Control and use of chroma quantization parameter values
Innovations in control and use of chroma quantization parameter (“QP”) values that depend on hum QP values. More generally, the innovations relate to control and use of QP values for a secondary color component that depend on QP values for a primary color component. For example, during encoding, an encoder determines a QP index from a primary component QP and secondary component QP offset. The encoder maps the QP index to a secondary component QP, which has an extended range. The encoder outputs at least part of a bitstream including the encoded content. A corresponding decoder receives at least part of a bitstream including encoded content. During decoding, the decoder determines a QP index from a primary component QP and secondary component QP offset, then maps the QP index to a secondary component QP, which has an extended range.
1. In a computer system that implements a video decoder, a method comprising:
receiving encoded data in at least part of a bitstream for a video sequence, the encoded data including syntax elements that indicate a picture-level chroma quantization parameter (QP) offset and a slice-level chroma QP offset; and
decoding the encoded data to reconstruct part of a picture of the video sequence for which values of QP vary according to a relationship between a luma component and one or more chroma components, wherein the decoding includes:
determining a luma QP;
determining a final chroma QP based at least in part on the luma QP, the picture-level chroma QP offset, and the slice-level chroma QP offset, wherein an upper limit of range of quantization step size (QSS) indicated by the final chroma QP substantially matches an upper limit of range of QSS indicated by the luma QP, including:
determining a QP index using the luma QP;
mapping the QP index to an intermediate chroma QP, including setting the intermediate chroma QP based on the QP index according to a piecewise mapping between different values of QP index and corresponding values of intermediate chroma QP; and
adjusting the intermediate chroma QP to determine the final chroma QP; and
performing inverse quantization operations for the part of picture based at least in part on the luma QP and the final chroma QP.
2. The method of claim 1 , wherein the piecewise mapping uses a function according to which a bottom range of values of QP index has a first linear mapping to corresponding values of intermediate chroma QP, a middle range of values of QP index has a non-linear mapping to corresponding values of intermediate chroma QP, and a top range of values of QP index has a linear mapping to corresponding values of intermediate chroma QP.
3. The method of claim 1 , wherein the piecewise mapping is implemented as a piecewise linear function.
4. The method of claim 3 , wherein, according to the piecewise linear function:
a first range of values of QP index has a first linear mapping to corresponding values of intermediate chroma QP;
a second range of values of QP index has a second linear mapping to corresponding values of intermediate chroma QP; and
a third range of values of QP index has a third linear mapping to corresponding values of intermediate chroma QP.
5. The method of claim 1 , wherein, for a range of values of QP index associated with high QSS, the relationship is characterized by change in value of the luma QP causing a change of the same size in value of chroma QP, such that ratio of change in the Juma QP to change in the chroma QP is 1.
6. The method of claim 1 , wherein values of the picture-level chroma QP offset and the slice-level chroma QP offset are constrained such that range of the picture-level chroma QP offset plus the slice-level chroma QP offset is −12 to 12.
7. The method of claim 1 , wherein values of the picture-level chroma QP offset and the slice-level chroma QP offset are constrained such that:
range of the picture-level chroma QP offset is −12 to 12; and
range of the slice-level chroma QP offset is −12 to 12.
8. The method of claim 1 , wherein the encoded data further includes a flag in a picture parameter set that indicates presence of slice-level chroma QP offsets in slice headers.
