| CPC H04N 19/132 (2014.11) [H04N 19/159 (2014.11); H04N 19/176 (2014.11); H04N 19/18 (2014.11); H04N 19/186 (2014.11)] | 12 Claims |
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1. An image decoding method performed by a decoding apparatus, the method comprising:
obtaining intra prediction mode information and an LFNST index from a bitstream;
deriving an intra prediction mode for a chroma block as a cross-component linear model (CCLM) mode based on the intra prediction mode information;
derives prediction samples for the chroma block based on the CCLM mode;
performing an LFNST to derive transform coefficients; and
deriving residual samples for the chroma block based on the transform coefficients,
wherein the performing of the LFNST comprising:
updating the intra prediction mode for the chroma block based on an intra prediction mode for a luma block corresponding to the chroma block;
modifying the updated intra prediction mode to a wide angle intra prediction mode based on a width of the chroma block not being equal to a height of the chroma block;
determining an LFNST set comprising LFNST matrices based on a modified intra prediction mode by the wide angle intra prediction mode mapping process; and
performing a matrix operation based on an LFNST matrix derived from the LFNST index and the LFNST set;
wherein the updated intra prediction mode is derived as an intra prediction mode corresponding to a specific position in the luma block,
wherein the specific position is set to ((xTbY+(nTbW*SubWidthC)/2), (yTbY+(nTbH*SubHeightC)/2)),
wherein xTbY and yTbY denote top-left coordinates of the luma block,
wherein nTbW and nTbH denote the width and the height of the chroma block, and
wherein SubWidthC and SubHeightC denote variables corresponding to a color format.
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