  32   , in the example illustrated in  FIG.  33   , when bi-prediction is to be applied to the generation of the prediction image of the current block, circuitry  260  determines that OBMC is not applicable to the generation of the prediction image of the current block. Then, circuitry  260  applies bi-prediction to the generation of the prediction image of the current block without applying OBMC. To put it differently, the example illustrated in  FIG.  33    includes the example illustrated in  FIG.  32   . 
     The example illustrated in  FIG.  33    produces the same effects as the example illustrated in  FIG.  32   . Furthermore, decoder  200  can generate, by the operations illustrated in  FIG.  33   , the prediction image in the one mode selected adaptively from among the three modes. 
       FIG.  34    is a flow chart illustrating an additional operation for the modification of the second example of the operations illustrated in  FIG.  33   . For example, decoder  200  illustrated in  FIG.  28    may perform the operation illustrated in  FIG.  34    in addition to the operations illustrated in  FIG.  33   . 
     Specifically, circuitry  260  of decoder  200  decodes a signal indicating, among a first value, a second value, and a third value respectively corresponding to the first mode, the second mode, and the third mode, one value corresponding to one mode (S 821 ). The one mode is selected from among the first mode, the second mode, and the third mode. 
     With this, decoder  200  can decode the one mode selected adaptively from among the three modes. Accordingly, encoder  100  and decoder  200  can select the same mode adaptively from among the three modes. 
     It should be noted that the decoding (S 821 ) is basically performed before the selecting (S 811 ). Specifically, circuitry  260  decodes a signal indicating one value corresponding to one mode, and selects the one mode corresponding to the one value indicated by the decoded signal. 
     The decoding and selecting of a mode may be performed for each block, may be performed for each slice, may be performed for each picture, and may be performed for each sequence. At least one of the first mode, the second mode, and the third mode may include sub-modes. 
     In the examples illustrated in  FIG.  32   ,  FIG.  33   , and  FIG.  34   , bi-prediction refers to two processed pictures. In these examples, however, bi-prediction may be limited to refer to a processed picture preceding a current picture in display order, and a processed picture following the current picture in display order. 
     [Supplemental Information] 
     Encoder  100  and decoder  200  according to the present embodiment may be used respectively as an image encoder and an image decoder, or may be used respectively as a video encoder and a video decoder. Alternatively, encoder  100  and decoder  200  can be each used as an inter prediction device (inter frame prediction device). 
     In other words, encoder  100  and decoder  200  may correspond only to inter predictor (inter frame predictor)  126  and inter predictor (inter frame predictor)  218 , respectively. Other constituent elements such as transformer  106  and inverse transformer  206  may be included in another device. 
     At least part of the present embodiment may be used as an encoding method, may be used as a decoding method, may be used as an inter frame prediction method, and may be used as another method. For example, at least part of the present embodiment may be used as an inter frame prediction method as indicated below. 
     For example, the inter frame prediction method is an inter frame prediction method of generating a prediction image, the inter frame prediction method including: generating a first image using a motion vector assigned to a current block; and generating a correction image using a motion vector assigned to a block surrounding the current block, and generating a second image by correcting the first image using the correction image. The generating of the first image is performed using one prediction image generation method selected from among prediction image generation methods including at least a first prediction image generation method and a second prediction image generation method. When the first prediction image generation method is used, determining the first image as the prediction image without performing the generating of the correction image or performing the generating of the correction image and determining the second image as the prediction image is selected. When the second prediction image generation method is used, the first image is determined as the prediction image without performing the generating of the correction image. 
     Alternatively, the inter frame prediction method is an inter frame prediction method of generating a prediction image, the inter frame prediction method including: generating a first image using a motion vector assigned to a current block; and generating a correction image using a motion vector assigned to a block surrounding the current block, and generating a second image by correcting the first image using the correction image. The generating of the first image is performed using one prediction image generation method selected from among prediction image generation methods including a first prediction image generation method and a second prediction image generation method. (1) Determining the first image generated by the first prediction image generation method as the prediction image, (2) determining the second image generated by correcting the first image generated by the first prediction image generation method, or (3) determining the first image generated by the second prediction image generation method as the prediction image is selected. 
     In the present embodiment, each of the constituent elements may be configured in the form of an exclusive hardware product, or may be implemented by executing a software program suitable for the constituent element. Each of the constituent elements may be implemented by means of a program execution unit, such as a CPU or a processor, reading and executing a software program recorded on a recording medium such as a hard disk or a semiconductor memory. 
     More specifically, encoder  100  and decoder  200  may each include processing circuitry and storage electrically connected to the processing circuitry and accessible from the processing circuitry. For example, the processing circuitry corresponds to circuitry  160  or  260 , and the storage corresponds to memory  162  or  262 . 
