Control and use of chroma quantization parameter values

In a computer system that implements a video encoder, a method comprising:

• encoding one or more pictures, thereby producing encoded data, including, as part of a motion compensation loop, for a portion of a given picture among the one or more pictures:
• determining a quantization parameter (QP) index indicated by a luma component QP and a chroma component QP offset, wherein the chroma component QP offset incorporates one of multiple picture-level chroma QP offsets and one of multiple slice-level chroma QP offsets, the QP index being determined in a manner consistent with:
• qPI=Clip3(a,b,QPY+qp_offset+slice_qp_delta),
• where qPI represents the QP index, QPY represents the luma component QP, qp_offset represents the one of the multiple picture-level chroma QP offsets, slice_qp_delta represents the one of the multiple slice-level chroma QP offsets, and Clip3(a, b, c) represents a function that clips the value of c to the range of a to b;
• and
• mapping the QP index to a chroma component QP;
• and
• outputting the encoded data in a bitstream for which values of QP vary according to a relationship between a luma component and chroma components, wherein the encoded data in the bitstream includes:
• a flag in a picture parameter set that indicates presence of the multiple slice-level chroma QP offsets in slice headers;
• syntax elements that indicate the multiple picture-level chroma QP offsets;
• and
• syntax elements that indicate the multiple slice-level chroma QP offsets.

One or more non-transitory computer-readable media having stored thereon encoded data in a bitstream for which values of quantization parameter (QP) vary according to a relationship between a luma component and chroma components, wherein the encoded data in the bitstream includes:

• a flag in a picture parameter set that indicates presence of slice-level chroma QP offsets in slice headers,
• syntax elements that indicate picture-level chroma QP offsets, and
• syntax elements that indicate the slice-level chroma QP offsets, and wherein the encoded data is organized to facilitate decoding, with a computer system that implements a video decoder, of at least some of the encoded data to reconstruct one or more pictures by operations that include, for a portion of a given picture among the one or more pictures:
• determining a QP index from a luma component QP and a chroma component QP offset, wherein the chroma component QP offset incorporates one of the picture-level chroma QP offsets and one of the slice-level chroma QP offsets, the QP index being determined according to:
• qPI=Clip3(a,b,QPY+qp_offset+slice_qp_delta),
• where qPI represents the QP index, QPY represents the luma component QP, qp_offset represents the one of the picture-level chroma QP offsets, slice_qp_delta represents the one of the slice-level chroma QP offsets, and Clip3(a, b, c) represents a function that clips the value of c to the range of a to b;
• and
• mapping the QP index to a chroma component QP.