Video signal processing method and apparatus using secondary transform

1. A video signal decoding apparatus, comprising a processor,

wherein the processor is configured to:

parse a syntax element related to a secondary transform of a coding unit from a bitstream of a video signal based on whether a prediction method of the coding unit is MIP (Matrix based Intra Prediction), wherein the syntax element related to the secondary transform is parsed when one or more preset conditions are satisfied;

check whether or not the secondary transform is applied to a transform block included in the coding unit based on the parsed syntax element;

obtain one or more inverse transform coefficients for a first sub-block by performing an inverse secondary transform based on one or more coefficients of the first sub-block which is one of one or more sub-blocks constituting the transform block when the secondary transform is applied to the transform block; and

obtain a residual sample for the transform block by performing an inverse primary transform based on the one or more inverse transform coefficients,

wherein the secondary transform is low frequency non-separable transform (LFNST),

wherein the transform block is a block to which a primary transform that is separable into a vertical transform and a horizontal transform is applied,

wherein the coding unit is composed of a plurality of coding blocks,

wherein transform blocks corresponding to each of the plurality of coding blocks are composed of a luma transform block, a chroma transform block of Cb color component, and a chroma transform block of Cr color component,

wherein the syntax element related to the secondary transform is parsed when at least one of the luma transform block, the chroma transform block of Cb color component, and the chroma transform block of Cr color component satisfies the one or more preset conditions, and

wherein a first condition of the one or more preset conditions is that an index value indicating a position of a first coefficient among the one or more coefficients of the first sub-block is greater than a preset threshold value.

2. The video signal decoding apparatus of claim 1 ,

wherein the syntax element includes information indicating whether or not the secondary transform is applied to the coding unit and information indicating a transform kernel used for the secondary transform.

3. The video signal decoding apparatus of claim 1 ,

wherein the first coefficient is a last significant coefficient among one or more significant coefficients according to a preset scan order, and

wherein the one or more significant coefficients are a non-zero coefficient.

4. The video signal decoding apparatus of claim 1 ,

wherein the first sub-block is a first sub-block according to a preset scan order.

5. The video signal decoding apparatus of claim 1 ,

wherein a second condition of the one or more preset conditions is that a width and height of the transform block are 4 pixels or more.

6. The video signal decoding apparatus of claim 1 ,

wherein the preset threshold value is 0.

7. The video signal decoding apparatus of claim 3 ,

wherein the preset scan order is an up-right diagonal scan order.

8. The video signal decoding apparatus of claim 1 ,

wherein a third condition of the one or more preset conditions is that a value of a transform skip flag included in the bitstream is not a specific value, and

wherein the transform skip flag indicates that the primary transform and the secondary transform are not applied to the transform block when the value of the transform skip flag has the specific value.

9. The video signal decoding apparatus of claim 1 ,

wherein a fourth condition of the one or more preset conditions is that at least one coefficient of the one or more coefficients of the first sub-block is not 0, and

wherein the at least one coefficient is present in a place other than a first position according to a preset scan order.

10. The video signal decoding apparatus of claim 1 ,

wherein when the secondary transform is applied, a last significant coefficient in a scan order is present only in the first sub-block of the transform block.

11. A video signal encoding apparatus, comprising a processor,

wherein the processor is configured to:

obtain a plurality of primary transform coefficients for a block by performing a primary transform on a residual sample of the block included in a coding unit;

obtain one or more secondary transform coefficients for a first sub-block which is one of sub-blocks constituting the block by performing a secondary transform based on one or more of the plurality of primary transform coefficients; and

obtain a bitstream by encoding information for the one or more secondary transform coefficients and a syntax element related to the secondary transform of the coding unit, and

wherein the secondary transform is low frequency non-separable transform (LFNST),

wherein the primary transform is separable into a vertical transform and a horizontal transform,

wherein the syntax element related to the secondary transform is encoded based on whether a prediction method of the coding unit is MIP (Matrix based Intra Prediction),

wherein the coding unit is composed of a plurality of coding blocks,

wherein transform blocks corresponding to each of the plurality of coding blocks are composed of a luma transform block, a chroma transform block of Cb color component, and a chroma transform block of Cr color component,

wherein the syntax element related to the secondary transform is encoded when at least one of the luma transform block, the chroma transform block of Cb color component, and the chroma transform block of Cr color component satisfies one or more preset conditions, and

wherein a first condition of the one or more preset conditions is that an index value indicating a position of a first coefficient of the one or more secondary transform coefficients is greater than a preset threshold value.

12. The video signal encoding apparatus of claim 11 ,

wherein the syntax element includes information indicating whether or not the secondary transform is applied to the coding unit and information indicating a transform kernel used for the secondary transform.

13. The video signal encoding apparatus of claim 11 ,

wherein the first coefficient is a last significant coefficient among one or more significant coefficients according to a preset scan order, and

wherein the one or more significant coefficients are a non-zero coefficient.

14. The video signal encoding apparatus of claim 11 ,

wherein the first sub-block is a first sub-block according to a preset scan order.

15. The video signal encoding apparatus of claim 11 ,

wherein a second condition of the one or more preset conditions is that a width and height of the primary transform block are 4 pixels or more.

16. The video signal encoding apparatus of claim 11 ,

wherein the preset threshold value is 0.

17. The video signal encoding apparatus of claim 13 ,

wherein the preset scan order is an up-right diagonal scan order.

18. The video signal encoding apparatus of claim 11 ,

wherein a third condition of the one or more preset conditions is that a value of a transform skip flag included in the bitstream is not a specific value, and

wherein the transform skip flag indicates that the primary transform and the secondary transform are not applied to the block when the transform skip flag value has the specific value.

19. The video signal encoding apparatus of claim 11 ,

wherein a fourth condition of the one or more preset conditions is that at least one coefficient of the one or more secondary transform coefficients is not 0, and

wherein the at least one coefficient is present in a place other than a first position according to a preset scan order.

20. A non-transitory computer-readable medium storing a bitstream, the bitstream being decoded by a decoding method, wherein the decoding method comprises:

parsing a syntax element related to a secondary transform of a coding unit from a bitstream of a video signal based on whether a prediction method of the coding unit is MIP (Matrix based Intra Prediction), wherein the syntax element related to the secondary transform is parsed when one or more preset conditions are satisfied;

checking whether or not the secondary transform is applied to a transform block included in the coding unit based on the parsed syntax element;

obtaining one or more inverse transform coefficients for a first sub-block by performing an inverse secondary transform based on one or more coefficients of the first sub-block which is one of one or more sub-blocks constituting the transform block when the secondary transform is applied to the transform block; and

obtaining a residual sample for the transform block by performing an inverse primary transform based on the one or more inverse transform coefficients,

wherein the secondary transform is low frequency non-separable transform (LFNST),

wherein the transform block is a block to which a primary transform that is separable into a vertical transform and a horizontal transform is applied,

wherein the coding unit is composed of a plurality of coding blocks,

wherein transform blocks corresponding to each of the plurality of coding blocks are composed of a luma transform block, a chroma transform block of Cb color component, and a chroma transform block of Cr color component,

wherein the syntax element related to the secondary transform is parsed when at least one of the luma transform block, the chroma transform block of Cb color component, and the chroma transform block of Cr color component satisfies the one or more preset conditions, and wherein a first condition of the one or more preset conditions is that an index value indicating a position of a first coefficient among the one or more coefficients of the first sub-block is greater than a preset threshold value.