Methods and apparatuses of video data processing with conditionally quantization parameter information signaling

1. A method of processing video data of slices in a video coding system, wherein video data of a color component in each slice is partitioned into Coding Blocks (CBs) according to a combined partitioning method, and the combined partitioning method partitions the video data according to a first partitioning method then a second partitioning method, comprising:

receiving input data associated with a current CB in a current slice;

determining a depth flag, wherein signaling of Quantization Parameter (QP) information for each CB in the current slice is conditioned by a comparing result of depths;

calculating a combined depth of the current CB according to a first partitioning depth and a second partitioning depth of the current CB, wherein the first partitioning depth corresponds to the first partitioning method and the second partitioning depth corresponds to the second partitioning method;

comparing the combined depth of the current CB with the depth flag;

determining and incorporating QP information for the current CB in a video bitstream or parsing the QP information for the current CB from the video bitstream if the combined depth of the current CB is smaller than or equal to the depth flag; and

quantizing a residual signal of the current CB or recovering a quantized residual signal of the current CB according to the QP information for the current CB.

2. The method of claim 1 , wherein the combined depth is calculated according to a weighted sum of the first partitioning depth and the second partitioning depth.

3. The method of claim 2 , wherein the combined partitioning method is Quad-Tree-plus-Binary-Tree (QTBT) partitioning method, the first partitioning depth is quad-tree depth of the current CB, the second partitioning depth is binary-tree depth of the current CB, and the combined depth is calculated by adding the quad-tree depth to a half of the binary-tree depth of the current CB.

4. The method of claim 1 , wherein the depth flag specifies a difference in depth values between a Coding Tree Block (CTB) size and a minimum block size for QP information signaling.

5. The method of claim 1 , wherein if the combined depth of the current CB is larger than the depth flag, the QP information for the current CB is derived from shared QP information sharing by a group of CBs, and no individual QP information is signaled in the video bitstream for each CB in the group.

6. The method of claim 5 , wherein the shared QP information is incorporated in the video bitstream or parsed from the video bitstream when processing a first coded CB of the group of CBs sharing the shared QP information, and the first coded CB is first coded in the group according to an encoding or decoding order that has at least one non-zero residual transform coefficient.

7. The method of claim 1 , wherein the QP information for the current CB is associated with a delta QP, a QP offset, or a final QP.

8. The method of claim 1 , wherein the color component carried in the current CB is a luma component, and the method further comprises:

signaling QP information for a chroma component separately with respect to signaling QP information for the luma component if block partitioning structures for the luma component and the chroma component are separately determined.

9. The method of claim 8 , wherein the QP information for the luma component is associated with delta QPs and the QP information for the chroma component is associated with chroma delta QPs.

10. The method of claim 1 , wherein the color component carried in the current CB is a chroma component, and the QP information for the current CB is determined by reusing QP information for a co-located luma block of the current CB if block partitioning structures for a luma component and the chroma component are separately determined.

11. The method of claim 10 , wherein a reuse flag is signaled at a sequence level, picture level, or slice level to adaptively enable or disable the reuse of QP information for the co-located luma block.

12. An apparatus of processing video data of slices in a video coding system, wherein video data of a color component in each slice is partitioned into Coding Blocks (CBs) according to a combined partitioning method, and the combined partitioning method partitions the video data according to a first partitioning method then a second partitioning method, the apparatus comprising one or more electronic circuits configured for:

receiving input data associated with a current CB in a current slice;

determining a depth flag, wherein signaling of Quantization Parameter (QP) information for each CB in the current slice is conditioned by a comparing result of depths;

calculating a combined depth of the current CB according to a first partitioning depth and a second partitioning depth of the current CB, wherein the first partitioning depth corresponds to the first partitioning method and the second partitioning depth corresponds to the second partitioning method;

comparing the combined depth of the current CB with the depth flag;

determining and incorporating QP information for the current CB in a video bitstream or parsing the QP information for the current CB from the video bitstream if the combined depth of the current CB is smaller than or equal to the depth flag; and

quantizing a residual signal of the current CB or recovering a quantized residual signal of the current CB according to the QP information for the current CB.

13. 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 data associated with a current CB in a current slice, wherein video data of a color component in the current slice is partitioned into Coding Blocks (CBs) according to a combined partitioning method, and the combined partitioning method partitions the video data according to a first partitioning method then a second partitioning method;

determining a depth flag, wherein signaling of Quantization Parameter (QP) information for each CB in the current slice is conditioned by a comparing result of depths;

calculating a combined depth of the current CB according to a first partitioning depth and a second partitioning depth of the current CB, wherein the first partitioning depth corresponds to the first partitioning method and the second partitioning depth corresponds to the second partitioning method;

comparing the combined depth of the current CB with the depth flag;

determining and incorporating QP information for the current CB in a video bitstream or parsing the QP information for the current CB from the video bitstream if the combined depth of the current CB is smaller than or equal to the depth flag; and

quantizing a residual signal of the current CB or recovering a quantized residual signal of the current CB according to the QP information for the current CB.