Image decoding method, image coding method, image decoding apparatus, image coding apparatus, and image coding and decoding apparatus
An image decoding method of decoding, on a block-by-block basis, image data included in a coded stream includes: deriving candidates for an intra prediction mode to be used for intra prediction for a decoding target block, the number of the candidates constantly being a plural number; obtaining, from the coded stream, an index for identifying one of the derived candidates for the intra prediction mode; and determining, based on the obtained index, one of the derived candidates for the intra prediction mode as the intra prediction mode to be used for intra prediction for the decoding target block.
1 . A transmission method for sending a bitstream,
the bitstream comprising a coded flag,
the bitstream being generated by performing encoding method comprising:
selecting candidates for an intra prediction mode to be used for intra prediction for a coding target block from a plurality of predetermined intra prediction modes, each of the plurality of predetermined intra prediction modes having a mode number;
deriving the selected candidates, the number of the candidates being a predetermined fixed number, the predetermined fixed number being at least two and less than a number of the plurality of predetermined intra prediction modes;
making a candidates list which includes indices and the derived candidates, the derived candidates corresponding on a one-to-one basis with the indices, and a number of the indices being equal to the predetermined fixed number;
generating the coded flag by coding a flag which indicating whether the intra prediction mode is inferred from a neighboring block or not;
adding the coded flag to the bitstream;
when the flag indicates that the intra prediction mode is inferred from the neighboring block,
(i) generating a coded specified index by coding a specified index which specifies an index of one of the derived candidates as the intra prediction mode to be used for intra prediction for the coding target block,
(ii) adding the coded specified index to the bitstream,
(iii) determining the one of the derived candidates using the specified index, the index of the one of the derived candidates being specified by the specified index in the candidates list, and
(iv) coding the image data using the determined one of the derived candidates; and
when the flag indicates that the intra prediction mode is not inferred from the neighboring block,
(i) generating a coded specified mode number by coding a specified mode number,
(ii) adding the coded specified mode number to the bitstream, and
(iii) comparing the mode number of one of the derived candidates with the specified mode number,
wherein the deriving includes:
deriving a first candidate for the intra prediction mode to be used for intra prediction for the coding target block from an intra prediction mode used for intra prediction for each of adjacent blocks that are adjacent to the coding target block; and
in a case that the number of the derived first candidates is smaller than the predetermined fixed number, further deriving a second candidate as a DC prediction mode to be used for intra prediction and a third candidate as a vertical (angular) prediction mode to be used for intra prediction.