9. One or more non-transitory computer-readable media having stored thereon encoded data organized as part of a bitstream for a video sequence, the encoded data including syntax elements that indicate a picture-level chroma quantization parameter (QP) offset and a slice-level chroma QP offset, the encoded data being organized to facilitate reconstruction, with a computer system that implements a video decoder, by operations comprising:
receiving the encoded data; and
decoding the encoded data to reconstruct a coding unit of a picture of the video sequence for which values of QP vary according to a relationship between a luma component and one or more chroma components, wherein the decoding includes:
determining a luma QP;
determining a final chroma QP based at least in part on the luma QP, the picture-level chroma QP offset, and the slice-level chroma QP offset, wherein an upper limit of range of quantization step size (QSS) indicated by the final chroma QP substantially matches an upper limit of range of QSS indicated by the luma QP, including:
determining a QP index using the luma QP;
mapping the QP index to an intermediate chroma QP, including setting the intermediate chroma QP based on the QP index according to a piecewise mapping between different values of QP index and corresponding values of intermediate chroma QP; and
adjusting the intermediate chroma QP to determine the final chroma QP; and
performing inverse quantization operations for the coding unit of picture based at least in part on the luma QP and the final chroma QP.
10. The one or more computer-readable media of claim 9 , wherein the piecewise mapping is implemented as a piecewise linear function.
11. The one or more computer-readable media of claim 9 , wherein values of the picture-level chroma QP offset and the slice-level chroma QP offset are constrained such that range of the picture-level chroma QP offset plus the slice-level chroma QP offset is −12 to 12.
12. The one or more computer-readable media of claim 9 , wherein values of the picture-level chroma QP offset and the slice-level chroma QP offset are constrained such that:
range of the picture-level chroma QP offset is −12 to 12; and
range of the slice-level chroma QP offset is −12 to 12.
13. The one or more computer-readable media of claim 9 , wherein the encoded data further includes a flag in a picture parameter set that indicates presence of slice-level chroma QP offsets in slice headers.
14. One or more computer-readable memory or storage devices having stored thereon computer-executable instructions for causing one or more processing units, when programmed thereby, to perform operations comprising:
encoding part of a picture of a video sequence for which values of quantization parameter (QP) vary according to a relationship between a luma component and one or more chroma components, thereby producing encoded data, wherein the encoding includes:
determining a luma QP;
determining a final chroma QP;
encoding the final chroma QP for the part of the picture based at least in part on the luma QP, a picture-level chroma QP offset, and a slice-level chroma QP offset, wherein an upper limit of range of quantization step size (QSS) indicated by the final chroma QP substantially matches an upper limit of range of QSS indicated by the luma QP, including:
determining a QP index using the luma QP; and
mapping the QP index to an intermediate chroma QP, including setting the intermediate chroma QP based on the QP index according to a piecewise mapping between different values of QP index and corresponding values of intermediate chroma QP; and
performing inverse quantization operations for the part of picture based at least in part on the luma QP and the final chroma QP; and
outputting the encoded data in at least part of a bitstream, the encoded data including syntax elements that indicate the picture-level chroma QP offset and the slice-level chroma QP offset.
15. The one or more computer-readable memory or storage devices of claim 14 , wherein, for a range of values of QP index associated with high QSS, the relationship is characterized by change in value of the luma QP causing a change of the same size in value of chroma QP, such that ratio of change in the luma QP to change in the chroma QP is 1.
16. The one or more computer-readable memory or storage devices of claim 14 , wherein the piecewise mapping is implemented as a piecewise linear function.
17. The one or more computer-readable memory or storage devices of claim 14 , wherein values of the picture-level chroma QP offset and the slice-level chroma QP offset are constrained such that range of the picture-level chroma QP offset plus the slice-level chroma QP offset is −12 to 12.
18. The one or more computer-readable memory or storage devices of claim 14 , wherein values of the picture-level chroma QP offset and the slice-level chroma QP offset are constrained such that:
range of the picture-level chroma QP offset is −12 to 12; and
range of the slice-level chroma QP offset is −12 to 12.
19. The one or more computer-readable memory or storage devices of claim 14 , wherein the encoded data further includes a flag in a picture parameter set that indicates presence of slice-level chroma QP offsets in slice headers.
20. The one or more computer-readable media of claim 9 , wherein, for a range of values of QP index associated with high QSS, the relationship is characterized by change in value of the luma QP causing a change of the same size in value of chroma QP, such that ratio of change in the luma QP to change in the chroma QP is 1.