JVET-P0164 CE5-related: Simplified ALF syntax regarding to alf_luma_coeff_signalled_flag and alf_luma_coeff_flag [C.-Y. Lai, C.-Y. Chen, T.-D. Chuang, O. Chubach, Y.-W. Huang, S.-M. Lei (MediaTek)]
In VTM6.0 adaptive loop filter (ALF), alf_luma_coeff_signalled_flag is used to indicate whether a zero filter exists in the filter set. If alf_luma_coeff_signalled_flag is true, an alf_luma_coeff_flag is signalled for each filter in the filter set to indicate whether the filter is a zero filter. In this contribution, two methods to simplify the related ALF syntax are proposed. In Method 1, alf_luma_coeff_signalled_flag is signalled before alf_luma_num_filters_signalled_minus1 with removal of alf_luma_coeff_flag. In Method 2, alf_luma_coeff_signalled_flag and alf_luma_coeff_flag are both removed. It is asserted that both methods simplify the related specification text. BD-rates are reported as follows.
Method 1:
AI: 0.00% (Y), 0.00% (Cb), 0.00% (Cr)
RA: 0.00% (Y), 0.00% (Cb), 0.00% (Cr)
LB: -0.01% (Y), 0.08% (Cb), 0.12% (Cr)
Method 2:
AI: 0.00% (Y), 0.00% (Cb), 0.00% (Cr)
RA: 0.00% (Y), 0.01% (Cb), 0.00% (Cr)
LB: -0.03% (Y), -0.03% (Cb), -0.08% (Cr)
Proponents prefer method 1.
It was mentioned that the proposed syntax in method 1would force the encoder to merge the zero-valued ALF filters.
Method 2 would require to signal coefficients for all filters.
BoG recommendation: It was recommended to adopt Method 2 (and SW from this proposal).