     The processing circuitry includes at least one of an exclusive hardware product and a program execution unit, and performs processing using the storage. When the processing circuitry includes a program execution unit, the storage stores a software program executed by the program execution unit. 
     Here, the software for implementing, for example, encoder  100  or decoder  200  in the present embodiment includes a program as indicated below. 
     Specifically, the program may cause a computer to execute an encoding method of encoding a video using inter picture prediction, the encoding method including: determining whether overlapped block motion compensation (OBMC) is applicable to generation of a prediction image of a current block, according to whether bi-directional optical flow (BIO) is to be applied to the generation of the prediction image of the current block, the current block being included in a current picture in the video; and when the BIO is to be applied to the generation of the prediction image of the current block, determining that the OBMC is not applicable to the generation of the prediction image of the current block, and applying the BIO to the generation of the prediction image of the current block without applying the OBMC. The BIO generates the prediction image of the current block by reference to a spatial gradient of luminance in an image obtained by performing motion compensation of the current block using a motion vector of the current block. The OBMC corrects the prediction image of the current block using an image obtained by performing motion compensation of the current block using a motion vector of a block surrounding the current block. 
     Alternatively, the program may cause a computer to execute a decoding method of decoding a video using inter picture prediction, the decoding method including: determining whether overlapped block motion compensation (OBMC) is applicable to generation of a prediction image of a current block, according to whether bi-directional optical flow (BIO) is to be applied to the generation of the prediction image of the current block, the current block being included in a current picture in the video; and when the BIO is to be applied to the generation of the prediction image of the current block, determining that the OBMC is not applicable to the generation of the prediction image of the current block, and applying the BIO to the generation of the prediction image of the current block without applying the OBMC. The BIO generates the prediction image of the current block by reference to a spatial gradient of luminance in an image obtained by performing motion compensation of the current block using a motion vector of the current block. The OBMC corrects the prediction image of the current block using an image obtained by performing motion compensation of the current block using a motion vector of a block surrounding the current block. 
     Alternatively, the program may cause a computer to execute an encoding method of encoding a video using inter picture prediction, the encoding method including: determining whether overlapped block motion compensation (OBMC) is applicable to generation of a prediction image of a current block, according to whether bi-prediction that refers to two processed pictures is to be applied to the generation of the prediction image of the current block, the current block being included in a current picture in the video; and when the bi-prediction is to be applied to the generation of the prediction image of the current block, determining that the OBMC is not applicable to the generation of the prediction image of the current block, and applying the bi-prediction to the generation of the prediction image of the current block without applying the OBMC. The OBMC corrects the prediction image of the current block using an image obtained by performing motion compensation of the current block using a motion vector of a block surrounding the current block. 
     Alternatively, the program may cause a computer to execute a decoding method of decoding a video using inter picture prediction, the decoding method including: determining whether overlapped block motion compensation (OBMC) is applicable to generation of a prediction image of a current block, according to whether bi-prediction that refers to two processed pictures is to be applied to the generation of the prediction image of the current block, the current block being included in a current picture in the video; and when the bi-prediction is to be applied to the generation of the prediction image of the current block, determining that the OBMC is not applicable to the generation of the prediction image of the current block, and applying the bi-prediction to the generation of the prediction image of the current block without applying the OBMC. The OBMC corrects the prediction image of the current block using an image obtained by performing motion compensation of the current block using a motion vector of a block surrounding the current block. 
     Alternatively, the program may cause a computer to execute an encoding method of encoding a video using inter picture prediction, the encoding method including: determining whether overlapped block motion compensation (OBMC) is applicable to generation of a prediction image of a current block, according to whether bi-prediction is to be applied to the generation of the prediction image of the current block, the current block being included in a current picture in the video, the bi-prediction referring to a processed picture preceding the current picture in display order, and a processed picture following the current picture in display order; and when the bi-prediction is to be applied to the generation of the prediction image of the current block, determining that the OBMC is not applicable to the generation of the prediction image of the current block, and applying the bi-prediction to the generation of the prediction image of the current block without applying the OBMC. The OBMC corrects the prediction image of the current block using an image obtained by performing motion compensation of the current block using a motion vector of a block surrounding the current block. 
     Alternatively, the program may cause a computer to execute a decoding method of decoding a video using inter picture prediction, the decoding method including: determining whether overlapped block motion compensation (OBMC) is applicable to generation of a prediction image of a current block, according to whether bi-prediction is to be applied to the generation of the prediction image of the current block, the current block being included in a current picture in the video, the bi-prediction referring to a processed picture preceding the current picture in display order, and a processed picture following the current picture in display order; and when the bi-prediction is to be applied to the generation of the prediction image of the current block, determining that the OBMC is not applicable to the generation of the prediction image of the current block, and applying the bi-prediction to the generation of the prediction image of the current block without applying the OBMC. The OBMC corrects the prediction image of the current block using an image obtained by performing motion compensation of the current block using a motion vector of a block surrounding the current block. 
