[FFmpeg-devel] [RFC] General design for subtitles demuxers

[Clément Bœsch]

On Wed, Jun 13, 2012 at 08:53:27AM +0200, Nicolas George wrote:
> Le sextidi 26 prairial, an CCXX, Clément Bœsch a écrit :
> > Yet another subtitles thread, addressing this time several issues in a row.
> > 
> > All the text subtitles formats are small files where it is interesting to
> > consider them as such, and not as a stream. In practice I propose the
> > following:
> > 
> > 1) Parse the whole file
> > 
> >    Just like lavf/assdec or lavf/jacosubdec, the whole file should be parsed on
> >    the read_header() callback.
> > 
> >    Why is it done like this? Because in "format" oriented ("ASS"), the events
> >    can be in any order, require looking ahead, or preprocessing the file; when
> >    they are muxed ("SSA") the timeline must makes sense, and it doesn't matter
> >    since this demuxing belongs to the video demuxer.
> > 
> >    JACOsub also has that issue, but SubRip as well (remember the event
> >    numbers?), and actually all the others (see below).
> > 
> > 2) Set missing durations
> > 
> >    Since we now have the complete list of events in a list, it's easy to work
> >    on them, and we can just set the packets duration when missing that
> >    information: this actually solves in a very simple way the
> >    last-to-next-subtitle feature I mentioned several times.
> > 
> > 3) Sort the events
> > 
> >    We should then export the sorting of all these subtitles (PTS based) raw
> 
> 
> Until now, I agree at 100%.
> 

Great. I'll see what I can do on this ASAP then.

> >    events from lavf/assdec.c.
> 
> This part I am not sure I understand. Do you mean that the demuxer is
> supposed to translate into ASS?
> 

I'm just refering here to:

    static int event_cmp(uint8_t **a, uint8_t **b)
    {
        return get_pts(*a) - get_pts(*b);
    }

    [...]

    qsort(ass->event, ass->event_count, sizeof(*ass->event), (void*)event_cmp);

> >    We should take out read_seek2() from lavf/assdec.
> 
> Looks reasonable.
> 

We just need to find a better way of storing the events & pts; maybe to
something simple like

    typedef struct {
        int64_t pts;
        uint8_t *data;
    } FFSubtitle;

We will BTW be able to have a generic read_packet() as well, and greatly
simplify again all this stuff.

-- 
Clément B.
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