Method and device for intra prediction

1. A video decoding method comprising the steps of:

receiving, by a decoding apparatus, index information indicating a candidate mode to be used as an intra prediction mode of a current block;

constructing, by the decoding apparatus, a candidate mode list including a plurality of candidate modes for the current block;

determining, by the decoding apparatus, as the intra prediction mode of the current block, the candidate mode indicated by the index information out of the plurality of candidate modes constituting the candidate mode list; and

generating, by the decoding apparatus, a predicted sample in the current block by performing intra prediction on the current block on the basis of the determined intra prediction mode,

wherein the constructing the candidate mode list includes:

setting a first candidate intra prediction mode corresponding to a first block adjacent to the left side of the current block and a second candidate intra prediction mode corresponding to a second block adjacent to the upper side of the current block, and

setting a first ordered candidate mode and a second ordered candidate mode among the candidate modes of the candidate mode list,

wherein the first ordered candidate mode is set equal to a planar mode and the second ordered candidate mode is set equal to a DC mode if the first candidate intra prediction mode is a planar mode and the second block is unavailable,

wherein the first ordered candidate mode is set equal to the planar mode and the second ordered candidate mode is set equal to the DC mode if the both of the first block and the second block are not available.

2. The video decoding method according to claim 1 , wherein the second candidate intra prediction mode is set to be the DC mode when the second block is not available.

3. The video decoding method according to claim 2 , wherein the constructing the candidate mode list includes determining that the first block is not available when the first block is located outside a current picture, a current slice or a current coding tree block and determining that the second block is not available when the second block is located outside of the current picture, the current slice or a current coding tree block.

4. The video decoding method according to claim 2 , wherein the constructing the candidate mode list includes determining that the first block is not available when the first block is not a block decoded with an intra mode and determining that the second block is not available when the second block is not a block decoded with an intra mode.

5. The video decoding method according to claim 1 , wherein the first candidate intra prediction mode is set to the first ordered candidate mode and the second candidate intra prediction mode is set to the second ordered candidate mode when the first candidate intra prediction mode and the second candidate intra prediction mode are not equal to each other.

6. The video decoding method according to claim 5 , wherein an index value of 0 is allocated to the first ordered candidate mode and an index value of 1 is allocated to the second ordered candidate mode.

7. The video decoding method according to claim 1 , wherein the constructing the candidate mode list includes:

setting a planar mode to a third candidate mode in the candidate mode list when neither of the first candidate intra prediction mode and the second candidate intra prediction mode is the planar mode,

setting a DC mode to the third candidate mode in the candidate mode list when one of the first candidate intra prediction mode and the second candidate intra prediction mode is the planar mode and neither of the first candidate intra prediction mode and the second candidate intra prediction mode is a DC mode, and

setting a vertical mode to the third candidate mode in the candidate mode list when one of the first candidate intra prediction mode and the second candidate intra prediction mode is the planar mode and the other is the DC mode.

8. The video decoding method according to claim 7 , wherein an index value of 2 is allocated to the third candidate mode.

9. The video decoding method according to claim 1 , wherein

the planar mode is set to the first ordered candidate mode and the DC mode is set to the second ordered candidate mode when the first candidate intra prediction mode and the second candidate intra prediction mode are the planar mode.

10. The video decoding method according to claim 1 , wherein

the planar mode is set to the first ordered candidate mode and the DC mode is set to the second ordered candidate mode when the first candidate intra prediction and the second candidate intra prediction mode are the DC mode.

11. The video decoding method according to claim 1 , wherein

the first candidate intra prediction mode is set to the first ordered candidate mode in the candidate mode list and a prediction mode having a prediction direction most similar or adjacent to a prediction direction of the first candidate intra prediction mode is set to the second ordered candidate mode when the first candidate intra prediction is neither a planar mode nor a DC mode.

12. The video decoding method according to claim 1 , further comprising receiving a flag specifying whether a candidate mode to be used as the intra prediction mode of the current block is present in the candidate modes constituting the candidate mode list.

13. The video decoding method according to claim 12 , further comprising receiving remaining prediction mode corresponding to the intra prediction mode of the current block when the flag specified that the candidate mode to be used as the intra prediction mode of the current block is not present in the plural candidate modes.