CCLM prediction-based image decoding method and apparatus in image coding system
An image decoding method executed by a decoding apparatus according to the present document comprises the steps of: deriving an intra prediction mode of the current chroma block by means of cross-component linear model (CCLM) mode; deriving downsampled luma samples on the basis of the current luma block; deriving downsampled neighboring luma samples on the basis of neighboring luma samples of the current luma block; and deriving the CCLM parameter on the basis of the downsampled neighboring luma samples and neighboring chroma samples of the current neighboring chroma block, wherein the number of downsampled neighboring luma samples and number of neighboring chroma samples are equal to the previously set thresh-hold value, and the thresh-hold value is derived on the basis of the width and height of the current chroma block.
1. An image decoding method performed by a decoding apparatus, the method comprising:
deriving a cross-component linear model (CCLM) mode as an intra prediction mode for a current chroma block based on prediction mode information on the current chroma block, wherein the derived CCLM mode is one of a left based CCLM mode or a top based CCLM mode;
deriving downsampled luma samples based on a current luma block;
deriving downsampled neighboring luma samples based on neighboring luma samples of the current luma block;
deriving a CCLM parameter based on the downsampled neighboring luma samples and neighboring chroma samples of the current chroma block; and
generating prediction samples of the current chroma block based on the CCLM parameter and the downsampled luma samples,
wherein a width and a height of the current chroma block is N,
wherein based on the CCLM mode being the left based CCLM mode and N being greater than a threshold value, downsampled left neighboring luma samples and left neighboring chroma samples are derived and a number of the downsampled left neighboring luma samples and a number of the left neighboring chroma samples are equal to twice the threshold value, and
wherein based on the CCLM mode being the top based CCLM mode and the N being greater than the threshold value, downsampled top neighboring luma samples and top neighboring chroma samples are derived and a number of the downsampled top neighboring luma samples and a number of the top neighboring chroma samples are equal to twice the threshold value.
2. An image encoding method performed by an encoding apparatus, the method comprising:
determining a cross-component linear model (CCLM) mode as an intra prediction mode for a current chroma block, wherein the determined CCLM mode is one of a left based CCLM mode or a top based CCLM mode;
deriving downsampled luma samples based on a current luma block;
deriving downsampled neighboring luma samples based on neighboring luma samples of the current luma block;
deriving a CCLM parameter based on the downsampled neighboring luma samples and neighboring chroma samples of the current chroma block; and
generating prediction samples of the current chroma block based on the CCLM parameter and the downsampled luma samples,
wherein a width and a height of the current chroma block is N,
wherein based on the CCLM mode being the left based CCLM mode and N being greater than a threshold value, downsampled left neighboring luma samples and left neighboring chroma samples are derived and a number of the downsampled left neighboring luma samples and a number of the left neighboring chroma samples are equal to twice the threshold value, and
wherein based on the CCLM mode being the top based CCLM mode and the N being greater than the threshold value, downsampled top neighboring luma samples and top neighboring chroma samples are derived and number of the downsampled top neighboring luma samples and the number of the top neighboring chroma samples are equal to twice the threshold value.
3. A transmission method of data for image, the method comprising:
obtaining a bitstream of image information including prediction mode information for a current chroma block; and
transmitting the data including the bitstream of the image information including the prediction mode information,
wherein the prediction mode information is generated by determining a cross-component linear model (CCLM) mode as an intra prediction mode for the current chroma block, wherein the determined CCLM mode is one of a left based CCLM mode or a top based CCLM mode, deriving downsampled luma samples based on a current luma block, deriving downsampled neighboring luma samples based on neighboring luma samples of the current luma block, deriving a CCLM parameter based on the downsampled neighboring luma samples and neighboring chroma samples of the current chroma block, generating prediction samples of the current chroma block based on the CCLM parameter and the downsampled luma samples, generating the prediction mode information for the prediction samples of the current chroma block,
wherein a width and a height of the current chroma block is N,
wherein based on the CCLM mode being the left based CCLM mode and N being greater than a threshold value, downsampled left neighboring luma samples and left neighboring chroma samples are derived and a number of the downsampled left neighboring luma samples and a number of the left neighboring chroma samples are equal to twice the threshold value, and
wherein based on the CCLM mode being the top based CCLM mode and the N being greater than the threshold value, downsampled top neighboring luma samples and top neighboring chroma samples are derived and a number of the downsampled top neighboring luma samples and a number of the top neighboring chroma samples are equal to twice the threshold value.