[FFmpeg-devel] libavfilter and subtitles (was: [RFC] Talk about subtitles)
Reimar.Doeffinger at gmx.de
Thu Dec 22 15:04:57 CET 2011
On Thu, Dec 22, 2011 at 02:32:09PM +0100, Nicolas George wrote:
> Le septidi 27 frimaire, an CCXX, Clément Bœsch a écrit :
> > First, writing the smovie source filter means adding the subtitles in the
> > filterchain.
> There are big issues that will arise with subtitles filtering if the
> architecture is kept the way it is.
> The problem I see is that subtitles, unlike audio and video, are sparse.
> Thus, with formats that mux the subtitles together with the video, accessing
> the next subtitle frame, as the result of request_frame, could trigger
> several megaoctets' worth of reads on the source file. For transcoding
> applications, this is a small drawback; for live playback this is a big no.
> Solving this issue in a satisfactory manner seems like a lot of work, but I
> believe in the long term it would be worth it.
> Filters with both continuous and sparse outputs should ignore request_frame
> and send on the sparse output when data is available. Filters with sparse
> inputs should not think that request_frame not triggering anything means the
> end of the stream.
> But for that to work, it is necessary that video and subtitles are demuxed
> together. If the filterchain is something like this:
> movie=input.vob [v]; smovie=input.mkv [s]; [v] [s] hardsub
> then smovie has only the sparse output, it can not keep it in sync with the
> video stream.
What would be the point of that? If the inputs are different you can
just read up to the next subtitle, for most formats that should not be
> IMHO, the real, clean, solution, would be to device a way for the movie
> source to be able to output both the video and the subtitles if necessary.
> And, while we are at it, the audio too.
That breaks down as soon as you _want_ to have a significant delay
between e.g. video and subtitle.
There are different options that seem more flexible to me:
1) request frame could have a time stamp. If there is no packet around
that time stamp, just return nothing. I think this will imply some
2) for sparse streams, we should automatically insert empty "virtual"
packets that have as pts the most recent time-stamp of which we know
there is no subtitle data for.
Though in light of seeking etc. I actually think that this would be much
easier if we re-introduced the end_pts again, but some people were
opposed enough to it to not want it despite its lack breaking MPlayer and
nobody really working on making things work properly without it.
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