Intra prediction-based image coding method and device using MPM list
An image decoding method according to the present document derives a left mode, which is a candidate intra prediction mode for a left peripheral block of a current block, derives an upper mode, which is a candidate intra prediction mode for an upper peripheral block of the current block, constructs an MPM list on the basis of the left mode and the upper mode, derives an intra prediction mode of the current block on the basis of the MPM list, generates prediction samples of the current block on the basis of the intra prediction mode, generates a restoration picture of the current block on the basis of the prediction samples, and derives the left mode or the upper mode as one from among candidate intra prediction modes of the MPM list, on the basis of when the left mode and the upper mode are not the same and the left mode and the upper mode are respectively a planar mode and a DC mode.
1. An image decoding method performed by a decoding apparatus, the image decoding method comprising:
obtaining intra prediction mode information related to an intra prediction mode of a current block;
deriving a left mode which is a candidate intra prediction mode for a left neighbouring block of the current block;
deriving an upper mode which is a candidate intra prediction mode for an upper neighbouring block of the current block;
constructing a Most Probable Mode (MPM) list based on the left mode and the upper mode;
deriving the intra prediction mode for the current block based on the MPM list and the intra prediction mode information;
generating prediction samples for the current block based on the intra prediction mode; and
generating a reconstructed picture based on the prediction samples,
wherein based on a case where the left mode and the upper mode are not the same and the left mode and the upper mode are a planar mode and a DC mode, respectively, the DC mode is derived as one of candidate intra prediction modes in the MPM list,
wherein a vertical intra prediction mode, a horizontal intra prediction mode, an intra prediction mode having a mode number being equal to a mode number of the vertical intra prediction mode−4, and an intra prediction mode having a mode number being equal to the mode number of the vertical intra prediction mode+4 are further derived as the candidate intra prediction modes in the MPM list,
wherein the candidate intra prediction modes in the MPM list are represented by index values, and
wherein an index value of the DC mode in the MPM list is smaller than index values of the vertical intra prediction mode, the horizontal intra prediction mode, the intra prediction mode having the mode number being equal to the mode number of the vertical intra prediction mode−4, and the intra prediction mode having the mode number being equal to the mode number of the vertical intra prediction mode+4 in the MPM list.
2. The image decoding method of claim 1 , wherein it is confirmed whether or not the planar mode is derived as a candidate intra prediction mode of the current block preferentially over the DC mode, the vertical intra prediction mode, the horizontal intra prediction mode, the intra prediction mode having a mode number of (mode number of the vertical intra prediction mode−4), and the intra prediction mode having a mode number of (mode number of the vertical intra prediction mode+4).
3. The image decoding method of claim 1 , wherein it is confirmed whether or not the planar mode is derived as a candidate intra prediction mode of the current block preferentially over the candidate intra prediction modes in the MPM list, based on whether or not 0th reference sample line for intra prediction is used for the current block.
4. The image decoding method of claim 3 , wherein the 0th reference sample line includes a left reference sample line located first closest to a left boundary of the current block, and an upper reference sample line located first closest to an upper boundary of the current block.
5. The image decoding method of claim 1 , wherein the constructing the MPM list includes deriving the left mode and the upper mode as the candidate intra prediction modes in the MPM list based on a case that the left mode and the upper mode are not the same and one of the left mode and the upper mode is a planar mode or a DC mode,
wherein a candidate intra prediction mode having a smaller mode number among the left mode and the upper mode is represented by a first index value in the MPM list, and
wherein a candidate intra prediction mode having a greater mode number among the left mode and the upper mode is represented by a second index value in the MPM list.
6. The image decoding method of claim 5 , wherein when one of the left mode and the upper mode is a planar mode, a DC mode is further derived as the candidate intra prediction modes in the MPM list, and the DC mode is represented by a third index value in the MPM list, or
wherein when one of the left mode and the upper mode is a DC mode, a planar mode is further derived as the candidate intra prediction modes in the MPM list, and the planar mode is represented by a third index value in the MPM list.
7. The image decoding method of claim 1 , wherein the deriving the left mode includes:
deriving an intra prediction mode of the left neighbouring block as the left mode when the left neighbouring block is available and intra prediction is applied to the left neighbouring block; and
deriving a planar mode as the left mode when the left neighbouring block is not available or the intra prediction is not applied to the left neighbouring block.
