Cross-component IPM derivation

1. A method of processing video data, comprising:

determining, for a conversion between a current coding unit of a video and a bitstream of the video, that a dimension of a luma block meet a certain condition, wherein the current coding unit includes the luma block and a chroma block;

determining to apply a partition operation only on the luma block but not on the chroma block based on partition parameters of the current coding unit;

determining a first intra-prediction mode for the unpartitioned chroma block based on a second intra-prediction mode for sub-regions obtained from the partition operation on the luma block; and

performing the conversion based on above determinations.

2. The method of claim 1 , wherein the first intra-prediction mode is derived as an intra-prediction mode for one sub-region of the luma block.

3. The method of claim 1 , wherein the first intra-prediction mode is derived based on an intra-prediction mode for a sub-region of the luma block at a predefined position.

4. The method of claim 3 , wherein the predefined position is one of a top-left position (TL) of the luma block, a top-right position (TR) of the luma block, or a bottom-left position (BL) of the luma block.

5. The method of claim 1 , wherein the first intra-prediction mode is coded separately from the second intra-prediction mode.

6. The method of claim 1 , wherein a coding of the first intra-prediction mode depends on a coding of the second intra-prediction mode.

7. The method of claim 1 , further comprising:

applying, to the luma block, at least one of a binary tree (BT) partitioning process, a ternary tree (TT) partitioning process or a quad tree (QT) partitioning process in a case that to apply the partition operation only on the luma block.

8. The method of claim 7 , wherein the BT partitioning process splits the luma block into two equally sized sub-regions, the TT partitioning process splits the luma block into three sub-regions, and the QT partitioning process splits the luma block into four equally sized sub-regions.

9. The method of claim 1 , wherein the dimension of the luma block meet the certain condition in a case that the luma block have a dimension 8×8.

10. The method of claim 1 , wherein the dimension of the luma block meet the certain condition in a case that the luma block have a dimension 4×8 or 8×4.

11. The method of claim 1 , wherein the chroma block is a Cb block or a Cr block.

12. The method of claim 1 , wherein each sub-region of the sub-regions of the luma block is a leaf node which could be a coding unit (CU) or a coding block.

13. The method of claim 1 , wherein the conversion includes encoding the current coding unit into the bitstream.

14. The method of claim 1 , wherein the conversion includes decoding the current coding unit from the bitstream.

15. 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, for a conversion between a current coding unit of a video and a bitstream of the video, that a dimension of a luma block meet a certain condition, wherein the current coding unit includes the luma block and a chroma block;

determine to apply a partition operation only on the luma block but not on the chroma block based on partition parameters of the current coding unit;

determine a first intra-prediction mode for the unpartitioned chroma block based on a second intra-prediction mode for sub-regions obtained from the partition operation onthe luma block; and

perform the conversion based on above determinations.

16. The apparatus of claim 15 , wherein the first intra-prediction mode is derived as an intra-prediction mode for one sub-region of the luma block.

17. The apparatus of claim 15 , wherein the first intra-prediction mode is derived based on an intra-prediction mode for a sub-region of the luma block at a predefined position.

18. The apparatus of claim 17 , wherein the predefined position is one of a top-left position (TL) of the luma block, a top-right position (TR) of the luma block, or a bottom-left position (BL) of the luma block.

19. 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, for a conversion between a current coding unit of a video and a bitstream of the video, that a dimension of a luma block meet a certain condition, wherein the current coding unit includes the luma block and a chroma block;

determining to apply a partition operation only on the luma block but not on the chroma block based on partition parameters of the current coding unit;

determining a first intra-prediction mode for the unpartitioned chroma block based on a second intra-prediction mode for sub-regions obtained from the partition operation onthe luma block; and

generating the bitstream from the current coding unit based on above determinations.

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

determine, for a conversion between a current coding unit of a video and a bitstream of the video, that a dimension of a luma block meet a certain condition, wherein the current coding unit includes the luma block and a chroma block;

determine to apply a partition operation only on the luma block but not on the chroma block based on partition parameters of the current coding unit;

determine a first intra-prediction mode for the unpartitioned chroma block based on a second intra-prediction mode for sub-regions obtained from the partition operation on the luma block; and

perform the conversion based on above determinations.