     The constituent elements may be circuits as described above. The circuits may constitute circuitry as a whole, or may be individual circuits. Each constituent element may be implemented by a general processor, or may be implemented by an exclusive processor. 
     Moreover, processing executed by a particular constituent element may be executed by another constituent element. The processing execution order may be modified, or a plurality of processes may be executed in parallel. Furthermore, an encoding and decoding device may include encoder  100  and decoder  200 . 
     The ordinal numbers such as “first” and “second” used in the description may be changed as appropriate. A new ordinal number may be given to the constituent elements, or the ordinal numbers of the constituent elements may be removed. 
     Although some aspects of encoder  100  and decoder  200  have been described above based on the embodiment, the aspects of encoder  100  and decoder  200  are not limited to this embodiment. Various modifications to the present embodiment that are conceivable to those skilled in the art, as well as embodiments resulting from combinations of constituent elements in different embodiments, may be included within the scope of the aspects of encoder  100  and decoder  200 , so long as they do not depart from the essence of the present disclosure. 
     The present aspect may be implemented in combination with one or more of the other aspects in the present disclosure. In addition, part of the processes in the flow charts, part of the constituent elements of the devices, and part of the syntax described in the present aspect may be implemented in combination with the other aspects. 
     Embodiment 2 
     As described in each of the above embodiments, each functional block can typically be realized as an MPU and memory, for example. Moreover, processes performed by each of the functional blocks are typically realized by a program execution unit, such as a processor, reading and executing software (a program) recorded on a recording medium such as ROM. The software may be distributed via, for example, downloading, and may be recorded on a recording medium such as semiconductor memory and distributed. Note that each functional block can, of course, also be realized as hardware (dedicated circuit). 
     Moreover, the processing described in each of the embodiments may be realized via integrated processing using a single apparatus (system), and, alternatively, may be realized via decentralized processing using a plurality of apparatuses. Moreover, the processor that executes the above-described program may be a single processor or a plurality of processors. In other words, integrated processing may be performed, and, alternatively, decentralized processing may be performed. 
     Embodiments of the present disclosure are not limited to the above exemplary embodiments; various modifications may be made to the exemplary embodiments, the results of which are also included within the scope of the embodiments of the present disclosure. 
     Next, application examples of the moving picture encoding method (image encoding method) and the moving picture decoding method (image decoding method) described in each of the above embodiments and a system that employs the same will be described. The system is characterized as including an image encoder that employs the image encoding method, an image decoder that employs the image decoding method, and an image encoder/decoder that includes both the image encoder and the image decoder. Other configurations included in the system may be modified on a case-by-case basis. 
     [Usage Examples] 
       FIG.  35    illustrates an overall configuration of content providing system ex 100  for implementing a content distribution service. The area in which the communication service is provided is divided into cells of desired sizes, and base stations ex 106 , ex 107 , ex 108 , ex 109 , and ex 110 , which are fixed wireless stations, are located in respective cells. 
     In content providing system ex 100 , devices including computer ex 111 , gaming device ex 112 , camera ex 113 , home appliance ex 114 , and smartphone ex 115  are connected to internet ex 101  via internet service provider ex 102  or communications network ex 104  and base stations ex 106  through ex 110 . Content providing system ex 100  may combine and connect any combination of the above elements. The devices may be directly or indirectly connected together via a telephone network or near field communication rather than via base stations ex 106  through ex 110 , which are fixed wireless stations. Moreover, streaming server ex 103  is connected to devices including computer ex 111 , gaming device ex 112 , camera ex 113 , home appliance ex 114 , and smartphone ex 115  via, for example, internet ex 101 . Streaming server ex 103  is also connected to, for example, a terminal in a hotspot in airplane ex 117  via satellite ex 116 . 
     Note that instead of base stations ex 106  through ex 110 , wireless access points or hotspots may be used. Streaming server ex 103  may be connected to communications network ex 104  directly instead of via internet ex 101  or internet service provider ex 102 , and may be connected to airplane ex 117  directly instead of via satellite ex 116 . 
     Camera ex 113  is a device capable of capturing still images and video, such as a digital camera. Smartphone ex 115  is a smartphone device, cellular phone, or personal handyphone system (PHS) phone that can operate under the mobile communications system standards of the typical 2G, 3G, 3.9G, and 4G systems, as well as the next-generation 5G system. 
     Home appliance ex 118  is, for example, a refrigerator or a device included in a home fuel cell cogeneration system. 
