Control and use of chroma quantization parameter values

One or more computer-readable memory or storage devices 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 by operations comprising:

• receiving the encoded data;
• 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.

In a computing system that implements a video encoder, a method 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.