JVET-P0371 AHG7/AHG15: signalling of corrective values for chroma residual scaling [E. François, F. Galpin, K. Naser, P. de Lagrange (InterDigital)]
This contribution relates to LMCS. It proposes to enable signalling in the APS corrective values for the chroma residual scaling. In current VTM, chroma residual scaling values are inferred as the inverse values of the luma mapping slopes. The table of luma mapping slopes is signalled in the APS. In order to provide higher encoder control flexibility, it is proposed to add in the APS a corrective chroma residual scaling parameters table. The corrective parameters are determined at the encoder and signalled in the APS as additive values to the luma mapping slopes. The final chroma residual scaling values are obtained by inverting the corrected luma mapping slope values.
It is reported that the proposed chroma residual scaling control allows much finer tuning of the luma/chroma bitrate balance than using the delta QP control. As an example, results of the tool-off test of LMCS in SDR, RA configuration, with VTM6.0, and with the proposal, are reported.
- VTM6.0 LMCS off: PSNR-Y, -U, -V: 1.63%, -1.30%, -1.42%
- Proposal LMCS off: PSNR-Y, -U, -V: 1.53%, -0.22%, -0.39%
Other test results for SDR and HDR PQ are also reported. Encoding and decoding runtime impact is said to be negligible.
The additional offset allows additional control of the relative benefit of LMCS on luma and chroma. Currently LM has more benefit on luma than CS has on chroma.
It is generally agreed that this is beneficial in terms of better controlling the effect of LMCS on luma and chroma separately.
There are two versions proposed, one with a table, and one with one single offset that would be applied to all current CS table entries. The latter is preferable.
In this, the offset is currently signalled by a fixed length code with max. amplitude of 63, which would be equivalent to changing the chroma QP by 8. It seems that a range which would correspond to a QP change of 2 would be sufficient.
Decision: Adopt JVET-P0371 (version with one offset value, restricted to a range which would correspond to a QP change of 2)
Decision (CTC): Enable JVET-P0371 in CTC. Values for HDR and SDR still to be determined by proponents, such that the gain/loss would be homogeneous for luma and chroma in tool on / tool off testing. Should be done based on VTM7 release 1.