     In content providing system ex 100 , a terminal including an image and/or video capturing function is capable of, for example, live streaming by connecting to streaming server ex 103  via, for example, base station ex 106 . When live streaming, a terminal (e.g., computer ex 111 , gaming device ex 112 , camera ex 113 , home appliance ex 114 , smartphone ex 115 , or airplane ex 117 ) performs the encoding processing described in the above embodiments on still-image or video content captured by a user via the terminal, multiplexes video data obtained via the encoding and audio data obtained by encoding audio corresponding to the video, and transmits the obtained data to streaming server ex 103 . In other words, the terminal functions as the image encoder according to one aspect of the present disclosure. 
     Streaming server ex 103  streams transmitted content data to clients that request the stream. Client examples include computer ex 111 , gaming device ex 112 , camera ex 113 , home appliance ex 114 , smartphone ex 115 , and terminals inside airplane ex 117 , which are capable of decoding the above-described encoded data. Devices that receive the streamed data decode and reproduce the received data. In other words, the devices each function as the image decoder according to one aspect of the present disclosure. 
     [Decentralized Processing] 
     Streaming server ex 103  may be realized as a plurality of servers or computers between which tasks such as the processing, recording, and streaming of data are divided. For example, streaming server ex 103  may be realized as a content delivery network (CDN) that streams content via a network connecting multiple edge servers located throughout the world. In a CDN, an edge server physically near the client is dynamically assigned to the client. Content is cached and streamed to the edge server to reduce load times. In the event of, for example, some kind of an error or a change in connectivity due to, for example, a spike in traffic, it is possible to stream data stably at high speeds since it is possible to avoid affected parts of the network by, for example, dividing the processing between a plurality of edge servers or switching the streaming duties to a different edge server, and continuing streaming. 
     Decentralization is not limited to just the division of processing for streaming; the encoding of the captured data may be divided between and performed by the terminals, on the server side, or both. In one example, in typical encoding, the processing is performed in two loops. The first loop is for detecting how complicated the image is on a frame-by-frame or scene-by-scene basis, or detecting the encoding load. The second loop is for processing that maintains image quality and improves encoding efficiency. For example, it is possible to reduce the processing load of the terminals and improve the quality and encoding efficiency of the content by having the terminals perform the first loop of the encoding and having the server side that received the content perform the second loop of the encoding. In such a case, upon receipt of a decoding request, it is possible for the encoded data resulting from the first loop performed by one terminal to be received and reproduced on another terminal in approximately real time. This makes it possible to realize smooth, real-time streaming. 
     In another example, camera ex 113  or the like extracts a feature amount from an image, compresses data related to the feature amount as metadata, and transmits the compressed metadata to a server. For example, the server determines the significance of an object based on the feature amount and changes the quantization accuracy accordingly to perform compression suitable for the meaning of the image. Feature amount data is particularly effective in improving the precision and efficiency of motion vector prediction during the second compression pass performed by the server. Moreover, encoding that has a relatively low processing load, such as variable length coding (VLC), may be handled by the terminal, and encoding that has a relatively high processing load, such as context-adaptive binary arithmetic coding (CABAC), may be handled by the server. 
     In yet another example, there are instances in which a plurality of videos of approximately the same scene are captured by a plurality of terminals in, for example, a stadium, shopping mall, or factory. In such a case, for example, the encoding may be decentralized by dividing processing tasks between the plurality of terminals that captured the videos and, if necessary, other terminals that did not capture the videos and the server, on a per-unit basis. The units may be, for example, groups of pictures (GOP), pictures, or tiles resulting from dividing a picture. This makes it possible to reduce load times and achieve streaming that is closer to real-time. 
     Moreover, since the videos are of approximately the same scene, management and/or instruction may be carried out by the server so that the videos captured by the terminals can be cross-referenced. Moreover, the server may receive encoded data from the terminals, change reference relationship between items of data or correct or replace pictures themselves, and then perform the encoding. This makes it possible to generate a stream with increased quality and efficiency for the individual items of data. 
     Moreover, the server may stream video data after performing transcoding to convert the encoding format of the video data. For example, the server may convert the encoding format from MPEG to VP, and may convert H.264 to H.265. 
     In this way, encoding can be performed by a terminal or one or more servers. Accordingly, although the device that performs the encoding is referred to as a “server” or “terminal” in the following description, some or all of the processes performed by the server may be performed by the terminal, and likewise some or all of the processes performed by the terminal may be performed by the server. This also applies to decoding processes. 
     [3D, Multi-Angle] 
     In recent years, usage of images or videos combined from images or videos of different scenes concurrently captured or the same scene captured from different angles by a plurality of terminals such as camera ex 113  and/or smartphone ex 115  has increased. Videos captured by the terminals are combined based on, for example, the separately-obtained relative positional relationship between the terminals, or regions in a video having matching feature points. 
