Method and apparatus for cross-component filtering

1. A method for video decoding in a decoder, comprising:

decoding coded information of a chroma coding block (CB) from a coded video bitstream, the coded information indicating that a cross-component filter is applied to the chroma CB, the coded information further indicating a chroma subsampling format and a chroma sample type that indicates a relative position of a chroma sample with respect to at least one luma sample in a corresponding luma CB;

determining a filter shape of the cross-component filter applied to the chroma CB based on at least one of the chroma subsampling format or the chroma sample type;

generating a first intermediate CB by applying a loop filter to the chroma CB;

generating a second intermediate CB by applying, to the corresponding luma CB, the cross-component filter applied to the chroma CB and having the determined filter shape; and

determining a filtered chroma CB based on the first intermediate CB and the second intermediate CB by combining the loop filtered chroma CB with the cross-component filtered luma CB, wherein

a number of filter coefficients of the cross-component filter is signaled in the coded video bitstream; and

the determining the filter shape includes determining the filter shape of the cross-component filter based on the number of filter coefficients and the at least one of the chroma subsampling format or the chroma sample type.

2. The method of claim 1 , wherein the chroma sample type is signaled in the coded video bitstream.

3. The method of claim 1 , wherein

the chroma subsampling format is 4:2:0;

the at least one luma sample includes four luma samples that are a top-left sample, a top-right sample, a bottom-left sample, and a bottom-right sample;

the chroma sample type is one of six chroma sample types 0-5 indicating six relative positions 0-5, respectively, and the six relative positions 0-5 of the chroma sample correspond to a left-center position between the top-left and the bottom-left samples, a center position of the four luma samples, a top left position co-located with the top-left sample, a top-center position between the top-left and the top-right samples, a bottom left position co-located with the bottom-left sample, and a bottom-center position between the bottom-left and the bottom-right samples, respectively; and

the determining the filter shape includes determining the filter shape of the cross-component filter based on the chroma sample type.

4. The method of claim 3 , wherein the coded video bitstream includes a cross-component linear model (CCLM) flag indicating that the chroma sample type is 0 or 2.

5. The method of claim 1 , wherein the cross-component filter is a cross-component adaptive loop filter (CC-ALF) and the loop filter is an adaptive loop filter (ALF).

6. The method of claim 1 , wherein a range of filter coefficients of the cross-component filter is less than or equal to K bits and K is a positive integer.

7. The method of claim 6 , wherein the filter coefficients of the cross-component filter are coded using fixed-length coding.

8. The method of claim 6 , further comprising:

shifting luma sample values of the corresponding luma CB to have a dynamic range of 8 bits based on the dynamic range of the luma sample values being larger than 8 bits, K being 8 bits, wherein

the generating the second intermediate CB includes applying the cross-component filter having the determined filter shape to the shifted luma sample values.

9. A method for video decoding in a decoder, comprising:

decoding coded information of a chroma coding block (CB) from a coded video bitstream, the coded information indicating that a cross-component filter is applied to the chroma CB based on a corresponding luma CB;

generating a down-sampled luma CB by applying a down-sampling filter to the corresponding luma CB, a chroma horizontal subsampling factor, and a chroma vertical subsampling factor between the chroma CB and the down-sampled luma CB being one;

generating a first intermediate CB by applying a loop filter to the chroma CB;

generating a second intermediate CB by applying, to the down-sampled luma CB, the cross-component filter applied to the chroma CB, a filter shape of the cross-component filter being independent of a chroma subsampling format and a chroma sample type of the chroma CB, the chroma sample type indicating a relative position of a chroma sample with respect to at least one luma sample in the corresponding luma CB; and

determining a filtered chroma CB based on the first intermediate CB and the second intermediate CB by combining the loop filtered chroma CB with the cross-component filtered down-sampled luma CB, wherein

a number of filter coefficients of the cross-component filter is signaled in the coded video bitstream; and

the filter shape of the cross-component filter is determined based on the number of filter coefficients.

10. The method of claim 9 , wherein the down-sampling filter corresponds to a filter applied to co-located luma samples in a CCLM mode.

11. The method of claim 9 , wherein the down-sampling filter is a {1, 2, 1; 1, 2, 1}/8 filter and the chroma subsampling format is 4:2:0.

12. The method of claim 9 , wherein the filter shape of the cross-component filter is one of a 7×7 diamond shape, a 7×7 square shape, a 5×5 diamond shape, a 5×5 square shape, a 3×3 diamond shape, and a 3×3 square shape.

13. An apparatus for video decoding, comprising:

processing circuitry configured to:

decode coded information of a chroma coding block (CB) from a coded video bitstream, the coded information indicating that a cross-component filter is applied to the chroma CB, the coded information further indicating a chroma subsampling format and a chroma sample type that indicates a relative position of a chroma sample with respect to at least one luma sample in a corresponding luma CB;

determine a filter shape of the cross-component filter applied to the chroma CB based on at least one of the chroma subsampling format or the chroma sample type;

generate a first intermediate CB by applying a loop filter to the chroma CB;

generate a second intermediate CB by applying, to the corresponding luma CB, the cross-component filter applied to the chroma CB and having the determined filter shape; and

determine a filtered chroma CB based on the first intermediate CB and the second intermediate CB by combining the loop filtered chroma CB with the cross-component filtered luma CB, wherein

a number of filter coefficients of the cross-component filter is signaled in the coded video bitstream; and

the filter shape of the cross-component filter is determined based on the number of filter coefficients and the at least one of the chroma subsampling format or the chroma sample type.

14. The apparatus of claim 13 , wherein the chroma sample type is signaled in the coded video bitstream.

15. The apparatus of claim 13 , wherein

the chroma subsampling format is 4:2:0;

the at least one luma sample includes four luma samples that are a top-left sample, a top-right sample, a bottom-left sample, and a bottom-right sample;

the chroma sample type is one of six chroma sample types 0-5 indicating six relative positions 0-5, respectively, and the six relative positions 0-5 of the chroma sample correspond to a left-center position between the top-left and the bottom-left samples, a center position of the four luma samples, a top left position co-located with the top-left sample, a top-center position between the top-left and the top-right samples, a bottom left position co-located with the bottom-left sample, and a bottom-center position between the bottom-left and the bottom-right samples, respectively; and

the processing circuitry is configured to determine the filter shape of the cross-component filter based on the chroma sample type.

16. The apparatus of claim 15 , wherein the coded video bitstream includes a cross-component linear model (CCLM) flag indicating that the chroma sample type is 0 or 2.

17. The apparatus of claim 13 , wherein the cross-component filter is a cross-component adaptive loop filter (CC-ALF) and the loop filter is an adaptive loop filter (ALF).

18. The apparatus of claim 13 , wherein a range of filter coefficients of the cross-component filter is less than or equal to K bits and K is a positive integer.