[FFmpeg-devel] [RFC] Talk about subtitles

Clément Bœsch ubitux at gmail.com
Sat Dec 17 23:34:56 CET 2011

On Tue, Nov 22, 2011 at 07:56:41PM +0100, Clément Bœsch wrote:
> Hi everyone,
> [TL;DR: I want to improve/rewrite a real subtitles support in FFmpeg and need
> some hints.]

Here is some news about the current state. To be honest I didn't produce
anything usable as a PoC yet, but I expected a few issues we need to

First, writing the smovie source filter means adding the subtitles in the
filterchain. Since the AVFrame is not in lavfi, maybe AVSubtitle hasn't
its place either.

Another related issue is what needs to be part of AVFilterBuffer and what
needs to be part of AVFilterBufferRef. I started with a AVSubtitle in
AVFilterBuffer, but I guess it won't be enough at some point.

Now the real blocking thing ATM seems to be the (apparent?) inability of
the filters to deal with different types between in & out: whatever
solution we will select with filters will lead to a filter which needs to
make the link between the subtitles and the video output. This is exactly
the same issue as the visualizations: audio input & video output. Maybe a
first simple step will be to update the API so this kind of thing work.

I tried to write some kind of video source filter, but added an audio
input; the filtergraph seems to deal fine with it ATM (-f lavfi
amovie=a.mp3,visualization) and just outputs video, but of course the
filter_samples() callback of the audio input is never called; so it's
actually pretty useless. I'm not sure the current state of the API could
deal with such issue; I'm trying right now to manually request some audio
directly in the video request_frame() callback...

Anyway, this is just a feedback on the current situation; any hint, advise
or other kind of help welcome ;)

Clément B.
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