8. The image decoding method of claim 1 , wherein the deriving the upper mode includes:
deriving an intra prediction mode of the upper neighbouring block as the upper mode when the upper neighbouring block is available, intra prediction is applied to the upper neighbouring block, and the upper neighbouring block is included in a current CTU; and
deriving a planar mode as the upper mode when the upper neighbouring block is not available, the intra prediction is not applied to the upper neighbouring block, or the upper neighbouring block is not included in the current CTU.
9. An image encoding method performed by an encoding apparatus, the image encoding method comprising:
deriving a left mode which is a candidate intra prediction mode for a left neighbouring block of a current block;
deriving an upper mode which is a candidate intra prediction mode for an upper neighbouring block of the current block;
constructing a Most Probable Mode (MPM) list based on the left mode and the upper mode;
generating intra prediction mode information related an intra prediction mode of the current block based on the MPM list; and
encoding image information including the intra prediction mode information,
wherein based on a case where the left mode and the upper mode are not the same and the left mode and the upper mode are a planar mode and a DC mode, respectively, the DC mode is derived as one of candidate intra prediction modes in the MPM list,
wherein a vertical intra prediction mode, a horizontal intra prediction mode, an intra prediction mode having a mode number being equal to a mode number of the vertical intra prediction mode−4, and an intra prediction mode having a mode number being equal to the mode number of the vertical intra prediction mode+4 are further derived as the candidate intra prediction modes in the MPM list,
wherein the candidate intra prediction modes in the MPM list are represented by index values, and
wherein an index value of the DC mode in the MPM list is smaller than index values of the vertical intra prediction mode, the horizontal intra prediction mode, the intra prediction mode having the mode number being equal to the mode number of the vertical intra prediction mode−4, and the intra prediction mode having the mode number being equal to the mode number of the vertical intra prediction mode+4 in the MPM list.
10. The image encoding method of claim 9 , wherein it is confirmed whether or not the planar mode is derived as a candidate intra prediction mode of the current block preferentially over the DC mode, the vertical intra prediction mode, the horizontal intra prediction mode, the intra prediction mode having a mode number of (mode number of the vertical intra prediction mode−4), and the intra prediction mode having a mode number of (mode number of the vertical intra prediction mode+4).
11. The image encoding method of claim 9 , wherein it is confirmed whether or not the planar mode is derived as a candidate intra prediction mode of the current block preferentially over the candidate intra prediction modes in the MPM list, based on whether or not 0th reference sample line for intra prediction is used for the current block.
12. The image encoding method of claim 11 , wherein the 0th reference sample line includes a left reference sample line located first closest to a left boundary of the current block, and an upper reference sample line located first closest to an upper boundary of the current block.
13. A non-transitory computer-readable digital storage medium storing a bitstream generated by a method, the method comprising:
deriving a left mode which is a candidate intra prediction mode for a left neighbouring block of a current block;
deriving an upper mode which is a candidate intra prediction mode for an upper neighbouring block of the current block;
constructing a Most Probable Mode (MPM) list based on the left mode and the upper mode;
generating intra prediction mode information related an intra prediction mode of the current block based on the MPM list; and
generating the bitstream by encoding image information including the intra prediction mode information,
wherein based on a case where the left mode and the upper mode are not the same and the left mode and the upper mode are a planar mode and a DC mode, respectively, the DC mode is derived as one of candidate intra prediction modes in the MPM list,
wherein a vertical intra prediction mode, a horizontal intra prediction mode, an intra prediction mode having a mode number being equal to a mode number of the vertical intra prediction mode−4, and an intra prediction mode having a mode number being equal to the mode number of the vertical intra prediction mode+4 are further derived as the candidate intra prediction modes in the MPM list,
wherein the candidate intra prediction modes in the MPM list are represented by index values, and
wherein an index value of the DC mode in the MPM list is smaller than index values of the vertical intra prediction mode, the horizontal intra prediction mode, the intra prediction mode having the mode number being equal to the mode number of the vertical intra prediction mode−4, and the intra prediction mode having the mode number being equal to the mode number of the vertical intra prediction mode+4 in the MPM list.