Method for decoding image on basis of CCLM prediction in image coding system, and device therefor
A video decoding method performed by a decoding apparatus according to the present disclosure includes deriving one of a plurality of cross-component linear model (CCLM) prediction mode as a CCLM prediction mode of the current chroma block, deriving a sample number of neighboring chroma samples of the current chroma block based on the CCLM prediction mode of the current chroma block, a size of the current chroma block, and a specific value; deriving the neighboring chroma samples of the sample number, calculating CCLM parameters based on the neighboring chroma samples and the down sampled neighboring luma samples, deriving prediction samples for the current chroma block based on the CCLM parameters and the down sampled luma samples and generating reconstructed samples for the current chroma block based on the prediction samples, wherein the specific value is derived as 2.
1. A video decoding method performed by a decoding apparatus, the method comprising:
obtaining video information comprising prediction mode information for a current chroma block;
deriving a left cross-component linear model (CCLM) prediction mode as an intra prediction mode of the current chroma block based on the prediction mode information;
deriving a sample number of left neighboring chroma samples of the current chroma block based on a height of the current chroma block and a specific value;
deriving the left neighboring chroma samples of the sample number;
deriving down sampled left neighboring luma samples related with the left neighboring chroma samples and down sampled luma samples of a current luma block;
deriving CCLM parameters based on the left neighboring chroma samples and the down sampled left neighboring luma samples;
deriving prediction samples for the current chroma block based on the CCLM parameters and the down sampled luma samples; and
generating reconstructed samples for the current chroma block based on the prediction samples,
wherein the specific value is derived as 2,
wherein the height of the current chroma block is N,
wherein based on 2N being less than or equal to twice the specific value, the sample number of the left neighboring chroma samples is derived as 2N, and
based on the 2N being greater than twice the specific value, the sample number of the left neighboring chroma samples is derived as 4.
2. The method of claim 1 , wherein the prediction mode information includes index information indicating one of CCLM prediction modes, and
wherein the CCLM prediction modes include a left top CCLM mode, a top CCLM mode and the left CCLM mode.
3. The method of claim 1 , wherein the specific value is derived as a predetermined value for the current chroma block to which the left CCLM mode is applied.
4. The method of claim 1 , wherein information on the specific value is signaled in a unit of coding unit (CU).
5. The method of claim 1 , wherein information on the specific value is signaled with a slice header, a Picture Parameter Set (PPS) and a Sequence Parameter Set (SPS).
6. A video encoding method performed by an encoding apparatus, the method comprising:
deriving a left cross-component linear model (CCLM) prediction mode as an intra prediction mode of a current chroma block;
deriving a sample number of left neighboring chroma samples of the current chroma block based on a height of the current chroma block and a specific value;
deriving the left neighboring chroma samples of the sample number;
deriving down sampled left neighboring luma samples related with the left neighboring chroma samples and down sampled luma samples of a current luma block;
deriving CCLM parameters based on the left neighboring chroma samples and the down sampled left neighboring luma samples;
deriving prediction samples for the current chroma block based on the CCLM parameters and the down sampled luma samples; and
encoding video information including prediction mode information for the current chroma block,
wherein the specific value is derived as 2,
wherein the height of the current chroma block is N,
wherein based on 2N being less than or equal to twice the specific value, the sample number of the left neighboring chroma samples is derived as 2N, and
based on the 2N being greater than twice the specific value, the sample number of the left neighboring chroma samples is derived as 4.
7. The method of claim 6 , wherein the prediction mode information includes index information indicating one of CCLM prediction modes, and
wherein the CCLM prediction modes include a left top CCLM mode, a top CCLM mode and the left CCLM mode.
8. The method of claim 6 , wherein the specific value is derived as a predetermined value for the current chroma block to which the left CCLM mode is applied.
9. The method of claim 6 , wherein information on the specific value is signaled in a unit of coding unit (CU).
10. The method of claim 6 , wherein information on the specific value is signaled with a slice header, a Picture Parameter Set (PPS) and a Sequence Parameter Set (SPS).
11. A non-transitory computer-readable storage medium storing a bitstream generated by a method, the method comprising:
deriving a left cross-component linear model (CCLM) prediction mode as an intra prediction mode of a current chroma block;
deriving a sample number of left neighboring chroma samples of the current chroma block based on a height of the current chroma block and a specific value;
deriving the left neighboring chroma samples of the sample number;
deriving down sampled left neighboring luma samples related with the left neighboring chroma samples and down sampled luma samples of a current luma block;
deriving CCLM parameters based on the left neighboring chroma samples and the down sampled left neighboring luma samples;
deriving prediction samples for the current chroma block based on the CCLM parameters and the down sampled luma samples;
encoding video information including prediction mode information for the current chroma block; and
generating the bitstream including the video information,
wherein the specific value is derived as 2,
wherein the height of the current chroma block is N,
wherein based on 2N being less than or equal to twice the specific value, the sample number of the left neighboring chroma samples is derived as 2N, and
based on the 2N being greater than twice the specific value, the sample number of the left neighboring chroma samples is derived as 4.
12. The non-transitory computer-readable storage medium of claim 11 , wherein the prediction mode information includes index information indicating one of CCLM prediction modes, and
wherein the CCLM prediction modes include a left top CCLM mode, a top CCLM mode and the left CCLM mode.
13. The non-transitory computer-readable storage medium of claim 11 , wherein the specific value is derived as a predetermined value for the current chroma block to which the left CCLM mode is applied.
14. The non-transitory computer-readable storage medium of claim 11 , wherein information on the specific value is signaled in a unit of coding unit (CU).
15. The non-transitory computer-readable storage medium of claim 11 , wherein information on the specific value is signaled with a slice header, a Picture Parameter Set (PPS) and a Sequence Parameter Set (SPS).