[FFmpeg-devel] [PATCH] Exposing forced flag for DVD and PGS subtitles

Michael Niedermayer michaelni at gmx.at
Wed Apr 18 01:15:14 CEST 2012

On Tue, Apr 17, 2012 at 06:52:00PM -0400, Erik Miranda wrote:
> On 17/04/2012 10:50 AM, Michael Niedermayer wrote:
> >On Mon, Apr 16, 2012 at 09:44:48PM +0200, Reimar Döffinger wrote:
> >>On 16 Apr 2012, at 12:45, Joakim Plate<elupus at ecce.se>  wrote:
> >>>Erik Miranda<erikmiranda<at>  gmail.com>  writes:
> >>>>This patch adds a "forced" member to the AVSubtitleRect structure and
> >>>>implements setting this flag in the DVD and PGS subtitle decoders, since
> >>>>these are the formats I'm aware of that include such a function. Since
> >>>>this patch changes AVSubtitleRect, it leads to a public API change. As
> >>>>far as I can tell, this is the best method to allow access to this
> >>>>information. Please advise me if otherwise, or if further changes are
> >>>>required as part of changing AVSubtitleRect. I think the libavcodec
> >>>>version would need a bump, correct?
> >>>There is already a content dispostion type for forced. That is stream
> >>>global thou. So one option would be to duplicate forced subs into a
> >>>secondary subtitle stream.
> >>What would be the point? What would someone do with that information except drop all non-forced ones?
> >>I guess it might be relevant if you want to preserve the info when reencoding.
> >>It should be possible to avoid the API/ABI breakage by adding an extra array to AVSubtitle instead of putting it into the AVSubtitleRect.
> >>Though we might consider changing the whole API so extending the rect is possible without changing the API in the future.
> >Is there really a ABI/API issue with adding fields to AVSubtitleRect ?
> >rect is a array of pointers
> >
> >assuming theres no inherent problem with adding fields to
> >AVSubtitleRect
> >to be able to maintain compatibility with forks of ffmpeg (which we
> >try to currently)
> >a bit of extra red tape is needed see fef411ef for how it was done for
> >AVFrame
> >Also just grep for avcodec_get_frame_class() to see code using this
> >
> >Code inside the lib declaring the AVSubtitleRect struct of course can
> >do direct accesses
> >its just applications which want to use shared libs that should avoid
> >direct access
> >This way shared libs can be exchanged between various forks in a binary
> >compatibile way.
> >
> >[...]
> Just for clarification, I took a look at fef411ef, and I presume the
> idea is to AVOption enable AVSubtitleRect. When I read the
> documentation on AVOption, normally the struct would begin with a
> "class" field, but this would break compatibility which is why I
> understand the class field was not added when doing the same for
> AVFrame in fef411ef, and instead a "get_frame_class" function was
> added instead, correct?


> Also, since this would be (from my
> understanding) a "fake" object, would I need to make any changes to
> follow the procedure of av_opt_set_defaults() and av_opt_free()?

do you need these 2 ?

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