     In addition to the encoding of two-dimensional moving pictures, the server may encode a still image based on scene analysis of a moving picture either automatically or at a point in time specified by the user, and transmit the encoded still image to a reception terminal. Furthermore, when the server can obtain the relative positional relationship between the video capturing terminals, in addition to two-dimensional moving pictures, the server can generate three-dimensional geometry of a scene based on video of the same scene captured from different angles. Note that the server may separately encode three-dimensional data generated from, for example, a point cloud, and may, based on a result of recognizing or tracking a person or object using three-dimensional data, select or reconstruct and generate a video to be transmitted to a reception terminal from videos captured by a plurality of terminals. 
     This allows the user to enjoy a scene by freely selecting videos corresponding to the video capturing terminals, and allows the user to enjoy the content obtained by extracting, from three-dimensional data reconstructed from a plurality of images or videos, a video from a selected viewpoint. Furthermore, similar to with video, sound may be recorded from relatively different angles, and the server may multiplex, with the video, audio from a specific angle or space in accordance with the video, and transmit the result. 
     In recent years, content that is a composite of the real world and a virtual world, such as virtual reality (VR) and augmented reality (AR) content, has also become popular. In the case of VR images, the server may create images from the viewpoints of both the left and right eyes and perform encoding that tolerates reference between the two viewpoint images, such as multi-view coding (MVC), and, alternatively, may encode the images as separate streams without referencing. When the images are decoded as separate streams, the streams may be synchronized when reproduced so as to recreate a virtual three-dimensional space in accordance with the viewpoint of the user. 
     In the case of AR images, the server superimposes virtual object information existing in a virtual space onto camera information representing a real-world space, based on a three-dimensional position or movement from the perspective of the user. The decoder may obtain or store virtual object information and three-dimensional data, generate two-dimensional images based on movement from the perspective of the user, and then generate superimposed data by seamlessly connecting the images. Alternatively, the decoder may transmit, to the server, motion from the perspective of the user in addition to a request for virtual object information, and the server may generate superimposed data based on three-dimensional data stored in the server in accordance with the received motion, and encode and stream the generated superimposed data to the decoder. Note that superimposed data includes, in addition to RGB values, an a value indicating transparency, and the server sets the a value for sections other than the object generated from three-dimensional data to, for example, 0, and may perform the encoding while those sections are transparent. Alternatively, the server may set the background to a predetermined RGB value, such as a chroma key, and generate data in which areas other than the object are set as the background. 
     Decoding of similarly streamed data may be performed by the client (i.e., the terminals), on the server side, or divided therebetween. In one example, one terminal may transmit a reception request to a server, the requested content may be received and decoded by another terminal, and a decoded signal may be transmitted to a device having a display. It is possible to reproduce high image quality data by decentralizing processing and appropriately selecting content regardless of the processing ability of the communications terminal itself. In yet another example, while a TV, for example, is receiving image data that is large in size, a region of a picture, such as a tile obtained by dividing the picture, may be decoded and displayed on a personal terminal or terminals of a viewer or viewers of the TV. This makes it possible for the viewers to share a big-picture view as well as for each viewer to check his or her assigned area or inspect a region in further detail up close. 
     In the future, both indoors and outdoors, in situations in which a plurality of wireless connections are possible over near, mid, and far distances, it is expected to be able to seamlessly receive content even when switching to data appropriate for the current connection, using a streaming system standard such as MPEG-DASH. With this, the user can switch between data in real time while freely selecting a decoder or display apparatus including not only his or her own terminal, but also, for example, displays disposed indoors or outdoors. Moreover, based on, for example, information on the position of the user, decoding can be performed while switching which terminal handles decoding and which terminal handles the displaying of content. This makes it possible to, while in route to a destination, display, on the wall of a nearby building in which a device capable of displaying content is embedded or on part of the ground, map information while on the move. Moreover, it is also possible to switch the bit rate of the received data based on the accessibility to the encoded data on a network, such as when encoded data is cached on a server quickly accessible from the reception terminal or when encoded data is copied to an edge server in a content delivery service. 
     [Scalable Encoding] 
     The switching of content will be described with reference to a scalable stream, illustrated in  FIG.  36   , that is compression coded via implementation of the moving picture encoding method described in the above embodiments. The server may have a configuration in which content is switched while making use of the temporal and/or spatial scalability of a stream, which is achieved by division into and encoding of layers, as illustrated in  FIG.  36   . Note that there may be a plurality of individual streams that are of the same content but different quality. In other words, by determining which layer to decode up to based on internal factors, such as the processing ability on the decoder side, and external factors, such as communication bandwidth, the decoder side can freely switch between low resolution content and high resolution content while decoding. For example, in a case in which the user wants to continue watching, at home on a device such as a TV connected to the internet, a video that he or she had been previously watching on smartphone ex 115  while on the move, the device can simply decode the same stream up to a different layer, which reduces server side load. 
