Image coding method on basis of transform, and apparatus therefor

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

receiving a bitstream including residual information;

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; and

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

wherein the deriving the modified transform coefficients comprises:

determining whether or not to parse an LFNST index based on whether or not a width and a height of the current block satisfy a condition that the LFNST is applied,

wherein whether or not the condition that the LFNST is applied is satisfied is determined based on a tree-type of the current block, a color format of the current block, and whether or not an intra sub partitions (ISP) is applied to the current block, and

wherein in case the ISP is applied, based on the width and the height of the current block corresponding to a subpartition block being equal to 4 or more, the LFNST index is parsed.

2. The method of claim 1 ,

wherein in case the tree-type of the current block is a dual-tree chroma, based on a width and a height of a chroma component block of the current block being equal to 4 or more, the LFNST index is parsed.

3. The method of claim 1 ,

wherein in case the tree-type of the current block is a single-tree or dual-tree luma, based on a width and a height of a luma component block of the current block being equal to 4 or more, the LFNST index is parsed.

4. The method of claim 1 ,

wherein case the tree-type of the current block is a dual-tree luma or single tree, based on a width and a height of a luma component block of the current block corresponding to the subpartition block are equal to 4 or more, the LFNST index is parsed.

5. The method of claim 1 ,

wherein the current block is a coding unit, and

wherein based on a width and a height of the coding unit being equal to or less than a maximum luma transform size that is available for transform, the LFNST index is parsed.

6. An image encoding method performed by an 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 for the residual samples;

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

encoding residual information on the modified transform coefficients, and an LFNST index related to an LFNST matrix that is applied to the LFNST,

wherein the LFNST index is encoded based on whether or not a width and a height of the current block satisfy a condition that the LFNST is applied,

wherein whether or not the condition that the LFNST is applied is satisfied is determined based on a tree-type of the current block, a color format of the current block, and whether or not an intra sub partitions (ISP) is applied to the current block, and

wherein in case the ISP is applied, based on the width and the height of the current block corresponding to a subpartition block being equal to 4 or more, the LFNST index is encoded.

7. The method of claim 6 ,

wherein in case the tree-type of the current block is a dual-tree chroma, based on a width and a height of a chroma component block of the current block being equal to 4 or more, the LFNST index is encoded.

8. The method of claim 6 ,

wherein in case the tree-type of the current block is a single-tree or dual-tree luma, based on a width and a height of a luma component block of the current block being equal to 4 or more, the LFNST index is encoded.

9. The method of claim 6 ,

wherein in case the tree-type of the current block is a dual-tree luma or single-tree, based on a width and a height of a luma component block of the current block corresponding to the subpartition block are equal to 4 or more, the LFNST index is encoded.

10. The method of claim 2 ,

wherein the current block is a coding unit, and

wherein based on a width and a height of the coding unit being equal to or less than a maximum luma transform size that is available for transform, the LFNST index is encoded.

11. A non-transitory computer-readable digital storage medium storing a bitstream generated by an image encoding 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 for the residual samples,

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

encoding residual information on the modified transform coefficients, and an LFNST index related to an LFNST matrix that is applied to the LFNST,

wherein the LFNST index is encoded based on whether or not a width and a height of the current block satisfy a condition that the LFNST is applied,

wherein whether or not the condition that the LFNST is applied is satisfied is determined based on a tree-type of the current block, a color format of the current block, and whether or not an intra sub partitions (ISP) is applied to the current block, and

wherein in case the ISP is applied, based on the width and the height of the current block corresponding to a subpartition block being equal to 4 or more, the LFNST index is encoded.