FIG. 1 also illustrates how offsets specified by chroma QGs are used to compute chroma QP values. In some embodiments such as those illustrated by FIG. 1, a first set of Chroma QP offset parameters is already signaled at the PPS 132 (and/or header for slice 112). In some embodiments, chroma QP offset values that are specified at levels above chroma QGs (e.g., PPS and/or slice header) are ignored when the QGs specify their own chroma QP offset values. In some embodiments, the higher level chroma QP offset syntax elements and the QG level chroma QP offsets are considered together to reduce overhead. That is, the adaptation of chroma QP values is done in relationship to both parameter sets together, and the signaled information can be seen as a “second-order” offset. In some of these embodiments, the chroma QP values for the current chroma QG is computed as:
QPchroma[i]=QPluma+QPoffset_pps[i]+QPoffset_quant_group[i]??(1)
(i=0 for the first chroma component; i=1 for the second chroma component)
Where QPluma is the QP of the luma component corresponding to the current chroma quantization group, QPoffset_pps[i] is the i-th chroma component's QP offset from the current PPS (or slice header), and QPoffset_quant_group[i] is the additional offset signaled at the QG level for that component. QPoffset_pps[i]+QPoffset_quant_group[i] thus constitute the overall chroma QP offset of the i-th chroma component.