     Furthermore, in addition to the configuration described above in which scalability is achieved as a result of the pictures being encoded per layer and the enhancement layer is above the base layer, the enhancement layer may include metadata based on, for example, statistical information on the image, and the decoder side may generate high image quality content by performing super-resolution imaging on a picture in the base layer based on the metadata. Super-resolution imaging may be improving the SN ratio while maintaining resolution and/or increasing resolution. Metadata includes information for identifying a linear or a non-linear filter coefficient used in super-resolution processing, or information identifying a parameter value in filter processing, machine learning, or least squares method used in super-resolution processing. 
     Alternatively, a configuration in which a picture is divided into, for example, tiles in accordance with the meaning of, for example, an object in the image, and on the decoder side, only a partial region is decoded by selecting a tile to decode, is also acceptable. Moreover, by storing an attribute about the object (person, car, ball, etc.) and a position of the object in the video (coordinates in identical images) as metadata, the decoder side can identify the position of a desired object based on the metadata and determine which tile or tiles include that object. For example, as illustrated in  FIG.  37   , metadata is stored using a data storage structure different from pixel data such as an SEI message in HEVC. This metadata indicates, for example, the position, size, or color of the main object. 
     Moreover, metadata may be stored in units of a plurality of pictures, such as stream, sequence, or random access units. With this, the decoder side can obtain, for example, the time at which a specific person appears in the video, and by fitting that with picture unit information, can identify a picture in which the object is present and the position of the object in the picture. 
     [Web Page Optimization] 
       FIG.  38    illustrates an example of a display screen of a web page on, for example, computer ex 111 .  FIG.  39    illustrates an example of a display screen of a web page on, for example, smartphone ex 1   l   5 . As illustrated in  FIG.  38    and  FIG.  39   , a web page may include a plurality of image links which are links to image content, and the appearance of the web page differs depending on the device used to view the web page. When a plurality of image links are viewable on the screen, until the user explicitly selects an image link, or until the image link is in the approximate center of the screen or the entire image link fits in the screen, the display apparatus (decoder) displays, as the image links, still images included in the content or I pictures, displays video such as an animated gif using a plurality of still images or I pictures, for example, or receives only the base layer and decodes and displays the video. 
     When an image link is selected by the user, the display apparatus decodes giving the highest priority to the base layer. Note that if there is information in the HTML code of the web page indicating that the content is scalable, the display apparatus may decode up to the enhancement layer. Moreover, in order to guarantee real time reproduction, before a selection is made or when the bandwidth is severely limited, the display apparatus can reduce delay between the point in time at which the leading picture is decoded and the point in time at which the decoded picture is displayed (that is, the delay between the start of the decoding of the content to the displaying of the content) by decoding and displaying only forward reference pictures (I picture, P picture, forward reference B picture). Moreover, the display apparatus may purposely ignore the reference relationship between pictures and coarsely decode all B and P pictures as forward reference pictures, and then perform normal decoding as the number of pictures received over time increases. 
     [Autonomous Driving] 
     When transmitting and receiving still image or video data such two- or three-dimensional map information for autonomous driving or assisted driving of an automobile, the reception terminal may receive, in addition to image data belonging to one or more layers, information on, for example, the weather or road construction as metadata, and associate the metadata with the image data upon decoding. Note that metadata may be assigned per layer and, alternatively, may simply be multiplexed with the image data. 
     In such a case, since the automobile, drone, airplane, etc., including the reception terminal is mobile, the reception terminal can seamlessly receive and decode while switching between base stations among base stations ex 106  through ex 110  by transmitting information indicating the position of the reception terminal upon reception request. Moreover, in accordance with the selection made by the user, the situation of the user, or the bandwidth of the connection, the reception terminal can dynamically select to what extent the metadata is received or to what extent the map information, for example, is updated. 
     With this, in content providing system ex 100 , the client can receive, decode, and reproduce, in real time, encoded information transmitted by the user. 
     [Streaming of Individual Content] 
     In content providing system ex 100 , in addition to high image quality, long content distributed by a video distribution entity, unicast or multicast streaming of low image quality, short content from an individual is also possible. Moreover, such content from individuals is likely to further increase in popularity. The server may first perform editing processing on the content before the encoding processing in order to refine the individual content. This may be achieved with, for example, the following configuration. 
