IMAGE DECODER, IMAGE DECODING METHOD, AND IMAGE DECODING PROGRAM

摘要

PROBLEM TO BE SOLVED: To provide an image decoding technique that further improves encoding efficiency by reducing a generated code amount of an intra-prediction mode.SOLUTION: An image decoder comprises: an intra-frame prediction mode storage unit; a prediction direction difference derivation unit that derives a prediction direction difference indicating a degree of difference between prediction directions of intra-frame prediction modes; a preferential prediction mode list creation unit that creates a list of preferential prediction modes to be candidates for an intra-frame prediction mode of a decoding object block on the basis of the intra-frame prediction modes of a plurality of reference blocks used for derivation of the prediction direction difference and the prediction direction difference; a preferential prediction mode determination flag decoding unit; a preferential prediction mode decoding unit; and a non-preferential prediction mode decoding unit. The preferential prediction mode list creation unit sets, as a preferential prediction mode, at least one of an intra-frame prediction mode having a prediction direction adjacent to the prediction directions of the intra-frame prediction modes of the reference blocks and an average value mode in addition to the same intra-frame prediction modes as the intra-frame prediction modes of the reference blocks.