Picture decoding device, picture decoding method, and picture decoding program

A picture decoding method for performing intra prediction decoding of a picture signal including a luma signal and a chroma signal in units of blocks by decoding information relating to an intra prediction mode, the picture decoding method comprising:

• deriving a value of a luma intra prediction mode;
• and
• deriving a value of a chroma intra prediction mode from a conversion table representing a value of the chroma intra prediction mode having a chroma format of 4:2:2 based on a value of the luma intra prediction mode of a prediction block located at a same position as the position of a prediction block of the chroma signal, which is derived in the deriving of a value of the luma intra prediction mode, in a case where the chroma format is 4:2:2 and deriving a value of the chroma intra prediction mode based on a value of the luma intra prediction mode of a prediction block located at a same position as the position of a prediction block of the chroma signal without using the conversion table in a case where the chroma format is not 4:2:2,
• wherein the conversion table shows at least values representing a plurality of chroma intra prediction modes having prediction directions that are respectively closest to a plurality of directions derived by performing ½ times scaling horizontally for prediction directions of a plurality of the luma intra prediction modes in which reference destinations are horizontally aligned and the conversion table shows a same unaltered value as a value representing the chroma intra prediction mode as to a value representing the luma intra prediction mode of vertical prediction in which a prediction direction is a vertical direction.

A recording medium in which a picture decoding program causing a computer to execute performing intra prediction decoding of a picture signal including a luma signal and a chroma signal in units of blocks by decoding information relating to an intra prediction mode is stored, the picture decoding program causing the computer to execute:

• deriving a value of a luma intra prediction mode;
• and
• deriving a value of a chroma intra prediction mode from a conversion table representing a value of the chroma intra prediction mode having a chroma format of 4:2:2 based on a value of the luma intra prediction mode of a prediction block located at a same position as the position of a prediction block of the chroma signal, which is derived in the deriving of a value of the luma intra prediction mode, in a case where the chroma format is 4:2:2 and deriving a value of the chroma intra prediction mode based on a value of the luma intra prediction mode of a prediction block located at a same position as the position of a prediction block of the chroma signal without using the conversion table in a case where the chroma format is not 4:2:2,
• wherein the conversion table shows at least values representing a plurality of chroma intra prediction modes having prediction directions that are respectively closest to a plurality of directions derived by performing ½ times scaling horizontally for prediction directions of a plurality of the luma intra prediction modes in which reference destinations are horizontally aligned and the conversion table shows a same unaltered value as a value representing the chroma intra prediction mode as to a value representing the luma intra prediction mode of vertical prediction in which a prediction direction is a vertical direction.