Clipping parameter derivation in adaptive loop filter

1. A method of processing video data, comprising:

determining, that a non-linear filtering operation is applied for a current video region of a video;

determining, for the current video region, a clipping index syntax element of a clipping operation that is part of the non-linear filtering operation; and

performing, based on the clipping index syntax element, a conversion between the current video region and a bitstream of the video,

wherein at least one first filtering index is derived,

wherein a first clipping parameter set is derived based on the at least one first filtering index and the clipping index syntax element, and

wherein the clipping index syntax element is coded with a fixed length of N bits in the bitstream, and wherein N is equal to 2.

2. The method of claim 1 , wherein a first filtering coefficient set is derived based on the at least one first filtering index and a coefficient parameter syntax element.

3. The method of claim 2 , wherein a value of the clipping index syntax element is determined independently from the first filtering coefficient set.

4. The method of claim 3 , wherein in a case that values of the first filtering coefficient set are zero, the clipping index syntax element is included in the bitstream.

5. The method of claim 1 , wherein the clipping index syntax element is included as a field in an adaptive parameter set.

6. The method of claim 1 , wherein the at least one first filtering index is derived based on multiple sample differences in different directions.

7. The method of claim 6 , wherein the current video region is split into multiple M×M video sub-regions, and wherein the multiple sample differences in different directions are derived for every M×M video sub-region.

8. The method of claim 7 , wherein M is equal to 2 or 4.

9. The method of claim 7 , wherein the multiple sample differences in different directions are derived based on a 1: N subsampling rate, and wherein N is greater than 1.

10. The method of claim 1 , wherein the current video region is a coding tree block or a slice.

11. The method of claim 1 , wherein the conversion includes encoding the current video region into the bitstream.

12. The method of claim 1 , wherein the conversion includes decoding the current video region from the bitstream.

13. 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, that a non-linear filtering operation is applied for a current video region of a video;

determine, for the current video region, a clipping index syntax element of a clipping operation that is part of the non-linear filtering operation; and

perform, based on the clipping index syntax element, a conversion between the current video region and a bitstream of the video,

wherein at least one first filtering index is derived,

wherein a first clipping parameter set is derived based on the at least one first filtering index and the clipping index syntax element, and

wherein the clipping index syntax element is coded with a fixed length of N bits in the bitstream, wherein N is equal to 2.

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

determining, that a non-linear filtering operation is applied for a current video region of a video;

determining, for the current video region, a clipping index syntax element of a clipping operation that is part of the non-linear filtering operation; and

performing, based on the clipping index syntax element, a conversion between the current video region and a bitstream of the video,

wherein at least one first filtering index is derived,

wherein a first clipping parameter set is derived based on the at least one first filtering index and the clipping index syntax element, and

wherein the clipping index syntax element is coded with a fixed length of N bits in the bitstream, wherein N is equal to 2.

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:

determining, that a non-linear filtering operation is applied for a current video region of a video;

determining, for the current video region, a clipping index syntax element of a clipping operation that is part of the non-linear filtering operation; and

generating, based on the clipping index syntax element for the current video region, a bitstream of the video,

wherein at least one first filtering index is derived,

wherein a first clipping parameter set is derived based on the at least one first filtering index and the clipping index syntax element, and

wherein the clipping index syntax element are coded with a fixed length of N bits in the bitstream, wherein N is equal to 2.

16. The apparatus of claim 13 , wherein a first filtering coefficient set is derived based on the at least one first filtering index and a coefficient parameter syntax element.

17. The apparatus of claim 16 , wherein a value of the clipping index syntax element is determined independently from the first filtering coefficient set.

18. The non-transitory computer-readable storage medium of claim 14 , wherein a first filtering coefficient set is derived based on the at least one first filtering index and a coefficient parameter syntax element.

19. The non-transitory computer-readable storage medium of claim 18 , wherein a value of the clipping index syntax element is determined independently from the first filtering coefficient set.

20. The non-transitory computer-readable recording medium of claim 15 , wherein a first filtering coefficient set is derived based on the at least one first filtering index and a coefficient parameter syntax element, and a value of the clipping index syntax element is determined independently from the first filtering coefficient set.