JVET-V0068 AHG8: A constraint of max CTU size for high bit depth and high bit rate coding [K. Kondo, M. Ikeda (Sony)]

This contribution proposes a constraint for max CTU size to 64 for high bit rate coding. When high throughput is required, wavefront parallel processing can be used by setting sps_entropy_coding_sync_enabled_flag to 1. In VVC version 1, max CTU size was extended from 64 (HEVC) to 128. It might make max delay of CABAC of a CTU row increased. This proposed constraint can decrease the max delay. To investigate the impact of coding efficiency, max CTU size 64 was tested. In the experiment, an impact of coding efficiency is reported as below. The numbers in the bracket mean BD rates for Y/G, U/B and V/R.

• Low QP range
  • {-0.05%, -0.20%, -0.26%} for wPSNR and PQ contents
  • {-0.04%, -0.17%, -0.23%} for PSNR and PQ contents
  • {0.01%, -0.07%, -0.13%} for HLG contents
  • {0.02%, 0.00%, 0.00%} for SVT contents.
• Normal QP range
  • {1.74%, 4.53%, 4.29%} overall in RA case
  • {0.35%, 1.49%, 0.46%} overall in AI case

In case of 4K video, 16 threads could be used at decoder with 128 CTU size. Is this not sufficient for the high bit rate range? Not known, further analysis would be necessary.

It is further pointed out that a decoder would not be able to do parsing in wavefront mode if it is not enabled in the bitstream. A profile intended for extreme high bit rate might need to enforce that.

Further study in the overall context of a possible definition of a 16 bit profile.