CCLM-based intra-prediction method and apparatus therefor
A picture decoding method, performed by a decoding apparatus according to the present disclosure, comprises the steps of: when an intra-prediction mode of a chroma block is a cross-component linear model (CCLM) mode, deriving neighboring chroma reference samples with respect to the chroma block; deriving neighboring luma reference samples of a luma block corresponding to the chroma block and luma samples in the luma block; deriving down-sampled neighboring luma reference samples and downsampled luma samples by down sampling the neighboring luma reference samples and the luma samples, respectively; deriving a linear model parameter on the basis of the down-sampled neighboring luma reference samples and the neighboring chroma reference samples; generating prediction samples with respect to the chroma block on the basis of the linear model parameter and the down-sampled luma samples of the luma block; and reconstructing the chroma block on the basis of the prediction samples with respect to the chroma block.
1. An image decoding method performed by a decoding apparatus, the method comprising:
obtaining residual information for a chroma block;
deriving neighboring chroma reference samples for the chroma block based on an intra prediction mode of the chroma block being a cross-component linear model (CCLM) mode;
deriving neighboring luma reference samples of a luma block corresponding to the chroma block and luma samples in the luma block;
deriving downsampled neighboring luma reference samples and downsampled luma samples by downsampling the neighboring luma reference samples and the luma samples;
deriving a linear model parameter based on the downsampled neighboring luma reference samples and the neighboring chroma reference samples;
generating prediction samples for the chroma block based on the linear model parameter and the downsampled luma samples of the luma block;
generating residual samples for the chroma block based on the residual information for the chroma block;
reconstructing the chroma block based on the prediction samples for the chroma block and the residual samples for the chroma block; and
performing a deblocking filtering on a reconstructed picture including the chroma block,
wherein the neighboring luma reference samples include top neighboring luma reference samples located above a top boundary of the luma block,
wherein the downsampled neighboring luma reference samples include a downsampled top neighboring luma reference sample, and
wherein based on the top boundary of the luma block being overlapped with a boundary of a coding tree unit (CTU), the downsampled top neighboring luma reference sample is derived based on an equation below,
p Top DsY[x ]=( pY[ 2* x− 1][−1]+2* pY[ 2* x][ −1]+ pY[ 2* x+ 1][−1]+2)>>2,
where pTopDsY[x] represents the downsampled top neighboring luma reference sample, x represents an x-axis direction position of the downsampled top neighboring luma reference sample, pY[2*x−1][−1], pY[2*x][−1] and pY[2*x+1][−1] represent sample values of the top neighboring luma reference samples, respectively, and a y-axis direction position of the top neighboring luma reference samples is −1.
2. An image encoding method performed by an encoding apparatus, the method comprising:
deriving neighboring chroma reference samples for a chroma block based on an intra prediction mode of the chroma block being a cross-component linear model (CCLM) mode;
deriving neighboring luma reference samples of a luma block corresponding to the chroma block and luma samples in the luma block;
deriving downsampled neighboring luma reference samples and downsampled luma samples by downsampling the neighboring luma reference samples and the luma samples;
deriving a linear model parameter based on the downsampled neighboring luma reference samples and the neighboring chroma reference samples;
generating prediction samples for the chroma block based on the linear model parameter and the downsampled luma samples of the luma block;
deriving residual samples for the chroma block based on the prediction samples for the chroma block; and
encoding image information including residual information related to the residual samples for the chroma block,
wherein the neighboring luma reference samples include top neighboring luma reference samples located above a top boundary of the luma block,
wherein the downsampled neighboring luma reference samples include a downsampled top neighboring luma reference sample, and
wherein based on the top boundary of the luma block being overlapped with a boundary of a coding tree unit (CTU), the downsampled top neighboring luma reference sample is derived based on an equation below,
p Top DsY[x ]=( pY[ 2* x− 1][−1]+2* pY[ 2* x][ −1]+ pY[ 2* x+ 1][−1]+2)>>2,
where pTopDsY[x] represents the downsampled top neighboring luma reference sample, x represents an x-axis direction position of the downsampled top neighboring luma reference sample, pY[2*x−1][−1], pY[2*x][−1] and pY[2*x+1][−1] represent sample values of the top neighboring luma reference samples, respectively, and a y-axis direction position of the top neighboring luma reference samples is −1.
3. A transmission method of data for an image, the method comprising:
obtaining a bitstream for the image, wherein the bitstream is generated based on deriving neighboring chroma reference samples for a chroma block based on an intra prediction mode of the chroma block being a cross-component linear model (CCLM) mode, deriving neighboring luma reference samples of a luma block corresponding to the chroma block and luma samples in the luma block, deriving downsampled neighboring luma reference samples and downsampled luma samples by downsampling the neighboring luma reference samples and the luma samples, deriving a linear model parameter based on the downsampled neighboring luma reference samples and the neighboring chroma reference samples, generating prediction samples for the chroma block based on the linear model parameter and the downsampled luma samples of the luma block, deriving residual samples for the chroma block based on the prediction samples for the chroma block, and encoding image information including residual information related to the residual samples for the chroma block to output the bitstream; and
transmitting the data comprising the bitstream,
wherein the neighboring luma reference samples include top neighboring luma reference samples located above a top boundary of the luma block,
wherein the downsampled neighboring luma reference samples include a downsampled top neighboring luma reference sample, and
wherein based on the top boundary of the luma block being overlapped with a boundary of a coding tree unit (CTU), the downsampled top neighboring luma reference sample is derived based on an equation below,
p Top DsY[x ]=( pY[ 2* x− 1][−1]+2* pY[ 2* x][ −1]+ pY[ 2* x+ 1][−1]+2)>>2,
where pTopDsY[x] represents the downsampled top neighboring luma reference sample, x represents an x-axis direction position of the downsampled top neighboring luma reference sample, pY[2*x−1][−1], pY[2*x][−1] and pY[2*x+1][−1] represent sample values of the top neighboring luma reference samples, respectively, and a y-axis direction position of the top neighboring luma reference samples is −1.