Method and device for image decoding on basis of cclm prediction in image coding system

An image encoding method performed by an encoding apparatus, the image encoding method comprising:

• determining a cross-component linear model (CCLM) mode as an intra prediction mode of a current chroma block;
• deriving a specific value for the current chroma block;
• deriving specific top neighboring chroma samples and specific left neighboring chroma samples of the current chroma block based on a width and a height of the current chroma block and the specific value;
• deriving CCLM parameters based on the specific top neighboring chroma samples and the specific left neighboring chroma samples;
• deriving prediction samples for the current chroma block based on the CCLM parameters;
• and
• encoding image information including prediction related information for the current chroma block,
• wherein the derived specific value is 2,
• wherein based on the width and the height being greater than or equal to the specific value, top neighboring chroma samples of specific top positions among top neighboring chroma samples of the current chroma block are selected as the specific top neighboring chroma samples,
• wherein the specific top positions are derived based on the number of the specific top neighboring chroma samples and the width of the current chroma block,
• wherein based on the width and the height being greater than or equal to the specific value, left neighboring chroma samples of specific left positions among left neighboring chroma samples of the current chroma block are selected as the specific left neighboring chroma samples,
• wherein the specific left positions are derived based on the number of the specific left neighboring chroma samples and the height of the current chroma block, and
• wherein when the width of the current chroma block is 4, the specific top positions are (1, −1) and (3, −1), and
• wherein when the height of the current chroma block is 4, the specific left positions are (−1, 1) and (−1,3).

A transmission method of data for image, the method comprising:

• obtaining a bitstream of image information including prediction related information for prediction samples for a current chroma block;
• and
• transmitting the data including the bitstream of the image information including the prediction related information,
• wherein the prediction samples for the current chroma block are derived based on cross-component linear model (CCLM) parameters,
• wherein the CCLM parameters are derived based on specific top neighboring chroma samples of the current chroma block, and specific left neighboring chroma samples of the current chroma block,
• wherein the specific top neighboring chroma samples and the specific left neighboring chroma samples of the current chroma block are derived based on a width and a height of the current chroma block and a specific value for the current chroma block,
• wherein the derived specific value is 2,
• wherein based on the width and the height being greater than or equal to the specific value, top neighboring chroma samples of specific top positions among top neighboring chroma samples of the current chroma block are selected as the specific top neighboring chroma samples,
• wherein the specific top positions are derived based on the number of the specific top neighboring chroma samples and the width of the current chroma block,
• wherein based on the width and the height being greater than or equal to the specific value, left neighboring chroma samples of specific left positions among left neighboring chroma samples of the current chroma block are selected as the specific left neighboring chroma samples,
• wherein the specific left positions are derived based on the number of the specific left neighboring chroma samples and the height of the current chroma block, and
• wherein when the width of the current chroma block is 4, the specific top positions are (1, −1) and (3, −1), and
• wherein when the height of the current chroma block is 4, the specific left positions are (−1, 1) and (−1,3).