JVET-R0244 AHG9: Coded Picture Buffer sizes and MinCr in VVC [S. Keating, A. Browne, K. Sharman (Sony)]
Discussion started here for Track A on Wednesday 22 April at 0500 (chaired by GJS & YKW).
See also the accompanying information document JVET-R0243.
This contribution describes proposed changes to maximum coded picture buffer (CPB) size and MinCrScalingFactor in order to guarantee that the buffer can always store a full picture when compressed at the minimum compression ratio (MinCr).
The contribution reports that a single picture coded at the MinCr bit rate would more than fill the entire CPB for some levels of the Main tier.
It was commented that we have the same issue in AVC and HEVC. At level 6 there is a “kink” in the table (60 000 rather than 80 000 for the max CPB size). This could be an errata report for AVC and HEVC, to change the MinCr limit to be derived from the CPB size limit or add a note for cases where the CPB size imposes a tighter limit than the MinCr does.
It was noted that levels 6, 6.1 and 6.2 have the same luma size limit, and that if a larger CPB size is needed, level 6.1 or 6.2 could be used.
See also contribution JVET-R0243, which showed that generated bit rates at low QPs could have coded sizes that scale to larger values (more than twice the number of coded bits) for 4:4:4 relative to 4:2:0.
Decision (cleanup): MaxCPB = 80 000 for level 6, 120 000 for level 6.1, 180 000 for level 6.2, and change MinCrScaleFactor for the 4:4:4 profile to 0.75, and change MinCrBase to 8 for level 6.2.