| JCTVC-O0044 RExt: CU-adaptive chroma QP offsets [D. Flynn, N. Nguyen, D. He, G. Martin-Cocher (BlackBerry), A. Tourapis, G. Cote, D. Singer (Apple)]
Chroma quantization step size adaptivity relative to luma is currently available at the slice level. This proposes to have some form of chroma-luma quantization step size control at a lower level.
Currently, changing the balance would require starting a new slice.
It was commented that adjustments of lambda may also be a way to deal with this.
It was noted that QP control is already something enabled at a low level, so the implementation burden of something like this would be less than for localized scaling list modification.
The proposal is to set up a set of offsets at the picture level and establish a segmentation structure granularity at which the chroma offset can be selected.
The prior contribution N0292 had a somewhat simpler form of this, using a single offset.
One suggestion was to make the segmentation the same for luma and chroma QP control. Mixed content (some areas using upsampled 4:2:0) was mentioned as a potential case where a differing level of control would make sense.
The scheme for segmentation that was proposed was the same as quantization groups (but for chroma transform blocks only and having its own level of control, and with the decision not propagating across quantization group regions).
The desirability of coupling of Cb and Cr quantization step sizes was also discussed. Setting up different offsets for QP in the two components in a picture-level table of selectable schemes was mentioned as a way to deal with this.
Deblocking is proposed to not include any adjustment for the low-level chroma adjustments.
It was commented that if we adopt it, we should do so only for 4:2:2 and 4:4:4 profiles, unless substantial evidence is provided.
Two "straw man" variants of the proposal:
Decision: Adopt the table approach as described, pending text review.
Profiling TBD – to be recorded elsewhere.
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