[x264-devel] x264 encoding corruption bug w/ AltiVec enabled

[Henrik Gramner]

On Wed, Jan 30, 2019 at 10:17 PM Timothy Pearson
<tpearson at raptorengineering.com> wrote:
> On 01/30/2019 03:08 PM, Timothy Pearson wrote:
> > Pull the Big Buck Bunny 23 second file from here:
> >
> > https://commons.wikimedia.org/wiki/File:Big_Buck_Bunny_first_23_seconds_1080p.ogv
> >
> > Encode with:
> >
> > x264 --threads 31 --fps 30 --keyint 30 --min-keyint 30 --scenecut 0
> > --preset ultrafast --tune grain --bitrate 25600000 -o
> > Big_Buck_Bunny_encoding_failure.ogv
> > Big_Buck_Bunny_first_23_seconds_1080p.ogv
> >
> > I'm noticing with some interest that the resulting file is unplayable
> > (black screen) in VLC while mplayer shows intermittent artifacts like
> > the one I attached.  mplayer also complains about numerous errors, which
> > would probably explain why each video seems to have different artifacts:
>
> <snip>
>
> Digging a bit further the artifacting is also present when rebuilt with
> --disable-asm.  It seems disable-asm might have made the bug shift
> somewhere else under ffmpeg but not disappear when using the direct x264
> application.  Also, the bug is readily apparent when using "-threads 16"
> instead of "threads 31", so it doesn't appear related to the threading
> support.

Seems to occur when using ultrafast in combination with a very high
bitrate (low qp). Also with several year old x264 versions so no
recent breakage.

Haven't tested more than that.