Transform-based image coding method, and device therefor

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

obtaining intra prediction mode information and low-frequency non-separable transform (LFNST) index information for a current block from a bitstream,

wherein the current block is divided into a plurality of sub-partition transform blocks based on an intra sub-partition mode in which intra prediction is performed on each of the sub-partition transform blocks in the current block, and

wherein the LFNST index information for the current block is obtained, from the bitstream, based on a width and a height of a sub-partition transform block resulting from dividing the current block being greater than or equal to 4;

deriving prediction samples of the sub-partition transform block based on an intra prediction mode derived from the intra prediction mode information for the current block;

determining an LFNST set including LFNST matrices based on the intra prediction mode;

selecting one of the LFNST matrices based on the LFNST set and the LFNST index information;

deriving transform coefficients for the sub-partition transform block based on the one of the LFNST matrices;

deriving residual samples for the sub-partition transform block based on the transform coefficients; and

generating reconstructed samples for the sub-partition transform block based on the prediction samples and the residual samples.

2. The image decoding method of claim 1 , wherein the LFNST set and the LFNST index information are applied to the sub-partition transform blocks divided from the current block.

3. The image decoding method of claim 1 , wherein the intra prediction mode is applied to the sub-partition transform blocks divided from the current block.

4. The image decoding method of claim 1 , wherein the transform coefficients are derived using the one of the LFNST matrices based on the width and the height of the sub-partition transform block being greater than or equal to 4.

5. The image decoding method of claim 1 , wherein the current block is vertically divided based on a size (width×height) of the current block being 8×4, and

wherein the current block is horizontally divided based on the size (width×height) of the current block being 4×8.

6. The image decoding method of claim 1 , wherein the current block is divided into four sub-partition transform blocks in a horizontal or vertical direction based on a size (width×height) of the current block being greater than 4×8 or 8×4.

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

deriving, based on an intra prediction mode of a current block, prediction samples for a sub-partition transform block resulting from dividing the current block,

wherein the current block is divided into a plurality of sub-partition transform blocks based on an intra sub-partition mode in which intra prediction is performed on each of the sub-partition transform blocks in the current block;

deriving residual samples for the sub-partition transform block based on the prediction samples;

applying a primary transform to the residual samples to derive transform coefficients;

deriving modified transform coefficients for the sub-partition transform block based on a low-frequency non-separable transform (LFNST) set mapped to the intra prediction mode and an LFNST matrix included in the LFNST set; and

encoding image information including LFNST index information related to the LFNST matrix and intra prediction mode information related to the intra prediction mode,

wherein the LFNST index information is encoded into a bitstream based on a width and a height of the sub-partition transform block being greater than or equal to 4.

8. The image encoding method of claim 7 , wherein the LFNST set and the LFNST matrix are applied to the sub-partition transform blocks.

9. The image encoding method of claim 7 , wherein the modified transform coefficients are derived using the LFNST matrix based on the width and the height of the sub-partition transform block being greater than or equal to 4.

10. The image encoding method of claim 7 , wherein the current block is vertically divided based on a size (width×height) of the current block being 8×4, and

wherein the current block is horizontally divided based on the size (width×height) of the current block being 4×8.

11. The image encoding method of claim 7 , wherein the current block is divided into four sub-partition transform blocks in a horizontal or vertical direction based on a size (width×height) of the current block being greater than 4×8 or 8×4.

12. A non-transitory computer-readable digital storage medium storing a bitstream generated by an image encoding method, the image encoding method comprising:

deriving, based on an intra prediction mode of a current block, prediction samples for a sub-partition transform block resulting from dividing the current block,

wherein the current block is divided into a plurality of sub-partition transform blocks based on an intra sub-partition mode in which intra prediction is performed on each of the sub-partition transform blocks in the current block;

deriving residual samples for the sub-partition transform block based on the prediction samples;

applying a primary transform to the residual samples to derive transform coefficients;

deriving modified transform coefficients for the sub-partition transform block based on a low-frequency non-separable transform (LFNST) set mapped to the intra prediction mode and an LFNST matrix included in the LFNST set; and

encoding image information including LFNST index information related to the LFNST matrix and intra prediction mode information related to the intra prediction mode,

wherein the LFNST index information is encoded into the bitstream based on a width and a height of the sub-partition transform block being greater than or equal to 4.