     In real-time while capturing video or image content or after the content has been captured and accumulated, the server performs recognition processing based on the raw or encoded data, such as capture error processing, scene search processing, meaning analysis, and/or object detection processing. Then, based on the result of the recognition processing, the server-either when prompted or automatically-edits the content, examples of which include: correction such as focus and/or motion blur correction; removing low-priority scenes such as scenes that are low in brightness compared to other pictures or out of focus; object edge adjustment; and color tone adjustment. The server encodes the edited data based on the result of the editing. It is known that excessively long videos tend to receive fewer views. Accordingly, in order to keep the content within a specific length that scales with the length of the original video, the server may, in addition to the low-priority scenes described above, automatically clip out scenes with low movement based on an image processing result. Alternatively, the server may generate and encode a video digest based on a result of an analysis of the meaning of a scene. 
     Note that there are instances in which individual content may include content that infringes a copyright, moral right, portrait rights, etc. Such an instance may lead to an unfavorable situation for the creator, such as when content is shared beyond the scope intended by the creator. Accordingly, before encoding, the server may, for example, edit images so as to blur faces of people in the periphery of the screen or blur the inside of a house, for example. Moreover, the server may be configured to recognize the faces of people other than a registered person in images to be encoded, and when such faces appear in an image, for example, apply a mosaic filter to the face of the person. Alternatively, as pre- or post-processing for encoding, the user may specify, for copyright reasons, a region of an image including a person or a region of the background be processed, and the server may process the specified region by, for example, replacing the region with a different image or blurring the region. If the region includes a person, the person may be tracked in the moving picture, and the head region may be replaced with another image as the person moves. 
     Moreover, since there is a demand for real-time viewing of content produced by individuals, which tends to be small in data size, the decoder first receives the base layer as the highest priority and performs decoding and reproduction, although this may differ depending on bandwidth. When the content is reproduced two or more times, such as when the decoder receives the enhancement layer during decoding and reproduction of the base layer and loops the reproduction, the decoder may reproduce a high image quality video including the enhancement layer. If the stream is encoded using such scalable encoding, the video may be low quality when in an unselected state or at the start of the video, but it can offer an experience in which the image quality of the stream progressively increases in an intelligent manner. This is not limited to just scalable encoding; the same experience can be offered by configuring a single stream from a low quality stream reproduced for the first time and a second stream encoded using the first stream as a reference. 
     [Other Usage Examples] 
     The encoding and decoding may be performed by LSI ex 500 , which is typically included in each terminal. LSI ex 500  may be configured of a single chip or a plurality of chips. Software for encoding and decoding moving pictures may be integrated into some type of a recording medium (such as a CD-ROM, a flexible disk, or a hard disk) that is readable by, for example, computer ex 111 , and the encoding and decoding may be performed using the software. Furthermore, when smartphone ex 115  is equipped with a camera, the video data obtained by the camera may be transmitted. In this case, the video data is coded by LSI ex 500  included in smartphone ex 115 . 
     Note that LSI ex 500  may be configured to download and activate an application. In such a case, the terminal first determines whether it is compatible with the scheme used to encode the content or whether it is capable of executing a specific service. When the terminal is not compatible with the encoding scheme of the content or when the terminal is not capable of executing a specific service, the terminal first downloads a codec or application software then obtains and reproduces the content. 
     Aside from the example of content providing system ex 100  that uses internet ex 101 , at least the moving picture encoder (image encoder) or the moving picture decoder (image decoder) described in the above embodiments may be implemented in a digital broadcasting system. The same encoding processing and decoding processing may be applied to transmit and receive broadcast radio waves superimposed with multiplexed audio and video data using, for example, a satellite, even though this is geared toward multicast whereas unicast is easier with content providing system ex 100 . 
     [Hardware Configuration] 
       FIG.  40    illustrates smartphone ex 115 .  FIG.  41    illustrates a configuration example of smartphone ex 115 . Smartphone ex 115  includes antenna ex 450  for transmitting and receiving radio waves to and from base station ex 110 , camera ex 465  capable of capturing video and still images, and display ex 458  that displays decoded data, such as video captured by camera ex 465  and video received by antenna ex 450 . Smartphone ex 115  further includes user interface ex 466  such as a touch panel, audio output unit ex 457  such as a speaker for outputting speech or other audio, audio input unit ex 456  such as a microphone for audio input, memory ex 467  capable of storing decoded data such as captured video or still images, recorded audio, received video or still images, and mail, as well as decoded data, and slot ex 464  which is an interface for SIM ex 468  for authorizing access to a network and various data. Note that external memory may be used instead of memory ex 467 . 
     Moreover, main controller ex 460  which comprehensively controls display ex 458  and user interface ex 466 , power supply circuit ex 461 , user interface input controller ex 462 , video signal processor ex 455 , camera interface ex 463 , display controller ex 459 , modulator/demodulator ex 452 , multiplexer/demultiplexer ex 453 , audio signal processor ex 454 , slot ex 464 , and memory ex 467  are connected via bus ex 470 . 
