As mentioned earlier by reference to FIGS. 2a-2b, quantization groups in different embodiments specify the chroma QP offset values for the two chroma components differently. FIG. 6 illustrates the specification of additional chroma QP offsets within the QG 122 using a different method for specifying the chroma QP offset values for the two chroma components. FIG. 6 illustrates a transform unit 600 that is described by pseudo code similar to those for the transform unit 500, the only difference being that lines 611-616 specify only one chroma QP offset value (magnitude+sign) for both chroma components. In some embodiments, specifying one chroma QP offset value for both chroma components is sufficient since in most cases the importance of the two color components might not change. The signaling of one or two such parameters can be prefixed, or could be signaled at different higher syntax level locations such as the SPS, PPS, or slice header.
For some embodiments, FIG. 7 conceptually illustrates a process 700 for determining chroma QP values from a video bitstream that allows additional chroma QP offset specifications. In some embodiments, the process 700 is performed by a video decoder, which uses this process to perform de-quantization of chroma components when decoding the video bitstream for display. In some embodiments, the process starts when the decoder has received a video bitstream and starts to decode a particular video picture for display or other purposes.
