Image coding method based on transform, and device therefor

1. An image decoding method performed by a decoding apparatus, comprising:

receiving residual information through a bitstream;

deriving transform coefficients for a current block based on the residual information;

deriving modified transform coefficients by applying a low frequency non-separable transform (LFNST) to the transform coefficients;

deriving residual samples for the current block based on an inverse primary transform for the modified transform coefficients; and

generating a reconstructed picture based on the residual samples,

wherein deriving the modified transform coefficients comprises:

parsing an LFNST index based on whether a tree type of the current block is a dual-tree chroma; and

deriving the modified transform coefficients based on the LFNST index and an LFNST matrix, and

wherein based on the tree type of the current block being the dual-tree chroma, the LFNST index is parsed from the bitstream regardless of whether a matrix-based intra prediction (MIP) mode is applied to the current block and whether an LFNST-applied width and an LFNST-applied height corresponding to the current block are greater than or equal to 16.

2. The image decoding method of claim 1 , wherein when the tree type of the current block is not the dual-tree chroma, the LFNST index is parsed from the bitstream based on the MIP mode being not applied to the current block.

3. The image decoding method of claim 1 , wherein when the tree type of the current block is not the dual-tree chroma and the MIP mode is applied to the current block, the LFNST index is parsed from the bitstream based on the LFNST-applied width and the LFNST-applied height corresponding to the current block being greater than or equal to 16.

4. The image decoding method of claim 3 , wherein the LFNST-applied width is set to a width of the current block divided by a number of sub-partitions when the current block is vertically split into the sub-partitions, and the LFNST-applied width is set to the width of the current block when the current block is not split into the sub-partitions, and

wherein the current block is a coding block.

5. The image decoding method of claim 3 , wherein the LFNST-applied height is set to a height of the current block divided by a number of sub-partitions when the current block is horizontally split into the sub-partitions, and the LFNST-applied height is set to the height of the current block when the current block is not split into the sub-partitions, and

wherein the current block is a coding block.

6. An image encoding method performed by an image encoding apparatus, comprising:

deriving prediction samples for a current block;

deriving residual samples for the current block based on the prediction samples;

deriving transform coefficients for the current block based on a primary transform of the residual samples;

deriving modified transform coefficients by applying a low frequency non-separable transform (LFNST) to the transform coefficients; and

encoding an LFNST index indicating an LFNST matrix applied to the LFNST,

wherein the LFNST index is encoded into a bitstream based on whether a tree type of the current block is a dual-tree chroma, and

wherein based on the tree type of the current block being the dual-tree chroma, the LFNST index is encoded into the bitstream regardless of whether a matrix-based intra prediction (MIP) mode is applied to the current block and whether an LFNST-applied width and an LFNST-applied height corresponding to the current block are greater than or equal to 16.

7. The image encoding method of claim 6 , wherein when the tree type of the current block is not the dual-tree chroma, the LFNST index is encoded into the bitstream based on the MIP mode being not applied to the current block.

8. The image encoding method of claim 6 , wherein when the tree type of the current block is not the dual-tree chroma and the MIP mode is applied to the current block, the LFNST index is encoded into the bitstream based on the LFNST-applied width and the LFNST-applied height corresponding to the current block being greater than or equal to 16.

9. The image encoding method of claim 8 , wherein the LFNST-applied width is set to a width of the current block divided by a number of sub-partitions when the current block is vertically split into the sub-partitions, and the LFNST-applied width is set to the width of the current block when the current block is not split into the sub-partitions, and

wherein the current block is a coding block.

10. The image encoding method of claim 8 , wherein the LFNST-applied height is set to a height of the current block divided by a number of sub-partitions when the current block is horizontally split into the sub-partitions, and the LFNST-applied height is set to the height of the current block when the current block is not split into the sub-partitions, and

wherein the current block is a coding block.

11. A non-transitory computer-readable digital storage medium that stores a bitstream generated by a method, the method comprising:

deriving prediction samples for a current block;

deriving residual samples for the current block based on the prediction samples;

deriving transform coefficients for the current block based on a primary transform of the residual samples;

deriving modified transform coefficients by applying a low frequency non-separable transform (LFNST) to the transform coefficients; and

encoding an LFNST index indicating an LFNST matrix applied to the LFNST to generate the bitstream,

wherein the LFNST index is encoded into the bitstream based on whether a tree type of the current block is a dual-tree chroma, and

wherein based on the tree type of the current block being the dual-tree chroma, the LFNST index is encoded into the bitstream regardless of whether a matrix-based intra prediction (MIP) mode is applied to the current block and whether an LFNST-applied width and an LFNST-applied height corresponding to the current block are greater than or equal to 16.