Chroma block vector and chroma coding mode derivation

1. A method for video decoding, comprising:

receiving a bitstream of a current region in a picture, the current region including a luma coding region and a chroma coding region, coding information in the bitstream indicating that the luma coding region and the chroma coding region are partitioned using separate coding tree structures;

determining that a first luma block associated with a first position in a reconstructed luma area that is collocated with a current chroma block is predicted by a first coding mode using a first luma block vector (BV) and a second coding mode that is one of an intra prediction mode, a local illumination compensation (LIC) mode, and a SKIP mode, the first coding mode comprising one of (i) an intra block copy (IBC) mode and (ii) an intra template matching prediction (IntraTMP) mode, the reconstructed luma area being in the luma coding region, the current chroma block being in the chroma coding region;

deriving a chroma prediction mode of the current chroma block based on a combination of (i) the first coding mode that uses the first BV and (ii) the second coding mode of the first luma block, the second coding mode being the one of the intra prediction mode, the LIC mode, and the SKIP mode;

deriving a chroma BV of the current chroma block based on the first luma BV of the first luma block; and

reconstructing the current chroma block based on the chroma prediction mode that is the combination of the first coding mode that uses the first luma BC and the second coding mode that is the one of the intra prediction mode, the LIC mode, and the SKIP mode used to code the first luma block.

2. The method of claim 1 , wherein

the first luma block is coded with a combined intra block copy and intra prediction (IBC-CIIP) mode;

the second coding mode is the intra prediction mode; and

the reconstructing the current chroma block includes

determining a first prediction of the current chroma block using the IBC mode or the IntraTMP mode and the chroma BV;

determining a second prediction of the current chroma block using the intra prediction mode;

determining a chroma prediction of the current chroma block based on a weighted sum of the first prediction and the second prediction; and

reconstructing the current chroma block based on the chroma prediction.

3. The method of claim 1 , wherein

the first luma block is coded with an IBC with a geometry partitioning mode (IBC-GPM);

the second coding mode is the intra prediction mode; and

the reconstructing the current chroma block includes

determining a first prediction of a first sub-partition of the current chroma block using the IBC mode or the IntraTMP mode and the chroma BV, the current chroma block being divided into the first sub-partition and a second sub-partition using the GPM mode;

determining a second prediction of the second sub-partition of the current chroma block using the intra prediction mode; and

reconstructing the current chroma block based on the first prediction and the second prediction.

4. The method of claim 1 , wherein

the second coding mode is the LIC mode;

the first luma block is coded with an IBC with LIC (IBC-LIC) mode; and

the reconstructing the current chroma block includes

determining an initial prediction using the IBC mode or the IntraTMP mode and the chroma BV, the initial prediction being from a reference chroma block indicated by the chroma BV;

determining LIC parameters for the current chroma block based on a current template of the current chroma block and a reference template of the reference chroma block;

determining a prediction based on the initial prediction and the LIC parameters using the LIC mode; and

reconstructing the current chroma block based on the prediction.

5. The method of claim 1 , wherein

the second coding mode is the SKIP mode, and the first luma block is coded with an IBC-SKIP mode; and

the reconstructing the current chroma block includes

determining a prediction using the IBC mode or the IntraTMP mode and the chroma BV; and

reconstructing the current chroma block from the prediction using the SKIP mode and with no residual.

6. The method of claim 1 , wherein the deriving the chroma BV comprises:

deriving the chroma BV based on the first luma BV of the first luma block and a second luma BV of a second luma block, the second luma block being associated with a second position in the reconstructed luma area that is collocated with the current chroma block.

7. The method of claim 1 , wherein the first luma block is coded with a valid BV in the IBC mode or the IntraTMP mode.

8. The method of claim 1 , wherein the bitstream includes intra prediction mode information or IBC mode information indicating that (i) the chroma prediction mode of the current chroma block is derived from the combination of the first coding mode and the second coding mode of the first luma block or (ii) the chroma BV is derived from the first luma BV of the first luma block.

9. The method of claim 1 , wherein the deriving the chroma BV comprises:

deriving the chroma BV by scaling the first luma BV with a scaling factor that depends on a chroma subsampling format.

10. The method of claim 1 , wherein

the current region is a current coding tree unit (CTU),

the luma coding region is a luma coding tree block (CTB) in the current CTU, and

the chroma coding region is a chroma CTB in the current CTU.

11. The method of claim 1 , wherein a current coding tree unit (CTU) in the picture is split into four coding units using a quad-tree split, and the current region is a coding unit of the four coding units.

12. The method of claim 1 , wherein the chroma BV of the current chroma blocks is derived based on a plurality of luma BVs associated with respective pre-defined positions in the reconstructed luma area, the plurality of BVs including the first luma BV.

13. The method of claim 12 , wherein the chroma BV of the current chroma block is derived based on a weighted average of the plurality of luma BVs.

