Low frequency non-separable transform signaling in video coding

1. A method of video processing, comprising:

performing a conversion between a video block of a video and a bitstream of the video,

wherein the video block uses a low frequency non-separable transform for the conversion,

wherein the bitstream conforms to a format rule that specifies that a first syntax element is included at a sequence level for the bitstream,

wherein the first syntax element is indicative of whether explicit scaling matrices is enabled for blocks coded with the low frequency non-separable transform;

wherein the format rule further specifies that a third syntax element is included in a slice header in the bitstream to control a scaling process applied to transform coefficients or non-transform coefficients, and

wherein whether the first syntax element is included in a sequence parameter set (SPS) depending on a value of a second syntax element included in the SPS, wherein the first syntax element is sps_scaling_matrix_for_lfnst_disabled_flag and the second syntax element is sps_lfnst_enabled_flag.

2. The method of claim 1 , wherein the first syntax element is included in the SPS based on whether the low frequency non-separable transform is enabled.

3. The method of claim 1 , wherein when the value of the second syntax element included in the SPS is equal to 1, the first syntax element is included in the SPS, and

wherein the value of the second syntax element equal to 1 indicates that the low frequency non-separable transform is enabled.

4. The method of claim 1 , further comprising:

determining whether to apply an explicit scaling matrix which is from the explicit scaling matrices or a default scaling matrix to the video block based on a value of the first syntax element and whether the low frequency non-separable transform is enabled for the video block.

5. The method of claim 4 , wherein when the value of the first syntax element is equal to 1 which indicates that use of the explicit scaling matrices for blocks coded with the low frequency non-separable transform is disabled and the low frequency non-separable transform is enabled for the video block, the default scaling matrix is applied to the video block.

6. The method of claim 5 , wherein scaling factors of the default scaling matrix are equal to 16.

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

8. The method of claim 1 , wherein the performing the conversion comprises 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:

perform a conversion between a video block of a video and a bitstream of the video,

wherein the video block uses a low frequency non-separable transform for the conversion,

wherein the bitstream conforms to a format rule that specifies that a first syntax element is included at a sequence level for the bitstream,

wherein the first syntax element is indicative of whether explicit scaling matrices is enabled for blocks coded with the low frequency non-separable transform;

wherein the format rule further specifies that a third syntax element is included in a slice header in the bitstream a to control a scaling process applied to transform coefficients or non-transform coefficients, and

wherein whether the first syntax element is included in a sequence parameter set (SPS) depending on a value of a second syntax element included in the SPS, wherein the first syntax element is sps_scaling_matrix_for_lfnst_disabled_flag and the second syntax element is sps_lfnst_enabled_flag.

10. The apparatus of claim 9 , wherein the first syntax element is included in the SPS based on whether the low frequency non-separable transform is enabled.

11. The apparatus of claim 9 , wherein when the value of the second syntax element included in the SPS is equal to 1, the first syntax element is included in the SPS, and

wherein the value of the second syntax element equal to 1 indicates that the low frequency non-separable transform is enabled.

12. The apparatus of claim 9 , wherein the instructions further cause the processor to:

determine whether to apply an explicit scaling matrix which is from explicit scaling matrices or a default scaling matrix to the video block based on a value of the first syntax element and whether the low frequency non-separable transform is enabled for the video block,

wherein when the value of the first syntax element is equal to 1 which indicates that use of the explicit scaling matrices for blocks coded with the low frequency non-separable transform is disabled and the low frequency non-separable transform is enabled for the video block, the default scaling matrix is applied to the video block.

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

perform a conversion between a video block of a video and a bitstream of the video,

wherein the video block uses a low frequency non-separable transform for the conversion,

wherein the bitstream conforms to a format rule that specifies that a first syntax element is included at a sequence level for the bitstream,

wherein the first syntax element is indicative of whether explicit scaling matrices is enabled for blocks coded with the low frequency non-separable transform;

wherein the format rule further specifies that a third syntax element is included in a slice header in the bitstream to control a scaling process applied to transform coefficients or non-transform coefficients, and

wherein whether the first syntax element is included in a sequence parameter set (SPS) depending on a value of a second syntax element included in the SPS, wherein the first syntax element is sps_scaling_matrix_for_lfnst_disabled_flag and the second syntax element is sps_lfnst_enabled_flag.

14. The non-transitory computer-readable storage medium of claim 13 , wherein the instructions further cause the processor to:

determine whether to apply an explicit scaling matrix which is from explicit scaling matrices or a default scaling matrix to the video block based on a value of the first syntax element and whether the low frequency non-separable transform is enabled for the video block,

wherein when the value of the first syntax element is equal to 1 which indicates that use of the explicit scaling matrices for blocks coded with the low frequency non-separable transform is disabled and the low frequency non-separable transform is enabled for the video block, the default scaling matrix is applied to the video block.

15. 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:

generating the bitstream of the video from a video block of the video,

wherein the video block uses a low frequency non-separable transform for generating the bitstream,

wherein the bitstream conforms to a format rule that specifies that a first syntax element is included at a sequence level for the bitstream,

wherein the first syntax element is indicative of whether explicit scaling matrices is enabled for blocks coded with the low frequency non-separable transform;

wherein the format rule further specifies that a third syntax element is included in a slice header in the bitstream to control a scaling process applied to transform coefficients or non-transform coefficients, and

wherein whether the first syntax element is included in a sequence parameter set (SPS) depending on a value of a second syntax element included in the SPS, wherein the first syntax element is sps_scaling_matrix_for_lfnst_disabled_flag and the second syntax element is sps_lfnst_enabled_flag.

16. The non-transitory computer-readable recording medium of claim 15 , wherein the method further comprises:

determining whether to apply an explicit scaling matrix which is from the explicit scaling matrices or a default scaling matrix to the video block based on a value of the first syntax element and whether the low frequency non-separable transform is enabled for the video block,

wherein when the value of the first syntax element is equal to 1 which indicates that use of the explicit scaling matrices for blocks coded with the low frequency non-separable transform is disabled and the low frequency non-separable transform is enabled for the video block, the default scaling matrix is applied to the video block.

17. The non-transitory computer-readable storage medium of claim 13 , wherein the first syntax element is included in the SPS based on whether the low frequency non-separable transform is enabled, and when the value of the second syntax element included in the SPS is equal to 1, the first syntax element is included in the SPS, and wherein the value of the second syntax element equal to 1 indicates that the low frequency non-separable transform is enabled.

18. The non-transitory computer-readable recording medium of claim 15 , wherein the first syntax element is included in the SPS based on whether the low frequency non-separable transform is enabled, and when the value of the second syntax element included in the SPS is equal to 1, the first syntax element is included in the SPS, and wherein the value of the second syntax element equal to 1 indicates that the low frequency non-separable transform is enabled.