JVET-C0042 EE2.3: NSST-PDPC Harmonization [S.-H. Kim, A. Segall (Sharp)]

This contribution proposes changes to the non-separable secondary transform (NSST) process in JEM 2.0, with (i) using a unified binarization for NSST index coding and (ii) adaptively signalling the NSST index on a CU and TU level. The specific changes are as follows: First, instead of using two binarization methods based on intra prediction mode and partition size as is done in JEM 2.0, the contribution proposes to code the NSST index with a truncated unary binarization method and adjust the context model to reflect the statistics of the index based on the intra prediction mode and partition size. Second, the contribution proposes to code the NSST index first at a CU level and then conditionally signal a TU-level flag to indicate whether NSST is applied. Finally, the contribution proposes to remove the bitstream restriction currently precluding enabling NSST and PDPC at the same time. Using these three proposals, it is reported that an improvement of 0.6%, 0.3%, 0.1%, and 0.1% luma BD-rate savings is observed for AI, RA, LD (B), and LD (P) configurations, respectively.

Unification of binarization does not give loss or gain, but is a slight simplification (also proposed in EE7).

Decision: Adopt this aspect.

The additional TU level flag for NSST gives no relevant gain (0.03%) (and anyway is obsolete if QTBT is adopted) – no action was taken on that.

The alternative NSST kernel gives approximately 0.2% BR reduction, but might increase memory usage (same as EE7, see further discussion there).

Decoupling PDPC and NSST gives approximately 0.4% gain, but increases encoder runtime by approximately 50% in AI. If it would not be implemented as a bitstream restriction but rather as an encoder option, it would increase the bit rate. No attractive tradeoff was shown, and thus no action was taken on that.