[Ffmpeg-devel] H.264 decoding bug in either lavc/JM (x264 too?)

[Loren Merritt]

On Fri, 22 Jul 2005, Tuukka Toivonen wrote:

> I encoded a video clip using x264 and decoded with both
> Mplayer (lavc) and JM 9.6. The two decoded videos differ
> slightly on some frames. I think this is bug, as
> H.264 should be bit exact, shouldn't it?
>
> Here's frame-by-frame comparison:
[...]
>
> x264 command line:
> --bframes 2 --8x8dct --b-pyramid --qp 26 --direct spatial
> The problem occurs only if using all three options --8x8dct, --b-pyramid,
> and --direct spatial. Most other options seem not to matter.

Possible cause:
h264.c:1207 should fall back to h->ref_list[1][0].motion_val[1][b4_xy] if 
h->ref_list[1][0] is a B-frame and the colocated block is L1 predicted.

I can't see what 8x8dct would have to do with it.

> For now, for encoding with x264, --direct temporal
> should be used (it increases bit rate around 3% at fixed
> PSNR, but removing --8x8dct or --b-pyramid has larger
> impact, 4-5%).

btw, I think the 3% penatly goes away if you use --ref > 1.

--Loren Merritt