Method and apparatus for encoding/decoding intra prediction mode

An image decoding method performed by an image decoding apparatus, the method comprising:

• decoding candidate mode set selecting information;
• selecting a candidate mode set based on the candidate mode set selecting information;
• decoding, in case the selected candidate mode set comprises two or more candidate modes, candidate mode selecting information;
• and
• selecting, as an intra-prediction mode of a current block, a candidate mode among one or more candidate modes included in the selected candidate mode set based on at least one among the selected candidate mode set and the candidate mode selecting information,
• wherein the candidate mode set selecting information is information for selecting, among one or more candidate mode sets, a candidate mode set to be used for deriving the intra prediction mode of the current block,
• wherein the one or more candidate mode sets comprises a first candidate mode set including M candidate modes, M being an integer equal to or greater than 1, a second candidate mode set including N candidate modes, N being an integer equal to or greater than 1, and a third candidate mode set including remaining candidate modes,
• wherein a candidate mode included in the second candidate mode set is derived based on a maximum value and a minimum value among an index value of an intra prediction mode of a left block adjacent to the current block and an index value of an intra prediction mode of an upper block adjacent to the current block, when the intra prediction mode of the left block and the intra prediction mode of the upper block are not equal to each other,
• wherein, in response to the index value of the intra prediction mode of the left block adjacent to the current block being larger than the index value of the intra prediction mode of the upper block adjacent to the current block, the maximum value is the index value of the intra prediction mode of the left block adjacent to the current block and the minimum value is the index value of the intra prediction mode of the upper block adjacent to the current block,
• wherein each of the index value of the intra prediction mode of the left block and the index value of the intra prediction mode of the upper block indicates one from among a DC mode, a planar mode and a plurality of angular modes, and
• wherein the candidate mode included in the second candidate mode set is derived by adding 2 to the maximum value or the minimum value, or derived by subtracting 2 from the maximum value or the minimum value.

A non-transitory computer-readable recording-medium storing a bitstream which is generated by an image encoding method,

• the method comprising:
• determining, among one or more candidate mode sets, a candidate mode set in which an intra prediction mode of a current block is included;
• encoding candidate mode set selecting information for selecting the candidate mode set in which the intra prediction mode of the current block is included;
• and
• encoding, in case the candidate mode set in which the intra prediction mode of the current block is included comprises two or more candidate modes, candidate mode selecting information for selecting the intra prediction mode of the current block among the two or mode candidate modes,
• wherein the one or more candidate mode sets comprises a first candidate mode set including M candidate modes, M being an integer equal to or greater than 1, a second candidate mode set including N candidate modes, N being an integer equal to or greater than 1, and a third candidate mode set including remaining candidate modes,
• wherein a candidate mode included in the second candidate mode set is derived based on a maximum value and a minimum value among an index value of an intra prediction mode of a left block adjacent to the current block and an index value of an intra prediction mode of an upper block adjacent to the current block, when the intra prediction mode of the left block and the intra prediction mode of the upper block are not equal to each other,
• wherein, in response to the index value of the intra prediction mode of the left block adjacent to the current block being larger than the index value of the intra prediction mode of the upper block adjacent to the current block, the maximum value is the index value of the intra prediction mode of the left block adjacent to the current block and the minimum value is the index value of the intra prediction mode of the upper block adjacent to the current block,
• wherein each of the index value of the intra prediction mode of the left block and the index value of the intra prediction mode of the upper block indicates one from among a DC mode, a planar mode and a plurality of angular modes, and
• wherein the candidate mode included in the second candidate mode set is derived by adding 2 to the maximum value or the minimum value, or derived by subtracting 2 from the maximum value or the minimum value.