JVET-AE0123 AHG10: Lagrange multiplier optimization for chroma ALF and CCALF [S.-W. Xie, Y. Gao, M.-H. Jia, Y.-L. Hu, C. Huang, P. Wu (ZTE)]

This contribution reports further results for the RDO method in ALF, which was proposed in JVET-AD0136. Regarding the proposed method, the chroma Lagrange multiplier will be adjusted by a scaling factor when the luma ALF APS transmission flag is ON and the number of history chroma ALF APS is insufficient. With VTM20.0 as anchor and test platform, the reported overall PSNR-Y,U,V BD-rate and {EncT, DecT} variations of this proposal are as follows:

• LDB: 0.05%, -0.80%, -1.18% {100%, 100%}
• LDP: 0.02%, -0.89%, -1.03% {100%, 101%}

As a general policy, encoder optimizations of this kind should equally be applied to the VTM and ECM. In the last meeting, it had specifically been requested to improve the performance of the ECM (where apparently losses had been observed, according to the JVET-AD1000 meeting notes. It was verbally reported that the ECM performance would be similar with the new proposal, but no results were provided.

For VTM, the results look beneficial, chroma gains outweigh the small luma losses.

It was pointed out that for ECM it might be necessary to adjust the multiplier differently.

Further study was encouraged on implementing the method on top of the ECM.