[MPlayer-users] ssa/ass rendering uses lots of cpu with -vo gl

Reimar Döffinger Reimar.Doeffinger at gmx.de
Sat Jan 22 16:46:31 CET 2011

On Mon, Jan 17, 2011 at 05:40:34PM +0300, Vladimir Mosgalin wrote:
> Well, what you describe does sound costly but it feels just not right
> that amount of cpu that's additionaly required to displaying subtitles
> is more than amount of cpu used for rendering movie itself, I mean we
> are talking about 1920x1080x29.97 video already, even if subtitles have
> to be uploaded each frame, one would expect it to consume less resources
> than rendering video itself.

The OpenGL drivers tend to be very inefficient at uploading lots (160 in this case)
tiny objects.
I tried using PBOs, but unfortunately as usually with ATI drivers this makes things
even worse.
You can just manually load -vf ass for those files where the OpenGL vo handles the
subtitles badly, I can not offer any better solution currently.
In case you have a card with open-source drivers optimizing those to also handle
this kind of workload might be possible.
Unfortunately I have a hard time getting any useful performance numbers in any way,
e.g. perf does not get useable stack traces, under valgrind the relative performance
is just fine (!) and the OSD handling does not use a significant number of cycles
(indicating that synchronous data transfer to the GPU probably is the issue) so
it's a huge pain to improve anything - if that's even possible.

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