JVET-O0494 AHG12: On filtering of independently coded region [R. Skupin, Y. Sanchez, K. Sühring, T. Schierl (HHI)]

Also included in the agenda item 6.18.

This contribution proposes to changes to the filtering of independently coded regions in order to allow drift free filtering and is twofold:

The first aspect is to reduce the number of in-loop filter affected samples at boundaries of independently coded regions by disabling certain in-loop filters. Two options are given to reduce the filter reach, i.e. the in-loop filter affected samples, to either 7 or 3 samples.

The second aspect is to apply padding in order to replace in-loop filter affected samples at the boundaries of independent regions for inter-prediction.

Revision 1 of the document adds the accumulated filter reach.

Revision 2 of the document adds specification text.

The independent coded regions are processed at the encoder such that loop filtering is applied across region boundaries, whereas a decoder would pad in case when it decodes an independent region but the neighbour region is not decoded. This may cause drift, which may be acceptable for an application.

This corresponds to MCTS of HEVC, where however the problem is less severe because

1. No ALF exists
2. No long deblocking filter exists
3. No padding is performed in MC, whereas the corresponding concept investigated in CE12 of Marrakesh pads samples to outside of the region, and these padded samples could already be wrong as they were generated by loop filters.

More study necessary to see how these problems can be resolved. There is a penalty in compression performance for this functionality. Therefore, the padding in MC which seems part of the problem might need to be re-considered (and simply disallow MV that point outside as in HEVC MCTS). Another aspect might be disallowing long deblocking filters (could this be done as encoder constraint?). For ALF, needs to be clarified if the virtual boundary concept helps.

Further study recommended.

JVET-O0494 AHG12: On filtering of independently coded region [R. Skupin, Y. Sanchez, K. Sühring, T. Schierl (HHI)]

Also included in the agenda item 6.5.