Use of chroma quantization parameter offsets in deblocking

1. A computing system that comprises one or more processing units and memory, wherein the computing system implements a video encoder configured to encode video content for which values of quantization parameter (QP) vary according to a relationship between a luma component and chroma components, the video encoder comprising:

a quantizer configured to quantize transform coefficients for one or more portions of a slice of a picture;

a deblock filter configured to perform deblock filtering of at least part of the slice, wherein derivation of a control parameter for the deblock filtering of the at least part of the slice considers a picture-level chroma QP offset for the picture but not a slice-level chroma QP offset for the slice, the control parameter depending on a variable qP I derived as:

qP I =((( QP Q +QP P +1)>>1)+ cqp _offset),

wherein variables QP Q and QP P represent luma QP values for blocks on either side of an edge in the at least part of the slice on which deblock filtering is performed, wherein variable cqp_offset represents the picture-level chroma QP offset for the picture, and wherein the variable qP I is used to determine a variable QP C that is used in the derivation of the control parameter; and

a buffer configured to store at least part of a bitstream including encoded content, the at least part of the bitstream including one or more syntax elements that indicate the picture-level chroma QP offset for the picture and one or more syntax elements that indicate the slice-level chroma QP offset for the slice.

2. The computing system of claim 1 , wherein, according to bitstream syntax, the slice-level chroma QP offset for the slice is separately signaled for two different chroma components for the slice.

3. The computing system of claim 1 , wherein, according to bitstream syntax, the picture-level chroma QP offset for the picture is separately signaled for two different chroma components for the picture.

4. The computing system of claim 3 , wherein the deblock filter is configured to separately derive values of the control parameter for the two different chroma components.

5. The computing system of claim 1 , wherein bitstream syntax permits signaling of different slice-level chroma QP offsets for different slices of the picture.

6. The computing system of claim 1 , wherein the video encoder further comprises a controller configured to adjust the picture-level chroma QP offset for the picture and/or the slice-level chroma QP offset for the slice during encoding for rate control.

7. The computing system of claim 1 , wherein the video encoder is configured to store the picture-level chroma QP offset for the picture after the quantizing, for use in the derivation of the control parameter for the deblock filtering.

8. A non-volatile memory or storage device having stored thereon at least part of a bitstream including encoded video content for which values of quantization parameter (QP) vary according to a relationship between a luma component and chroma components, wherein the bitstream includes one or more syntax elements that indicate a picture-level chroma QP offset for a picture and one or more syntax elements that indicate a slice-level chroma QP offset for a slice of the picture, and wherein the bitstream results from encoding that includes:

setting the picture-level chroma QP offset for the picture and the slice-level chroma QP offset for the slice;

quantizing transform coefficients for one or more portions of the slice;

performing deblock filtering of at least part of the slice, wherein derivation of a control parameter for the deblock filtering of the at least part of the slice considers the picture-level chroma QP offset for the picture but not the slice-level chroma QP offset for the slice, the control parameter depending on a variable qP I derived as:

qP I =((( QP Q +QP P +1)>>1)+ cqp _offset),

wherein variables QP Q and QP P represent luma QP values for blocks on either side of an edge in the at least part of the slice on which deblock filtering is performed, wherein variable cqp_offset represents the picture-level chroma QP offset for the picture, and wherein the variable qP I is used to determine a variable QP C that is used in the derivation of the control parameter; and

outputting the at least part of a bitstream including the encoded content.

9. The non-volatile memory or storage device of claim 8 , wherein, according to bitstream syntax, the slice-level chroma QP offset for the slice is separately signaled for two different chroma components for the slice.

10. The non-volatile memory or storage device of claim 8 , wherein bitstream syntax permits signaling of different slice-level chroma QP offsets for different slices of the picture.

11. The non-volatile memory or storage device of claim 8 , wherein, according to bitstream syntax, the picture-level chroma QP offset for the picture is separately signaled for two different chroma components for the picture.

12. The non-volatile memory or storage device of claim 11 , wherein values of the control parameter are derived separately for the two different chroma components.

13. The non-volatile memory or storage device of claim 8 , wherein the variable qP I is also clipped according to a clipping function.

14. In a computing device that implements a video decoder, a method comprising:

receiving at least part of a bitstream including encoded video content for which values of quantization parameter (QP) vary according to a relationship between a luma component and chroma components, wherein the bitstream includes one or more syntax elements that indicate a picture-level chroma QP offset for a picture and one or more syntax elements that indicate a slice-level chroma QP offset for a slice of the picture; and

decoding at least some of the encoded content, wherein the decoding includes:

using the picture-level chroma QP offset for the picture and the slice-level chroma QP offset for the slice to regulate inverse quantization, according to a chroma QP, of transform coefficients for one or more portions of the slice; and

performing deblock filtering of at least part of the slice, wherein derivation of a control parameter for the deblock filtering of the at least part of the slice considers the picture-level chroma QP offset for the picture but not the slice-level chroma QP offset for the slice, the control parameter depending on a variable qP I derived as:

qP I =((( QP Q +QP P +1)>>1)+ cqp _offset),

wherein variables QP Q and QP P represent luma QP values for blocks on either side of an edge in the at least part of the slice on which deblock filtering is performed, wherein variable cqp_offset represents the picture-level chroma QP offset for the picture, and wherein the variable qP I is used to determine a variable QP C that is used in the derivation of the control parameter.

15. The method of claim 14 , wherein, according to bitstream syntax, the slice-level chroma QP offset for the slice is separately signaled for two different chroma components for the slice.

16. The method of claim 14 , wherein bitstream syntax permits signaling of different slice-level chroma QP offsets for different slices of the picture.

17. The method of claim 14 , wherein, according to bitstream syntax, the picture-level chroma QP offset for the picture is separately signaled for two different chroma components for the picture.

18. The method of claim 17 , wherein values of the control parameter are derived separately for the two different chroma components.

19. The method of claim 14 , wherein the variable qP I is also clipped according to a clipping function.

20. The method of claim 14 , wherein the decoding further comprises:

storing the picture-level chroma QP offset for the picture after the inverse quantization, for use in the derivation of the control parameter for the deblock filtering; and

discarding the slice-level chroma QP offset for the slice after the inverse quantization.