Cross-component quantization in video coding

1. A method for processing visual information, comprising:

parsing a bitstream to determine a quantization parameter (QP) for a luma component of a region of the visual information from a data unit of a parameter set included in the bitstream;

determining a value of a flag from the data unit of the parameter set, the value of the flag being a first value indicating an absence of an offset to the OP for the luma component in the bitstream or a second value indicating a presence of the offset to the QP for the luma component in the bitstream;

determining, in response to the flag being equal to the first value, a QP for a chroma component of the region as a first function of the QP for the luma component and a default chroma delta QP indicated by the flag;

determining, in response to the flag being equal to the second value different from the first value, the QP for the chroma component of the region by:

(a) obtaining a delta chroma QP from the data unit; and

(b) determining the QP for the chroma component as a second function of the QP for the luma component and the delta chroma QP; and

decoding the chroma component using the QP for the chroma component in a case that the parameter set is activated for decoding the region, and

wherein the QP for the chroma component of the region is determined based on the first function or the second function by using the default chroma delta QP indicated by the flag or the delta chroma QP depending on whether the flag has the first value or the second value.

2. The method of claim 1 , wherein the first function and the second function are addition functions.

3. The method of claim 1 , wherein the region corresponds to a picture or a portion smaller than the picture.

4. The method of claim 1 , wherein the default chroma delta QP indicated by the flag is determined based on a mapping, and wherein the mapping remains unchanged in the bitstream.

5. The method of claim 1 , wherein the default chroma delta QP indicated by the flag is determined based on a mapping, and wherein the mapping is changed by a signaling in the bitstream.

6. A method of generating a coded representation of a visual information, comprising:

determining a quantization parameter (QP) for a luma component of a region of the visual information; and

signaling a QP for a chroma component in the coded representation of the visual information by:

including, in response to the QP for the chroma component being equal to a value of a first function of a default QP for the chroma component and the QP for the luma component, in the coded representation, a flag equal to a first value indicating that the default QP for the chroma component is to be used for decoding, and coding the flag in a data unit of a parameter set of the coded representation; and

including, in response to the QP for the chroma component being not equal to the value of the first function of the default QP for the chroma component and the QP for the luma component, in the coded representation, the flag equal to a second value and a value in the data unit indicative of a delta chroma QP such that the QP for the chroma component is a second function of the luma component and the delta chroma QP, and

wherein whether the flag has the first value or the second value depends on whether the QP for the chroma component of the region is equal to or different from the value of the first function of the default QP for the chroma component and the QP for the luma component, and

wherein the OP for the chroma component of the region is determined based on the first function or the second function by using the default OP indicated by the flag or the delta chroma QP depending on whether the flag has the first value or the second value.

7. The method of claim 6 , wherein the first function and second functions are addition functions.

8. The method of claim 6 , wherein the region corresponds to a picture or a portion smaller than the picture.

9. The method of claim 6 , wherein the default QP for the chroma component is determined based on a mapping, and wherein the mapping remains unchanged in a bitstream.

10. The method of claim 6 , wherein the default QP for the chroma component is determined based on a mapping, and wherein the mapping is changed by a signaling in a bitstream.

11. A video decoder apparatus comprising a processor and a memory storing instructions, which upon execution by the processor, causes the video decoder apparatus to:

parse a bitstream to determine a quantization parameter (QP) for a luma component of a region of visual information from a data unit of a parameter set included in the bitstream;

determine a value of a flag from the data unit of the parameter set, the value of the flag being a first value indicating an absence of an offset to the OP for the luma component in the bitstream or a second value indicating a presence of the offset to the QP for the luma component in the bitstream;

determine, in response to the flag being equal to the first value, a QP for a chroma component of the region as a first function of the QP for the luma component and a default chroma delta QP indicated by the flag;

determine, in response to the flag being equal to the second value different from the first value, the QP for the chroma component of the region by:

(a) obtaining a delta chroma QP from the data unit; and

(b) determining the QP for the chroma component as a second function of the QP for the luma component and the delta chroma QP; and

decode the chroma component using the QP for the chroma component in a case that the parameter set is activated for decoding the region, and

wherein the QP for the chroma component of the region is determined based on the first function or the second function by using the default chroma delta QP indicated by the flag or the delta chroma QP depending on whether the flag has the first value or the second value.

12. The video decoder apparatus of claim 11 , wherein the first function and the second function are addition functions.

13. The video decoder apparatus of claim 11 , wherein the region corresponds to a picture or a picture region.

14. The video decoder apparatus of claim 11 , wherein the default chroma delta QP indicated by the flag is determined based on a mapping, and wherein the mapping remains unchanged in the bitstream.

15. The video decoder apparatus of claim 11 , wherein the default chroma delta QP indicated by the flag is determined based on a mapping, and wherein the mapping is changed by a signaling in the bitstream.

16. A video encoder apparatus comprising a processor and a memory storing instructions, which upon execution by the processor, causes the video encoder apparatus to:

determine a quantization parameter (QP) for a luma component of a region of visual information; and

signal a QP for a chroma component in a coded representation of the visual information by:

including, in response to the QP for the chroma component being equal to a value of a first function of a default QP for the chroma component and the QP for the luma component, in the coded representation, a flag equal to a first value indicating that the default QP for the chroma component is to be used for decoding, and coding the flag in a data unit of a parameter set of the coded representation; and

including, in response to the QP for the chroma component being not equal to the value of the first function of the default QP for the chroma component and the QP for the luma component, in the coded representation, the flag equal to a second value and a value in the data unit indicative of a delta chroma QP such that the QP for the chroma component is a second function of the luma component and the delta chroma QP, and

wherein whether the flag has the first value or the second value depends on whether the QP for the chroma component of the region is equal to or different from the value of the first function of the default QP for the chroma component and the QP for the luma component, and

wherein the OP for the chroma component of the region is determined based on the first function or the second function by using the default QP indicated by the flag or the delta chroma QP depending on whether the flag has the first value or the second value.

17. The video encoder apparatus of claim 16 , wherein the first function and second functions are addition functions.

18. The video encoder apparatus of claim 16 , wherein the region corresponds to a picture or a portion smaller than the picture.

19. The video encoder apparatus of claim 16 , wherein the default QP for the chroma component is determined based on a mapping, and wherein the mapping remains unchanged in a bitstream.

20. The video encoder apparatus of claim 16 , wherein the default QP for the chroma component is determined based on a mapping, and wherein the mapping is changed by a signaling in a bitstream.