Chroma quantization in video coding
A method of signaling additional chroma QP offset values that are specific to quantization groups is provided, in which each quantization group explicitly specifies its own set of chroma QP offset values. Alternatively, a table of possible sets of chroma QP offset values is specified in the header area of the picture, and each quantization group uses an index to select an entry from the table for determining its own set of chroma QP offset values. The quantization group specific chroma QP offset values are then used to determine the chroma QP values for blocks within the quantization group in addition to chroma QP offset values already specified for higher levels of the video coding hierarchy.
1 . A apparatus comprising:
one or more processing units; and
memory storing instructions that, when executed, are configured to cause the one or more processing units to perform operations comprising:
receiving, in a bitstream, an encoded group of video pictures, including an encoded first video picture with a hierarchical coding structure having at least a first coding level, a second coding level and a third coding level, the encoded first video picture comprising two or more sets of chroma quantization parameter (QP) offset values at the first coding level and the second coding level, each set of chroma QP offset values providing information about chroma QPs of video units encompassed by one level of the hierarchical coding structure, the encoded first video picture further comprising one or more chroma quantization groups (QGs) at the third coding level;
for a particular chroma QG of the one or more chroma QGs at the third coding level, identifying an additional set of chroma QP offsets, the identifying comprising:
determining, based on information included in the first coding level or the second coding level, a parameter specifying the maximum length of a binary string in the bitstream, and
identifying the additional set of chroma QP offsets using the parameter; and
decoding the encoded first video picture by computing chroma QP values corresponding to the one or more chroma QGs of the encoded first video picture, the decoding comprising, for the particular chroma QG:
selecting the identified additional set of chroma QP offsets, and
computing the chroma QP values corresponding to the particular chroma QG using the selected set of the chroma QP offsets.
2 . The apparatus of claim 1 , wherein computing the chroma QP values further comprises combining the chroma QP offsets in the selected set with one or more luma QP values corresponding to the particular chroma QG.
3 . The apparatus of claim 2 , wherein at least the particular chroma QG is associated with a plurality of luma QP values, and wherein computing the chroma QP values for the particular chroma QG comprises computing a plurality of different chroma QP values corresponding to the plurality of luma QP values.
4 . The apparatus of claim 2 , wherein at least the particular chroma QG is associated with a single luma QP value, and wherein computing the chroma QP values for the particular chroma QG comprises computing a single chroma QP value corresponding to the single luma QP value.
5 . The apparatus of claim 1 , wherein receiving in the bitstream the encoded first video picture comprising two or more sets of chroma QP offsets at the first coding level and the second coding level comprises receiving, in the bitstream, first chroma QP offsets corresponding to the first coding level of the encoded first video picture and second chroma QP offsets corresponding to the second coding level of the encoded first video picture, the operations further comprising:
receiving, in the bitstream, third chroma QP offsets corresponding to the third coding level of the encoded first video picture.
6 . The apparatus of claim 5 , wherein the first and second coding levels are higher in a hierarchy of coding levels in the hierarchical coding structure compared to the third coding level, and
wherein computing the chroma QP values corresponding to the particular chroma QG comprises using the first chroma QP offsets.
7 . The apparatus of claim 6 , wherein computing the chroma QP values corresponding to the one or more chroma QGs of the encoded first video picture comprises computing chroma QP values for the one or more chroma QGs associated with the third coding level of the encoded first video picture using the first chroma QP offsets.
8 . The apparatus of claim 6 , wherein the first coding level corresponds to a picture level, the second coding level corresponds to a slice level, and the third coding level corresponds to one of a coding tree unit level, a coding unit level, or a transform unit level, and
wherein computing the chroma QP values corresponding to the particular chroma QG comprises using chroma QP offsets specified in a picture parameter set (PPS) of the encoded first video picture.
9 . The apparatus of claim 1 , the operations further comprising:
receiving, in the bitstream, one or more indices corresponding to the one or more chroma QGs of the encoded first video picture; and
selecting, for at least a first chroma QG of the one or more chroma QGs, a chroma QP offset value from the set of chroma QP offset values using an index corresponding to the first chroma QG.
10 . The apparatus of claim 1 , wherein the parameter specifying the maximum length of a binary string in the bitstream comprises the cMax parameter used in a binarization process.
11 . The apparatus of claim 10 , wherein the encoded first video picture is encoded using Context Adaptive Binary Arithmetic (CABAC) coding.