Image coding method on basis of non-separable secondary transform and device therefor

1. A video decoding method performed by a decoding apparatus, comprising:

deriving transform coefficients of a target block from a bitstream;

deriving a Non-Separable Secondary Transform (NSST) index for the target block;

deriving residual samples of the target block by performing an inverse transform for the transform coefficients of the target block based on the NSST index; and

generating a reconstructed picture based on the residual samples,

wherein based on a value of the NSST index not being 0, the residual samples are derived by performing NSST for the transform coefficients located in a top-left target area of the target block based on a transform matrix indicated by the NSST index,

wherein a size of the transform matrix is RxN,

wherein the N is a sample number of the top-left target area, and

wherein the R is smaller than the N.

2. The method of claim 1 , wherein the step of deriving the residual samples of the target block by performing the inverse transform for the transform coefficients of the target block based on the NSST index includes:

based on a value of the NSST index being 0, performing core transform for the transform coefficients of the target block and deriving the residual samples; and

based on the value of the NSST index not being 0, deriving a modified transform coefficients by performing NSST for the transform coefficients located in the top-left target area of the target block based on the transform matrix indicated by the NSST index and deriving the residual samples by performing the core transform for the target block including the modified transform coefficients.

3. The method of claim 1 , wherein the step of deriving the NSST index for the target block, includes:

scanning R+1 th to N th transform coefficients among the transform coefficients of the target block; and

based on a non-zero transform coefficient being included in the R+1 th to N th transform coefficients, deriving the value of the NSST index as 0.

4. The method of claim 1 , wherein the step of deriving the NSST index for the target block, includes:

based on a non-zero transform coefficient not being included in the R+1 th to N th transform coefficients, deriving the value of the NSST index by parsing a syntax element for the NSST index included in the bitstream.

5. The method of claim 1 , wherein a range of the NSST index is derived based on a minimum width and a width of the target block, a minimum height and a height of the target block, and

wherein the minimum width and the minimum height are preconfigured.

6. The method of claim 1 , wherein a range of the NSST index is derived based on a specific value and a sample number of the target block, and

wherein the sample number is a value of multiplying a width and a height of the target block, and the specific value is preconfigured.

7. The method of claim 1 , wherein a range of the NSST index is derived based on whether the target block is a non-square block.

8. The method of claim 1 , based on the NSST index being used for the inverse transform for transform coefficients of a luma block of the target block, transform coefficients of a chroma Cb block and transform coefficients of a chroma Cr block, R+1 th to N th transform coefficients of the luma block, R+1 th to N th transform coefficients of the chroma Cb block and R+1 th to N th transform coefficients of the chroma Cr block are scanned, and

based on a non-zero transform coefficient being included in the scanned transform coefficients, the value of the NSST index is derived as 0.

9. The method of claim 1 , wherein an NSST index indicator for a target CTU including the target block is generated from the bitstream, and

wherein the NSST index indicator is related to whether NSST is enabled in the target CTU.

10. The video decoding method of claim 1 , wherein deriving the transform coefficients comprises decoding information regarding the transform coefficients of the target block received from the bitstream to derive the transform coefficients of the target block.

11. The video decoding method of claim 1 , wherein the NSST index is used for a plurality of components of the target block.

12. A non-transitory computer-readable storage medium storing encoded information causing a decoding apparatus to perform a video decoding method, the method comprising:

deriving transform coefficients of a target block from a bitstream;

deriving a Non-Separable Secondary Transform (NSST) index for the target block;

deriving residual samples of the target block by performing an inverse transforming for the transform coefficients of the target block based on the NSST index; and

generating a reconstructed picture based on the residual samples,

wherein, based on a value of the NSST index not being 0, the residual samples are derived by performing NSST for the transform coefficients located in a top-left target area of the target block based on a transform matrix indicated by the NSST index,

wherein a size of the transform matrix is RxN,

wherein the N is a sample number of the top-left target area, and

wherein the R is smaller than the N.

13. The non-transitory computer-readable storage medium of claim 12 , wherein deriving the transform coefficients comprises decoding information regarding the transform coefficients of the target block received from the bitstream to derive the transform coefficients of the target block.

14. The non-transitory computer-readable storage medium of claim 13 , wherein the information is residual information.

15. The non-transitory computer-readable storage medium of claim 12 , wherein the NSST index is used for a plurality of components of the target block.

16. A video encoding method performed by an encoding apparatus, comprising:

deriving residual samples of a target block;

deriving transform coefficients of the target block by performing transform for the residual samples;

determining whether to encode a Non-Separable Secondary Transform (NSST) index for the transform coefficients; and

encoding information for the transform coefficients,

wherein based on a value of the NSST index not being 0, the transform coefficients are derived by performing the transform based on NSST for coefficients located in a top-left target area of the target block based on a transform matrix indicated by the NSST index,

wherein a size of the transform matrix is RxN,

wherein the N is a sample number of the top-left target area, and

wherein the R is smaller than the N.

17. The method of claim 16 , wherein the step of determining whether to encode the NSST index further includes:

based on the non-zero transform coefficient being not included in the R+1 th to N th transform coefficients, determining to encode the NSST index,

based on the non-zero transform coefficient being not included in the R+1 th to N th transform coefficients, the information for the transform coefficient includes a syntax element for the NSST index.

18. The method of claim 16 , wherein the step of deriving the transform coefficients of the target block by performing the transform for the residual samples includes:

determining whether to apply NSST for the target block;

based on the NSST being applied to the target block, deriving modified transform coefficients by performing core transform for the residual samples and deriving the transform coefficients of the target block by performing the NSST for the modified transform coefficients located in a top-left target area of the target block based on a transform matrix; and

based on the NSST being not applied to the target block, derive the transform coefficients of the target block by performing the core transform for the residual samples.

19. The method of claim 16 , wherein a range of the NSST index is derived based on a minimum width and a width of the target block, a minimum height and a height of the target block, and

wherein the minimum width and the minimum height are preconfigured.

20. The video decoding method of claim 10 , wherein the information is residual information.