Picture coding method, picture coding apparatus, picture decoding method, and picture decoding apparatus

A picture coding apparatus which codes a picture, on a block-by-block basis, to generate a bitstream, the apparatus comprising:

• a processor;
• and
• a non-transitory memory having stored thereon executable instructions, which when executed by the processor, cause the processor to perform:
• performing a third derivation process for deriving third candidates, each of the third candidates including a third motion vector corresponding to a third direction number, for use in coding of a current block, the third motion vector used for coding another block, the other block being coded before the current block is coded, and the other block being different from the current block;
• performing a first derivation process for deriving a first candidate which includes a candidate set of a first prediction direction number which indicates bi-prediction, a first motion vector, and a fourth motion vector for use in the coding of the current block, the first motion vector being included in one of the third candidates, the fourth motion vector being included in another one of the third candidates;
• performing a second derivation process for deriving a second candidate which includes a second motion vector corresponding to a second prediction direction for use in the coding of the current block, the second motion vector being a zero motion vector;
• selecting a candidate to be used in the coding of the current block from among the first candidate, the second candidate, and the third candidates;
• and
• attaching information for identifying the selected candidate to the bitstream,
• wherein the first derivation process is performed so that a total number of the first candidates does not exceed a first predetermined number,
• the second derivation process is performed until a total number of the first to third candidates equals to a second predetermined number, the second predetermined number being a maximum number of the candidates, and
• the second predetermined number is greater than the first predetermined number wherein the second derivation process is performed regardless the first derivation process.