Next, the process assigns (at 1740) chroma QP offset values based on the analysis of the region. In some embodiments, the process first identifies the suitable chroma QP value for the region based on the analysis performed in 1730, then subtracts the chosen luma QP value from the identified chroma QP value to obtain the desired overall chroma QP offset value. The process 1740 in some embodiments also dissolves the overall chroma QP offset value into chroma QP offset values at various levels of the video coding hierarchy (e.g., picture, slice, and quantization group). Some embodiments identify the chroma QP offset values for quantization groups by subtracting chroma QP offset values for higher level syntax elements from the overall chroma QP offset value. In some embodiments, this operation is performed by a rate controller as described by reference to FIG. 18 below.

The process then populates (at 1750) the chroma QP offset table with the chroma QP offset values identified for the quantization groups in the region. The process 1700 also encodes their corresponding index values in the quantization groups as discussed in Section I.b above. For some embodiments that specify chroma QP offset values explicitly, the process 1700 encodes the chroma QP offset values within the quantization groups themselves as discussed in Section I.a above.

The process then determines (at 1760) whether there are more regions in the picture that remain to be analyzed. If so, the process returns to 1720. Otherwise, the process 1700 ends.

III. Video Systems