IMAGE DECODING METHOD, IMAGE CODING METHOD, IMAGE DECODING APPARATUS, IMAGE CODING APPARATUS, AND IMAGE CODING AND DECODING APPARATUS

1 . An image decoding method for decoding, on a block-by-block basis, image data included in a coded bitstream, the method comprising:

obtaining a fixed number of merging candidates each of which is a candidate set of a prediction direction, a motion vector, and a reference picture index which are to be referenced in decoding of a current block, the fixed number being greater than or equal to two;

obtaining, from the coded bitstream, an index for identifying a merging candidate among the fixed number of merging candidates, the identified merging candidate being a merging candidate to be referenced in the decoding of the current block; and

identifying the merging candidate using the obtained index, and decoding the current block using the identified merging candidate,

wherein the fixed number of merging candidates include: one or more first candidates each derived based on a prediction direction, a motion vector, and a reference picture index which have been used for decoding a neighboring block spatially or temporally neighboring the current block; and one or more second candidates having a predetermined fixed value.

2 . The image decoding method according to claim 1 ,

wherein the obtaining of a fixed number of merging candidates includes:

deriving the one or more first candidates and including the one or more first candidates in the fixed number of merging candidates;

deriving one or more third candidates and including the one or more third candidates in the fixed number of merging candidates, when a total number of the first candidates is smaller than the fixed number, the third candidates each having a picture index for a picture referable in the decoding of the current block; and

deriving the one or more second candidates and including the one or more second candidates in the fixed number of merging candidates so that a total number of the first candidates, the second candidates, and the third candidates equals the fixed number, when a total number of the first candidates and the third candidates is smaller than the fixed number.

3 . The image decoding method according to claim 2 ,

wherein in the deriving of one or more third candidates, the one or more third candidates are derived by selecting, according to a predetermined priority order, one or more candidates from among a plurality of prepared candidates different from each other.

4 . The image decoding method according to claim 1 ,

wherein the obtaining of a fixed number of merging candidates includes:

initializing the fixed number of merging candidates by setting all the fixed number of merging candidates to the second candidates;

deriving the one or more first candidates and updating part of the fixed number of merging candidates so as to include the one or more first candidates in the fixed number of merging candidates; and

deriving one or more third candidates and updating part of the fixed number of merging candidates so as to include the one or more third candidates in the fixed number of merging candidates, when a total number of the first candidates is smaller than the fixed number, the third candidates each having a picture index for a picture referable in the decoding of the current block.

5 . An image coding method for coding an image on a block-by-block basis to generate a coded bitstream, the method comprising:

obtaining a fixed number of merging candidates each of which is a candidate set of a prediction direction, a motion vector, and a reference picture index which are to be referenced in coding of a current block, the fixed number being greater than or equal to two; and

attaching, to the coded bitstream, an index for identifying a merging candidate among the fixed number of merging candidates, the identified merging candidate being a merging candidate to be referenced in the coding of the current block,

wherein the fixed number of merging candidates include: one or more first candidates each derived based on a prediction direction, a motion vector, and a reference picture index which have been used for coding a neighboring block spatially or temporally neighboring the current block; and one or more second candidates having a predetermined fixed value.

6 . The image coding method according to claim 5 ,

wherein the obtaining of a fixed number of merging candidates includes:

deriving the one or more first candidates and including the one or more first candidates in the fixed number of merging candidates;

deriving one or more third candidates and including the one or more third candidates in the fixed number of merging candidates, when a total number of the first candidates is smaller than the fixed number, the third candidates each having a picture index for a picture referable in the decoding of the current block; and

deriving the one or more second candidates and including the one or more second candidates in the fixed number of merging candidates so that a total number of the first candidates, the second candidates, and the third candidates equals the fixed number, when a total number of the first candidates and the third candidates is smaller than the fixed number.

7 . The image coding method according to claim 6 ,

wherein in the deriving of one or more third candidates, the one or more third candidates are derived by selecting, according to a predetermined priority order, one or more candidates from among a plurality of prepared candidates different from each other.

8 . The image coding method according to claim 5 ,

wherein the obtaining of a fixed number of merging candidates includes:

initializing the fixed number of merging candidates by setting all the fixed number of merging candidates to the second candidates;

deriving the one or more first candidates and updating part of the fixed number of merging candidates so as to include the one or more first candidates in the fixed number of merging candidates; and

deriving one or more third candidates and updating part of the fixed number of merging candidates so as to include the one or more third candidates in the fixed number of merging candidates, when a total number of the first candidates is smaller than the fixed number, the third candidates each having a picture index for a picture referable in the decoding of the current block.

9 . An image decoding apparatus which decodes, on a block-by-block basis, image data included in a coded bitstream, the apparatus comprising:

an merging candidate obtaining unit configured to obtain a fixed number of merging candidates each of which is a candidate set of a prediction direction, a motion vector, and a reference picture index which are to be referenced in decoding of a current block, the fixed number being greater than or equal to two;

an index obtaining unit configured to obtain, from the coded bitstream, an index for identifying a merging candidate among the fixed number of merging candidates, the identified merging candidate being a merging candidate to be referenced in the decoding of the current block; and

a decoding unit configured to identify the merging candidate using the obtained index and decode the current block using the identified merging candidate,

wherein the fixed number of merging candidates include: one or more first candidates each derived based on a prediction direction, a motion vector, and a reference picture index which have been used for decoding a neighboring block spatially or temporally neighboring the current block; and one or more second candidates having a predetermined fixed value.

10 . An image coding apparatus which codes an image on a block-by-block basis to generate a coded bitstream, the apparatus comprising:

an merging candidate obtaining unit configured to obtain a fixed number of merging candidates each of which is a candidate set of a prediction direction, a motion vector, and a reference picture index to be referenced in decoding of a current block, the fixed number being greater than or equal to two; and

a coding unit configured to attach, to the coded bitstream, an index for identifying a merging candidate among the fixed number of merging candidates, the identified merging candidate being a merging candidate to be referenced in the coding of the current block,

wherein the fixed number of merging candidates include: one or more first candidates each derived based on a prediction direction, a motion vector, and a reference picture index which have been used for coding a neighboring block spatially or temporally neighboring the current block; and one or more second candidates having a predetermined fixed value.

11 . An image coding and decoding apparatus comprising:

the image decoding apparatus according to claim 9 ; and

the image coding apparatus according to claim 10 .