[FFmpeg-devel] subtitles filter and -ss

[Manolis Stamatogiannakis]

Hello,

I've noticed what appears to be a bug/missing feature in the subtitles
filter: when "-ss" is used for the input, it is not applied to the
subtitles stream. E.g., for the following command line, the video playback
will start on 20:10, but the subtitles will start from 00:00.

ffmpeg -re -ss 20:10.00 -i input.mp4
        -vf subtitles=subs.srt \
        -c:v libx264 crf 25 -c:a aac -ab 160k \
        -strict experimental \
        -f flv $RTMP_URL

This should be fairly straightforward to fix and review, so I'd like to
directly submit a patch rather than opening a bug and having someone else
fix it. As I'm not an expert with the ffmpeg internals, I'd like some
guidance on implementing a proper fix.

What would be considered more appropriate:
(a) automatically skipping the subtitles for the time specified by -ss?
(b) adding a shift option to the subtitles filter that allows shifting
subtitles time by specific time?
(c) something else I can't think of?

Although (a) would probably be a bit more elegant to use, I'm not sure if
it always makes sense as ffmpeg may use several input files.

Thanks,
Manolis