Change #271411

Comments

avcodec/snowenc: fix chroma reference edge extension for odd width
When extending the edges of the reconstructed reference frame, the chroma
plane width and height were passed to draw_edges() as w >> chroma_h_shift
(floor) instead of AV_CEIL_RSHIFT(). For an odd width with subsampled chroma
(e.g. 4:2:0) the chroma plane is one column wider than the floor, and
draw_edges() replicates the edge columns, so it overwrote the last real
chroma column with the previous one. The decoder does not draw_edges() its
reference, so its copy kept the correct value: the encoder and decoder
references diverged and every following inter frame was reconstructed
differently by the two, breaking bit-exact (lossless) round-tripping and
drifting lossy inter coding, for any odd-width 4:2:0 snow stream.

Use AV_CEIL_RSHIFT(), matching the input_picture edge extension just above.

Found-by: Claude
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

Changed files

• libavcodec/snowenc.c