Method and apparatus for cross-component filtering

1. A method for video decoding, comprising:

generating first reconstructed samples of a current block in a current frame;

obtaining reconstructed samples in a temporal source frame that is different from the current frame;

applying a filter to at least the reconstructed samples in the temporal source frame to generate offsets; and

combining the first reconstructed samples of the current block with the generated offsets to generate second reconstructed samples of the current block,

wherein the temporal source frame is an inter coded frame that satisfies at least one of:

a quantization parameter (QP) requirement that requires the inter coded frame to have a lower QP value than the current frame;

a temporal distance requirement that requires a temporal distance between the inter coded frame and the current frame is less than a threshold; and

a reference picture requirement that requires the inter coded frame to be a reference picture that is used in motion compensation during a reconstruction of the current frame.

2. The method of claim 1 , further comprising at least one of:

applying the filter to a luma component of the temporal source frame to generate the offsets;

applying the filter to a chroma component of the temporal source frame to generate the offsets; and

applying the filter to a weighted sum of the luma component and the chroma component of the temporal source frame to generate the offsets.

3. The method of claim 1 , further comprising:

applying the filter to a first color component of the temporal source frame to generate the offsets; and

combining a second color component of the first reconstructed samples with the generated offsets.

4. The method of claim 1 , further comprising:

identifying motion information of the current block;

determining, in the temporal source frame, a corresponding block to the current block based on the identified motion information; and

applying the filter to at least reconstructed samples of the determined corresponding block to generate the offsets.

5. The method of claim 4 , further comprising at least one of:

deriving the motion information as a motion vector used for motion compensation of the current block;

performing a motion estimation process to identify the motion information; and

performing the motion estimation process with the motion vector used for motion compensation of the current block as a starting point to identify the motion information.

6. The method of claim 4 , further comprising at least one of:

performing a motion estimation process using a luma component of the current block and a luma component of the temporal source frame to identify the motion information;

performing the motion estimation process using a current color component of the current block and a current color component of the temporal source frame to identify the motion information; and

performing the motion estimation process using two or more color components of the current block and the two or more color components of the temporal source frame to identify the motion information.

7. The method of claim 4 , further comprising:

applying the filter to a weighed sum of the reconstructed samples of the corresponding block and the first reconstructed samples of the current block to generate the offsets.

8. The method of claim 1 , further comprising:

decoding prediction information of the current block from a coded video bitstream, the prediction information being indicative of characteristics of the filter.

9. The method of claim 8 , further comprising at least one of:

determining inputs of the filter based on a flag decoded from the coded video bitstream;

determining a weighting factor based on a weight index decoded from the coded video bitstream; and

determining the weighting factor based on quantization parameter values of the current block in the current frame and a corresponding block of the current block in the temporal source frame.

10. The method of claim 1 , wherein the filter includes at least one of a cross component filter (CCF), an adaptive loop filter (ALF), a loop restoration filter, and an in loop constrained directional enhanced filter (CDEF).

11. The method of claim 1 , further comprising:

determining filter coefficients based on at least the reconstructed samples in the temporal source frame; and

encoding information of the determined filter coefficients in a coded video bitstream.

12. An apparatus for video processing, comprising:

processing circuitry configured to:

generate first reconstructed samples of a current block in a current frame;

obtain reconstructed samples in a temporal source frame that is different from the current frame;

apply a filter to at least the reconstructed samples in the temporal source frame to generate offsets; and

combine the first reconstructed samples of the current block with the generated offsets to generate second reconstructed samples of the current block,

wherein the temporal source frame is an inter coded frame that satisfies at least one of:

a quantization parameter (QP) requirement that requires the inter coded frame to have a lower QP value than the current frame;

a temporal distance requirement that requires a temporal distance between the inter coded frame and the current frame is less than a threshold; and

a reference picture requirement that requires the inter coded frame to be a reference picture that is used in motion compensation during a reconstruction of the current frame.

13. The apparatus of claim 12 , wherein the processing circuitry is further configured to:

apply the filter to at least one of:

a luma component of an intra coded frame having a lower quantization parameter value than the current frame;

a chroma component of the intra coded frame having a lower quantization parameter value than the current frame; and

a weighted sum of the luma component and the chroma component of the intra coded frame having a lower quantization parameter value than the current frame.

14. The apparatus of claim 12 , wherein the processing circuitry is further configured to:

apply the filter to a first color component of the temporal source frame to generate the offsets; and

combine a second color component of the first reconstructed samples with the generated offsets to generate a second color component of the second reconstructed samples of the current block.

15. The apparatus of claim 12 , wherein the processing circuitry is further configured to:

identify motion information of the current block;

determine, in the temporal source frame, a corresponding block to the current block based on the identified motion information; and

apply the filter to at least reconstructed samples of the determined corresponding block to generate the offsets.

16. The apparatus of claim 15 , wherein the processing circuitry is further configured to perform at least one of:

deriving the motion information as a motion vector used for motion compensation of the current block;

a motion estimation process to identify the motion information; and

the motion estimation process with the motion vector used for motion compensation of the current block as a starting point to identify the motion information.

17. The apparatus of claim 15 , wherein the processing circuitry is further configured to perform at least one of:

a motion estimation process using a lama component of the current block and a lima component of the temporal source frame to identify the motion information;

the motion estimation process using a current color component of the current block and a current color component of the temporal source frame to identify the motion information; and

the motion estimation process using two or more color components of the current block and two or more color components of the temporal source frame to identify the motion information.