Method for decoding image on basis of CCLM prediction in image coding system, and device therefor

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 one of a plurality of cross-component linear model (CCLM) prediction modes 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 a width or height of the current chroma block and a specific value;

deriving the neighboring chroma samples of the sample number;

deriving down sampled neighboring luma samples related with the neighboring chroma samples and down sampled luma samples of a current luma block;

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,

wherein the width or height of the current chroma block is N,

wherein based on the N being less than or equal to the specific value, the sample number of the neighboring chroma samples is derived as 2N, and

based on the N being greater than the specific value, the sample number of the neighboring chroma samples is derived as 4.

2. The method of claim 1 , wherein the CCLM prediction modes includes a left top LM mode, a top LM mode and a left LM mode.

3. The method of claim 2 , wherein

based on the CCLM prediction mode of the current chroma block being the left LM mode and the N being greater than the specific value, the neighboring chroma samples include 4 left neighboring chroma samples of the current chroma block.

4. The method of claim 2 , wherein based on the CCLM prediction mode of the current chroma block being the left top LM mode, and the N being less than or equal to the specific value, the neighboring chroma samples include N left neighboring chroma samples and N top neighboring chroma samples of the current chroma block.

5. The method of claim 3 , wherein the prediction mode information includes index information indicating one of the CCLM prediction modes, and

wherein the CCLM prediction mode of the current chroma block is derived based on the index information.

6. The method of claim 1 , wherein the specific value is derived as a predetermined value for the current chroma block to which the CCLM mode is applied.

7. The method of claim 2 , wherein based on the CCLM prediction mode of the current chroma block being the top LM mode, and the N being less than or equal to the specific value, the neighboring chroma samples include 2N top neighboring chroma samples of the current chroma block.

8. The method of claim 2 , wherein based on the CCLM prediction mode of the current chroma block being the left LM mode, and the N being less than or equal to the specific value, the neighboring chroma samples include 2N left neighboring chroma samples of the current chroma block.

9. The method of claim 8 , wherein the information representing the specific value is signaled in a unit of coding unit (CU).

10. The method of claim 8 , wherein the information representing the specific value is signaled with a slice header, a Picture Parameter Set (PPS) and a Sequence Parameter Set (SPS).

11. A video encoding method performed by an encoding apparatus, the method comprising:

determining a cross-component linear model (CCLM) prediction mode among a plurality of CCLM prediction modes;

deriving a sample number of neighboring chroma samples of the current chroma block based on a width or height of the current chroma block and a specific value;

deriving the neighboring chroma samples of the sample number;

deriving down sampled neighboring luma samples related with the neighboring chroma samples and down sampled luma samples of a current luma block;

deriving 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

encoding video information including prediction mode information for the current chroma block,

wherein the specific value is derived as 2,

wherein the width or height of the current chroma block is N,

wherein based on the N being less than or equal to the specific value, the sample number of the neighboring chroma samples is derived as 2N, and

based on the N being greater than the specific value, the sample number of the neighboring chroma samples is derived as 4.

12. The method of claim 11 , wherein the CCLM prediction modes includes a left top LM mode, a top LM mode and a left LM mode, and

wherein the prediction mode information includes index information indicating the CCLM prediction mode of the current chroma block among the CCLM prediction modes.

13. A non-transitory computer-readable storage medium storing a bitstream, the bitstream, when executed, causing a decoding apparatus to perform the following steps:

obtaining video information comprising prediction mode information for a current chroma block;

deriving one of a plurality of cross-component linear model (CCLM) prediction modes 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 a width or height of the current chroma block and a specific value;

deriving the neighboring chroma samples of the sample number;

deriving down sampled neighboring luma samples related with the neighboring chroma samples and down sampled luma samples of a current luma block;

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,

wherein the width or height of the current chroma block is N,

wherein based on the N being less than or equal to the specific value, the sample number of the neighboring chroma samples is derived as 2N, and

based on the N being greater than the specific value, the sample number of the neighboring chroma samples is derived as 4.

14. The method of claim 2 , wherein

based on the CCLM prediction mode of the current chroma block being the top LM mode and the N being greater than the specific value, the neighboring chroma samples include 4 top neighboring chroma samples of the current chroma block.

15. The method of claim 2 , wherein

based on the CCLM prediction mode of the current chroma block being the left top LM mode and the N being greater than the specific value, the neighboring chroma samples include the 2 left neighboring chroma samples and the 2 top neighboring chroma samples of the current chroma block.

16. The method of claim 12 , wherein

based on the CCLM prediction mode of the current chroma block being the left LM mode and the N being greater than the specific value, the neighboring chroma samples include 4 left neighboring chroma samples of the current chroma block.

17. The method of claim 12 , wherein

based on the CCLM prediction mode of the current chroma block being the top LM mode and the N being greater than the specific value, the neighboring chroma samples include 4 top neighboring chroma samples of the current chroma block.

18. The method of claim 12 , wherein

based on the CCLM prediction mode of the current chroma block being the left top LM mode and the N being greater than the specific value, the neighboring chroma samples include the 2 left neighboring chroma samples and the 2 top neighboring chroma samples of the current chroma block.