     When the user turns the power button of power supply circuit ex 461  on, smartphone ex 115  is powered on into an operable state by each component being supplied with power from a battery pack. 
     Smartphone ex 115  performs processing for, for example, calling and data transmission, based on control performed by main controller ex 460 , which includes a CPU, ROM, and RAM. When making calls, an audio signal recorded by audio input unit ex 456  is converted into a digital audio signal by audio signal processor ex 454 , and this is applied with spread spectrum processing by modulator/demodulator ex 452  and digital-analog conversion and frequency conversion processing by transmitter/receiver ex 451 , and then transmitted via antenna ex 450 . The received data is amplified, frequency converted, and analog-digital converted, inverse spread spectrum processed by modulator/demodulator ex 452 , converted into an analog audio signal by audio signal processor ex 454 , and then output from audio output unit ex 457 . In data transmission mode, text, still-image, or video data is transmitted by main controller ex 460  via user interface input controller ex 462  as a result of operation of, for example, user interface ex 466  of the main body, and similar transmission and reception processing is performed. In data transmission mode, when sending a video, still image, or video and audio, video signal processor ex 455  compression encodes, via the moving picture encoding method described in the above embodiments, a video signal stored in memory ex 467  or a video signal input from camera ex 465 , and transmits the encoded video data to multiplexer/demultiplexer ex 453 . Moreover, audio signal processor ex 454  encodes an audio signal recorded by audio input unit ex 456  while camera ex 465  is capturing, for example, a video or still image, and transmits the encoded audio data to multiplexer/demultiplexer ex 453 . Multiplexer/demultiplexer ex 453  multiplexes the encoded video data and encoded audio data using a predetermined scheme, modulates and converts the data using modulator/demodulator (modulator/demodulator circuit) ex 452  and transmitter/receiver ex 451 , and transmits the result via antenna ex 450 . 
     When video appended in an email or a chat, or a video linked from a web page, for example, is received, in order to decode the multiplexed data received via antenna ex 450 , multiplexer/demultiplexer ex 453  demultiplexes the multiplexed data to divide the multiplexed data into a bitstream of video data and a bitstream of audio data, supplies the encoded video data to video signal processor ex 455  via synchronous bus ex 470 , and supplies the encoded audio data to audio signal processor ex 454  via synchronous bus ex 470 . Video signal processor ex 455  decodes the video signal using a moving picture decoding method corresponding to the moving picture encoding method described in the above embodiments, and video or a still image included in the linked moving picture file is displayed on display ex 458  via display controller ex 459 . Moreover, audio signal processor ex 454  decodes the audio signal and outputs audio from audio output unit ex 457 . Note that since real-time streaming is becoming more and more popular, there are instances in which reproduction of the audio may be socially inappropriate depending on the user&#39;s environment. Accordingly, as an initial value, a configuration in which only video data is reproduced, i.e., the audio signal is not reproduced, is preferable. Audio may be synchronized and reproduced only when an input, such as when the user clicks video data, is received. 
     Although smartphone ex 115  was used in the above example, three implementations are conceivable: a transceiver terminal including both an encoder and a decoder; a transmitter terminal including only an encoder; and a receiver terminal including only a decoder. Further, in the description of the digital broadcasting system, an example is given in which multiplexed data obtained as a result of video data being multiplexed with, for example, audio data, is received or transmitted, but the multiplexed data may be video data multiplexed with data other than audio data, such as text data related to the video. Moreover, the video data itself rather than multiplexed data maybe received or transmitted. 
     Although main controller ex 460  including a CPU is described as controlling the encoding or decoding processes, terminals often include GPUs. Accordingly, a configuration is acceptable in which a large area is processed at once by making use of the performance ability of the GPU via memory shared by the CPU and GPU or memory including an address that is managed so as to allow common usage by the CPU and GPU. This makes it possible to shorten encoding time, maintain the real-time nature of the stream, and reduce delay. In particular, processing relating to motion estimation, deblocking filtering, sample adaptive offset (SAO), and transformation/quantization can be effectively carried out by the GPU instead of the CPU in units of, for example pictures, all at once. 
     Although only some exemplary embodiments of the present disclosure have been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the exemplary embodiments without materially departing from the novel teachings and advantages of the present disclosure. Accordingly, all such modifications are intended to be included within the scope of the present disclosure. 
     INDUSTRIAL APPLICABILITY 
     The present disclosure is applicable to, for example, television receivers, digital video recorders, car navigation systems, mobile phones, digital cameras, digital video cameras, teleconferencing systems, and electronic mirrors.