JVET-V0131 AHG8: Fixing the forward transform matrices for high bit depth coding [K. Naser, F. Galpin, F. F. Le Léannec, P. De Lagrange (InterDigital)]

In High Bit-depth coding of VVC, the forward transform matrices are represented with higher precision than the standard coding. This modification is reported to alter the properties of DST-VII and DCT-VIII 4x4 transforms that leads to difference between matrix multiplication- and fast algorithm-based computation. This contribution proposes modifying these transform matrices such that both matrix multiplication and fast algorithm are identical and therefore restore the dual-implementation property.

The following experimental results of bdrate gain are reported:

Low QP

• 0.00%, 0.00% and -0.01% in bdrateY in PQ, HLG and SVT in AI
• -0.01%, -0.01% and 0.00% in bdrateY in PQ, HLG and SVT in LDB
• X.XX%, X.XX% and X.XX% in bdrateY in PQ, HLG and SVT in RA

Normal QP

• -0.07% and 0.00% in bdrateY in H1 and H2 in AI
• - 0.04% and X.XX% in bdrateY in H1 and H2 in RA

Presented in session 13 at 1405 UTC (chaired by JRO)

This does not have any impact on the spec, as the inverse transform does not require increased precision. It is however useful to have it in the software, as a reference for implementers.

Decision (SW): Adopt JVET-V0131