Method and device for decoding images using CCLM prediction in image coding system
An image decoding method performed by a decoding device according to the present document comprises: a step for deriving the number of samples of upper peripheral chroma samples and left peripheral chroma samples of a current chroma block on the basis of a specific value and the width and height of the current chroma block; a step for deriving said number of upper peripheral chroma samples and said number of left peripheral chroma samples; and a step for deriving CCLM parameters on the basis of the upper peripheral chroma samples, the left peripheral chroma samples, and down-sampled peripheral luma samples, wherein, when the specific value is derived as 2 and the width and the height of the current chroma block are larger than the specific value, the number of samples is derived as the specific value.
1. An image decoding method performed by a decoding apparatus, the method comprising:
obtaining image information including prediction mode information for a current luma block and prediction mode information for a current chroma block;
deriving an intra prediction mode of the current luma block based on the prediction mode information;
generating luma samples for the current luma block based on the intra prediction mode and neighboring luma samples of the current luma block;
deriving an intra prediction mode of the current chroma block as a cross-component linear model (CCLM) mode based on the prediction mode information for the current chroma block;
deriving a sample number for top neighboring chroma samples and left neighboring chroma samples of the current chroma block based on a specific value, a width and a height of the current chroma block;
deriving the top neighboring chroma samples of the sample number and the left neighboring chroma samples of the sample number;
deriving down-sampled neighboring luma samples based on the neighboring luma samples of the current luma block;
deriving down-sampled luma samples based on the luma samples of the current luma block;
deriving CCLM parameters based on the top neighboring chroma samples, the left 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, and
wherein based on the width and the height of the current chroma block being greater than the specific value, the sample number is derived as the specific value.
2. The image decoding method of claim 1 , wherein based on the width and the height being less than or equal to the specific value, the sample number is derived as one value of the width and the height.
3. The image decoding method of claim 1 , wherein the down-sampled neighboring luma samples include down-sampled top neighboring luma samples of the sample number related with the top neighboring chroma samples, and down-sampled left neighboring luma samples of the sample number related with the left neighboring chroma samples.
4. The image decoding method of claim 1 , wherein the specific value is derived as a preset value for the current chroma block to which the CCLM mode is applied.
5. The image decoding method of claim 1 , wherein the specific value is derived based on the size of the current chroma block.
6. The image decoding method of claim 1 , wherein the image information includes information indicating the specific value, and the specific value is derived based on the information indicating the specific value.
7. The image decoding method of claim 6 , wherein the information indicating the specific value is signaled in units of a coding unit (CU).
8. The image decoding method of claim 6 , wherein the information indicating the specific value is signaled in a slice header, a picture parameter set (PPS), or a sequence parameter set (SPS).
9. The image decoding method of claim 1 , wherein:
the image information includes flag information indicating whether to derive the sample number based on the specific value;
based on the value of the flag information being 1 , the image information includes information indicating the specific value and the specific value is derived based on the information indicating the specific value.
10. The image decoding method of claim 1 , wherein the specific value is derived based on whether a smaller value between the width and the height of the current block is greater than a specific threshold value.
11. The image decoding method of claim 10 , wherein the specific threshold is preset, and, based on the smaller value between the width and the height being less than or equal to the specific threshold value, the specific value is derived as 2.
12. An image encoding method performed by an encoding apparatus, the method comprising:
deriving an intra prediction mode of a current luma block;
generating luma samples for the current luma block based on the intra prediction mode and neighboring luma samples of the current luma block;
determining an intra prediction mode of a current chroma block as a cross-component linear model (CCLM) mode;
deriving a sample number for top neighboring chroma samples and left neighboring chroma samples of the current chroma block based on a specific value, a width and a height of the current chroma block;
deriving the top neighboring chroma samples of the sample number and the left neighboring chroma samples of the sample number;
deriving down-sampled neighboring luma samples based on the neighboring luma samples of the current luma block;
deriving down-sampled luma samples based on the luma samples of the current luma block;
deriving CCLM parameters based on the top neighboring chroma samples, the left 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
encoding image information including prediction mode information for the current luma block and prediction mode information for the current chroma block,
wherein the specific value is derived as 2, and
wherein based on the width and the height of the current chroma block being greater than the specific value, the sample number is derived as the specific value.
13. The image encoding method of claim 12 , wherein based on the width and the height being less than or equal to the specific value, the sample number is derived as one value of the width and the height.
14. The image encoding method of claim 12 , wherein the down-sampled neighboring luma samples include down-sampled top neighboring luma samples of the sample number related with the top neighboring chroma samples, and down-sampled left neighboring luma samples of the sample number related with the left neighboring chroma samples.
15. The image encoding method of claim 12 , wherein the specific value is derived as a preset value for the current chroma block to which the CCLM mode is applied.
16. A non-transitory computer-readable storage medium storing a bitstream generated by a method, the method comprising:
deriving an intra prediction mode of a current luma block;
generating luma samples for the current luma block based on the intra prediction mode and neighboring luma samples of the current luma block;
determining an intra prediction mode of a current chroma block as a cross-component linear model (CCLM) mode;
deriving a sample number for top neighboring chroma samples and left neighboring chroma samples of the current chroma block based on a specific value, a width and a height of the current chroma block;
deriving the top neighboring chroma samples of the sample number and the left neighboring chroma samples of the sample number;
deriving down-sampled neighboring luma samples based on the neighboring luma samples of the current luma block;
deriving down-sampled luma samples based on the luma samples of the current luma block;
deriving CCLM parameters based on the top neighboring chroma samples, the left 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;
encoding image information including prediction mode information for the current luma block and prediction mode information for the current chroma block; and
generating the bitstream including the image information,
wherein the specific value is derived as 2, and
wherein based on the width and the height of the current chroma block being greater than the specific value, the sample number is derived as the specific value.