Methods and apparatuses of chroma quantization parameter derivation in video processing system

1. A method of processing video data in a video encoding or decoding system, comprising:

receiving input video data associated with a current chroma block, wherein the input video data is partitioned according to two separate Coding Unit (CU) partitioning structures for luma and chroma components when dual tree coding is enabled;

determining a luma Quantization Parameter (QP) of a collocated luma block, wherein the collocated luma block is a block that covers a collocated sample of one predefined sample of the current chroma block;

reusing the luma QP of the collocated luma block to derive a chroma QP for the current chroma block; and

encoding or decoding one or more Transform Units (TUs) associated with the current chroma block using the chroma QP.

2. The method of claim 1 , further comprising deriving a chroma QP offset from one or more syntax elements signaled in or parsed from a Transform Unit (TU) associated with the current chroma block in the video decoding system or deriving and signaling a chroma QP offset in a TU associated with the current chroma block in the video encoding system, wherein the chroma QP is derived by the luma QP of the collocated luma block and the chroma QP offset.

3. The method of claim 2 , further comprising determining a depth of the current chroma block, and deriving the chroma QP offset from one or more syntax elements or signaling the chroma QP offset when the depth of the current chroma block is smaller than or equal to a threshold for QP signaling.

4. The method of claim 3 , wherein the depth of the current chroma block is derived by an area of the current chroma block and the threshold for QP signaling is derived by a maximum QP signaling depth.

5. The method of claim 3 , wherein the depth of the current chroma block is a depth of a parent node plus 2 if the current chroma block is a first or third split node of Ternary Tree (TT) splitting, the depth of the current chroma block is a depth of a parent node plus 1 if the current split chroma node is a second split node of TT splitting.

6. The method of claim 3 , wherein the depth of the current chroma block is a depth of a parent node plus 1 if the current chroma block is partitioned from the parent node by Binary Tree (BT) splitting.

7. The method of claim 3 , wherein the depth of the current chroma block is a depth of a parent node plus 2 if the current chroma block is partitioned from the parent node by quadtree splitting.

8. The method of claim 4 , wherein the current chroma block is partitioned from a parent node by TT splitting and the depth of the current chroma block is larger than the threshold for QP signaling, a depth of a second split node partitioned from the parent node is equal to the threshold for QP signaling, and the chroma QP offset derived from the one or more syntax elements is shared by all three split nodes partitioned from the parent node.

9. The method of claim 3 , wherein one chroma QP offset is derived for all children nodes partitioned from the current chroma block by Ternary Tree (TT) splitting if the depth of the current chroma block is smaller than or equal to the maximum delta QP signaling depth and a depth of at least one children node partitioned from the current chroma block is larger than the maximum delta QP signaling depth, and the chroma QP offset derived from the one or more syntax elements is used by one or more of the children nodes partitioned from the current chroma block.

10. The method of claim 3 , wherein when the current chroma block is a leaf CU, the QP offset is derived from the one or more syntax elements if the depth of the current chroma block is smaller than or equal to the maximum delta QP signaling depth and the current chroma block has at least one non-zero coefficient or at least one escape palette.

11. The method of claim 3 , wherein when the current chroma block is not a leaf CU and is further partitioned into multiple leaf CUs, the QP offset is derived from the one or more syntax elements if the depth of the current chroma block is smaller than or equal to the maximum delta QP signaling depth, a depth of at least one children node partitioned from the current chroma block is larger than the maximum delta QP signaling depth, and at least one of the leaf CUs in the current chroma block has at least one non-zero coefficient or at least one escape palette, wherein the QP offset is used by one or more of the leaf CUs within the current chroma block.

12. The method of claim 2 , wherein the chroma QP offset is derived from one or more syntax elements signaled in or parsed from a TU when there is at least one non-zero coefficient or at least one escape palette in the current chroma block.

13. The method of claim 2 , wherein parsing or signaling the chroma QP offset is skipped when transform and quantization are bypassed for the current chroma block.

14. The method of claim 1 , wherein the collocated luma block is a collocated CU, Prediction Unit (PU), TU, or delta QP unit of the current chroma block.

15. The method of claim 1 , wherein the predefined sample of the current chroma block is a top-left sample, top-right sample, bottom-left sample, bottom-right sample, or a center sample of the current chroma block.

16. The method of claim 1 , wherein a part or all of parameters related to QP signaling are duplicated for the luma and chroma components.

17. The method of claim 16 , wherein the parameters related to QP signaling include one or more thresholds, decision results, control flags from parent nodes, conditions, and a minimum delta QP signaling CU area or a maximum delta QP signaling depth.

18. The method of claim 1 , wherein the luma QP of the collocated luma block is derived from one or more syntax elements associated with a delta QP signaled or parsed in a TU associated with the collocated luma block.

19. An apparatus of processing video data in a video encoding or decoding system, the apparatus comprising one or more electronic circuits configured for:

receiving input video data associated with a current chroma block, wherein the input video data is partitioned according to two separate Coding Unit (CU) partitioning structures for luma and chroma components when dual tree coding is enabled;

determining a luma Quantization Parameter (QP) of a collocated luma block, wherein the collocated luma block is a block that covers a collocated sample of one predefined sample of the current chroma block;

reusing the luma QP of the collocated luma block to derive a chroma QP for the current chroma block; and

encoding or decoding one or more Transform Units (TUs) associated with the current chroma block using the chroma QP.

20. A non-transitory computer readable medium storing program instruction causing a processing circuit of an apparatus to perform video processing method, and the method comprising:

receiving input video data associated with a current chroma block, wherein the input video data is partitioned according to two separate Coding Unit (CU) partitioning structures for luma and chroma components when dual tree coding is enabled;

determining a luma Quantization Parameter (QP) of a collocated luma block, wherein the collocated luma block is a block that covers a collocated sample of one predefined sample of the current chroma block;

reusing the luma QP of the collocated luma block to derive a chroma QP for the current chroma block; and

encoding or decoding one or more Transform Units (TUs) associated with the current chroma block using the chroma QP.