Use of secondary transform in coded video

1. A method of processing video data, comprising:

determining, for a conversion between a current block of a video and a bitstream of the video, that a secondary transform is applicable to the current block, wherein the secondary transform comprises at least one of a forward secondary transform and an inverse secondary transform, wherein the forward secondary transform is performed between a forward primary transform and a quantization, and the inverse secondary transform is performed between a de-quantization and an inverse primary transform;

determining, in response to a dimension of the current block satisfying a first condition, that the secondary transform with an 8×8 secondary transform size is applicable to a first single top-left sub-block of the current block with a dimension of 8×8, and wherein the first condition requires that the dimension of the current block is W1×H1, and wherein H1≥8 and W1≥8;

determining, in response to the dimension of the current block satisfying a second condition, that the secondary transform with a 4×4 secondary transform size is applicable to a second single top-left sub-block of the current block with a dimension of 4×4 and that no secondary transform is applied to a sub-block having a dimension of 4×4 and adjacent to the second single top-left sub-block, and wherein the second condition requires that the dimension of the current block is 4×H1 or W1×4, wherein H1>8 and W1>8; and

performing the conversion based on the determining,

wherein in response to a block being coded with a non intra prediction mode, the secondary transform is not applied to the block.

2. The method of claim 1 , wherein a matrix for the secondary transform is selected from four transform sets, and each of the four transform sets consists of two transform matrices.

3. The method of claim 2 , wherein in response to the current block being a chroma block and one of three cross-component linear model intra prediction modes being used for the current block, transform set 0 is selected for the current block.

4. The method of claim 1 , wherein whether to apply the secondary transform depends on a coding mode of a block.

5. The method of claim 1 , wherein in response to a block being coded with a transform skip mode, the secondary transform is not applied to the block.

6. The method of claim 1 , wherein in response to the secondary transform not being applied to a block, syntax elements to indicate information related the secondary transform in the block is not included in the bitstream.

7. The method of claim 1 , wherein the conversion includes encoding the video into the bitstream.

8. The method of claim 1 , wherein the conversion includes decoding the video from the bitstream.

9. An apparatus for processing video data comprising a processor and a non-transitory memory with instructions thereon, wherein the instructions upon execution by the processor, cause the processor to:

determine, for a conversion between a current block of a video and a bitstream of the video, that a secondary transform is applicable to the current block, wherein the secondary transform comprises at least one of a forward secondary transform and an inverse secondary transform, wherein the forward secondary transform is performed between a forward primary transform and a quantization, and the inverse secondary transform is performed between a de-quantization and an inverse primary transform;

determine, in response to a dimension of the current block satisfying at least one a first condition, that the secondary transform with an 8×8 secondary transform size is applicable to a first single top-left sub-block of the current block with a dimension of 8×8, and wherein the first condition requires that the dimension of the current block is W1×H1, and wherein H1≥8 and W1≥8;

determine, in response to the dimension of the current block satisfying a second condition, that the secondary transform with a 4×4 secondary transform size is applicable to a second single top-left sub-block of the current block with a dimension of 4×4 and that no secondary transform is applied to a sub-block having a dimension of 4×4 and adjacent to the second single top-left sub-block, and wherein the second condition requires that the dimension of the current block is 4×H1 or W1×4, wherein H1>8 and W1>8; and

perform the conversion based on the determining,

wherein in response to a block being coded with a non intra prediction mode, the secondary transform is not applied to the block.

10. The apparatus of claim 9 , wherein a matrix for the secondary transform is selected from four transform sets, and each of the four transform sets consists of two transform matrices.

11. The apparatus of claim 10 , wherein in response to the current block being a chroma block and one of three cross-component linear model intra prediction modes being used for the current block, transform set 0 is selected for the current block.

12. The apparatus of claim 9 , wherein whether to apply the secondary transform depends on a coding mode of a block.

13. The apparatus of claim 9 , wherein in response to a block being coded with a transform skip mode, the secondary transform is not applied to the block.

14. The apparatus of claim 9 , wherein in response to the secondary transform not being applied to a block, syntax elements to indicate information related the secondary transform in the block is not included in the bitstream.

15. A non-transitory computer-readable storage medium storing instructions that cause a processor to:

determine, for a conversion between a current block of a video and a bitstream of the video, that a secondary transform is applicable to the current block, wherein the secondary transform comprises at least one of a forward secondary transform and an inverse secondary transform, wherein the forward secondary transform is performed between a forward primary transform and a quantization, and the inverse secondary transform is performed between a de-quantization and an inverse primary transform;

determine, in response to a dimension of the current block satisfying a first condition, that the secondary transform with an 8×8 secondary transform size is applicable to a first single top-left sub-block of the current block with a dimension of 8×8, and wherein the first condition requires that the dimension of the current block is W1×H1, and wherein H1≥8 and W1≥8;

determine, in response to the dimension of the current block satisfying a second condition, that the secondary transform with a 4×4 secondary transform size is applicable to a second single top-left sub-block of the current block with a dimension of 4×4 and that no secondary transform is applied to a sub-block having a dimension of 4×4 and adjacent to the second single top-left sub-block, and wherein the second condition requires that the dimension of the current block is 4×H1 or W1×4, wherein H1>8 and W1>8; and

perform the conversion based on the determining,

wherein in response to a block being coded with a non intra prediction mode, the secondary transform is not applied to the block.

16. The non-transitory computer-readable storage medium of claim 15 , wherein a matrix for the secondary transform is selected from four transform sets, and each of the four transform sets consists of two transform matrices.

17. The non-transitory computer-readable storage medium of claim 16 , wherein in response to the current block being a chroma block and one of three cross-component linear model intra prediction modes being used for the current block, transform set 0 is selected for the current block.

18. A non-transitory computer-readable recording medium storing a bitstream of a video which is generated by a method performed by a video processing apparatus, wherein the method comprises:

determining that a secondary transform is applicable to a current block of a video, wherein the secondary transform comprises at least one of a forward secondary transform and an inverse secondary transform, wherein the forward secondary transform is performed between a forward primary transform and a quantization, and the inverse secondary transform is performed between a de-quantization and an inverse primary transform;

determining, in response to a dimension of the current block satisfying a first condition, that the secondary transform with an 8×8 secondary transform size is applicable to a first single top-left sub-block of the current block with a dimension of 8×8, and wherein the first condition requires that the dimension of the current block is W1×H1, and wherein H1≥8 and W1≥8;

determining, in response to the dimension of the current block satisfying a second condition, that the secondary transform with a 4×4 secondary transform size is applicable to a second single a top-left sub-block of the current block with a dimension of 4×4 and that no secondary transform is applied to a sub-block having a dimension of 4×4 and adjacent to the second single top-left sub-block, and wherein the second condition requires that the dimension of the current block is 4×H1 or W1×4, wherein H1>8 and W1>8; and

generating the bitstream of the video based on the determining,

wherein in response to a block being coded with a non intra prediction mode, the secondary transform is not applied to the block.

19. The non-transitory computer-readable recording medium of claim 18 , wherein a matrix for the secondary transform is selected from four transform sets, and each of the four transform sets consists of two transform matrices.

20. The non-transitory computer-readable recording medium of claim 19 , wherein in response to the current block being a chroma block and one of three cross-component linear model intra prediction modes being used for the current block, transform set 0 is selected for the current block.