Using loop filter level from adjacent superblock
Please see libaom issue 2585: https://bugs.chromium.org/p/aomedia/issues/detail?id=2585
dav1d and libaom do the same thing, but is it correct? In the C versions of the loop filter in dav1d (loopfilter_tmp.c) the question is whether the lines
const int L = l[0][0] ? l[0][0] : l[-1][0]; const int L = l[0][0] ? l[0][0] : l[-b4_stride][0];
are correct. Does the spec mandate anywhere that l[0][0] == 0 is a special case where adjacent levels should be used instead?