14. A method for video encoding, comprising:

determining that a first luma block associated with a first position in a reconstructed luma area is predicted by (i) a first coding mode using a luma block vector (BV) and (ii) a second coding mode that is one of an in intra prediction mode, a local illumination compensation (LIC) mode, and a SKIP mode, the first coding mode comprising one of (i) an intra block copy (IBC) mode and (ii) an intra template matching prediction (IntraTMP) mode, a current region including a luma coding region and a chroma coding region that are partitioned using separate coding tree structures, the luma coding region including the reconstructed luma area;

deriving a chroma prediction mode of a current chroma block in the chroma coding region based on a combination of (i) the first coding mode that uses the first luma BV and (ii) the second coding mode of the first luma block, the second coding mode being the one of the intra prediction mode, the LIC mode, and the SKIP mode, the current chroma block being collocated with the reconstructed luma area;

deriving a chroma BV of the current chroma block based on the first luma BV of the first luma block;

encoding the current chroma block based on the chroma prediction mode that is the combination of the first coding mode that uses the first luma BV and the second coding mode that is the one of the intra prediction mode, the LIC mode, and the SKIP mode used to code the first luma block; and

encoding coding information in a bitstream indicating that the luma coding region and the chroma coding region are partitioned using the separate coding tree structures.

15. The method of claim 14 , wherein

the first luma block is coded with a combined intra block copy and intra prediction (IBC-CIIP) mode;

the second coding mode is the intra prediction mode; and

the encoding the current chroma block includes

determining a first prediction of the current chroma block using the IBC mode or the IntraTMP mode and the chroma BV:

determining a second prediction of the current chroma block using the intra prediction mode;

determining a chroma prediction of the current chroma block based on a weighted sum of the first prediction and the second prediction; and

encoding the current chroma block based on the chroma prediction.

16. The method of claim 14 , wherein

the first luma block is coded with an IBC with a geometry partitioning mode (IBC-GPM);

the second coding mode is the intra prediction mode; and

the encoding the current chroma block includes

determining a first prediction of a first sub-partition of the current chroma block using the IBC mode or the IntraTMP mode and the chroma BC, the current chroma block being divided into the first sub-partition and a second sub-partition using the GPM mode;

determining a second prediction of the second sub-partition of the current chroma block using the intra prediction mode; and

encoding the current chroma block based on the first prediction and the second prediction.

17. The method of claim 14 , wherein

the second coding mode is the LIC mode;

the first luma block is coded with an IBC with LIC (IBC-LIC) mode; and

the encoding the current chroma block includes

determining an initial prediction using the IBC mode or the IntraTMP mode and the chroma BC, the initial prediction being from a reference chroma block indicated by the chroma BV;

determining LIC parameters for the current chroma block based on a current template of the current chroma block and a reference template of the reference chroma block;

determining a prediction based on the initial prediction and the LIC parameters using the LIC mode; and

encoding the current chroma block based on the prediction.

18. The method of claim 14 , wherein

the second coding mode is the SKIP mode, and the first luma block is coded with an IBC-SKIP mode; and

the encoding the current chroma block includes

determining a prediction using the IBC mode or the IntraTMP mode and the chroma BC; and

encoding the current chroma block from the prediction using the SKIP mode and with no residual.

19. The method of claim 14 , wherein the deriving the chroma BV comprises:

deriving the chroma BV based on the first luma BV of the first luma block and a second luma BV of a second luma block, the second luma block being associated with a second position in the reconstructed luma area that is collocated with the current chroma block.

20. A method of processing visual media data, the method comprising:

processing a bitstream of the visual media data according to a format rule, wherein

the bitstream includes coding information indicating that a luma coding region and a chroma coding region in a current region in a picture are partitioned using separate coding tree structures; and

the format rule specifies that:

a first luma block associated with a first position in a reconstructed luma area that is collocated with a current chroma block is predicted by a first coding mode using a first luma block vector (BV) and a second coding mode that is one of an intra prediction mode, a local illumination compensation (LIC) mode, and a SKIP mode, the first coding mode comprising one of (i) an intra block copy (IBC) mode and (ii) an intra template matching prediction (IntraTMP) mode, the reconstructed luma area being in the luma coding region, the current chroma block being in the chroma coding region;

a chroma prediction mode of the current chroma block is derived based on a combination of (i) the first coding mode that uses the first luma BV and (ii) the second coding mode of the first luma block, the second coding mode being the one of the intra prediction mode, the LIC mode, and the SKIP mode;

a chroma BV of the current chroma block is derived based on the first luma BV of the first luma block; and

the current chroma block is reconstructed based on the chroma prediction mode that is the combination of the first coding mode that uses the first luma BV and the second coding mode that is the one of the intra prediction mode, the LIC mode, and the SKIP mode used to code the first luma block.