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

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

• deriving a top cross-component linear model (CCLM) prediction mode as an intra prediction mode of a current chroma block;
• deriving a sample number of top neighboring chroma samples of the current chroma block based on a width of the current chroma block and a specific value;
• deriving the top neighboring chroma samples of the sample number;
• deriving down sampled top neighboring luma samples related with the top neighboring chroma samples and down sampled luma samples of a current luma block;
• deriving CCLM parameters based on the top neighboring chroma samples and the down sampled top 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 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 top neighboring chroma samples is derived as 2N, and
• based on the 2N being greater than twice the specific value, the sample number of the top neighboring chroma samples is derived as 4.

A non-transitory computer-readable storage medium storing a bitstream generated by a method, the method comprising:

• deriving a top cross-component linear model (CCLM) prediction mode as an intra prediction mode of a current chroma block;
• deriving a sample number of top neighboring chroma samples of the current chroma block based on a width of the current chroma block and a specific value;
• deriving the top neighboring chroma samples of the sample number;
• deriving down sampled top neighboring luma samples related with the top neighboring chroma samples and down sampled luma samples of a current luma block;
• deriving CCLM parameters based on the top neighboring chroma samples and the down sampled top 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 width 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 top neighboring chroma samples is derived as 2N, and
• based on the 2N being greater than twice the specific value, the sample number of the top neighboring chroma samples is derived as 4.

A method of transmitting a bitstream generated by a method, the method comprising:

• deriving a top cross-component linear model (CCLM) prediction mode as an intra prediction mode of a current chroma block;
• deriving a sample number of top neighboring chroma samples of the current chroma block based on a width of the current chroma block and a specific value;
• deriving the top neighboring chroma samples of the sample number;
• deriving down sampled top neighboring luma samples related with the top neighboring chroma samples and down sampled luma samples of a current luma block;
• deriving CCLM parameters based on the top neighboring chroma samples and the down sampled top 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 width 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 top neighboring chroma samples is derived as 2N, and
• based on the 2N being greater than twice the specific value, the sample number of the top neighboring chroma samples is derived as 4.