JVET-M0301 Non-CE: Loop filter line buffer reduction [A. M. Kotra, S. Esenlik, B. Wang, H. Gao, J. Chen (Huawei)]

The current contribution proposes a mechanism of reducing the line buffer requirement of ALF (adaptive loop filter). The contribution uses the concept of virtual boundaries (VBs) which are upward shifted horizontal CTU boundaries by “N” samples. Modified ALF block classification and modified ALF filtering are applied for the samples which are near the virtual boundary to reduce the number of line buffers required. Modified ALF block classification only uses the samples which are above the VB to classify the given 4 x 4 block which is above VB. Similarly for the classification of the 4 x 4 block below VB, samples belonging to the lines below the VB are used. Modified ALF filtering uses a combination of conditional disabling and truncated versions of the original ALF filter.

Objective results are as follows: Over VTM 3.0 Anchor, for the case when “N” is 4 (AI, RA, LDB, LDP): Luma BD-Rate of 0.07%, 0.09%, 0.08%, 0.07% with no increase in EncT and DecT.

Over VTM 3.0 Anchor, for the case when “N” is 6 (AI, RA, LDB, LDP): Luma BD-Rate of 0.10%, 0.12%, 0.16%, 0.16% with no increase in EncT and DecT.

In terms of virtual boundary, the method is following the same concept as JVET-M0164. It is however somehow different in how the classification for padded samples is determined.

Investigate in CE, and this should include subjective testing.