| JCTVC-O0207 AhG5 and AhG18: Bypass bins alignment depending on the Rice parameter [J. Sole, R. Joshi, M. Karczewicz (Qualcomm)]
A method was presented in JCTVC-M0178 (an earlier basis for JCTVC-N0190 and JCTVC-O0046) for aligning the CABAC process prior to coding bypass data by setting the range equal to 256. The method reportedly allows simultaneous decoding of multiple bypass bins. Each alignment incurs a penalty of 0.55 bits on average. This contribution proposes to apply the aligning only when the Rice parameter is above a threshold and the sign data hiding condition is met, thus applying the alignment when large amount of bypass data is expected to be coded. The method is also combined with the Rice initialization method D1 of RCE2 (JCTVC-O0239). The performance impact is reported to be negligible for the CTC settings and an average loss of 0.3% for the high bit-depth AhG18 settings, which goes down to 0.1% on top of RCE2 D1.
The proponent asserted that a simpler approach from JCTVC-M0178 with a modification of the conditions for applying the scheme, based on the selected Rice parameter can be simpler and perform about the same.
In this scheme or N0190, there would be 3 categories of bins:
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context coded (not affected)
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ordinary bypass bins
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"raw" bins
JCTVC-N0190 reduces category 2 to a maximum of 16 per coefficient block
JCTVC-O0207 reduces category 2 to 125.
The change to the internal design of CABAC is asserted to be smaller with JCTVC-O0207.
Both schemes introduce a dependency between the bin processing stage and the context modelling stage.
JCTVC-O0209 discusses basically coding all coefficient data without using CABAC at all, and does roughly just as well or better. It was further remarked that eliminating CABAC altogether for everything would also work just as well – just output the binarizations directly for everything (and potentially do away with the significance map and other features of the coding representation).
Plan a CE and some other further study.
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