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

Erik Miranda erikmiranda at gmail.com
Thu Apr 19 12:02:57 CEST 2012

On 18/04/2012 5:48 PM, Reimar Döffinger wrote:
> On Tue, Apr 17, 2012 at 04:53:57AM -0400, Erik Miranda wrote:
>> On Apr 16 21:44:48 CEST 2012, Reimar Döffinger<Reimar.Doeffinger at gmx.de>   wrote:
>>> On 16 Apr 2012, at 12:45, Joakim Plate<elupus at ecce.se<http://ffmpeg.org/mailman/listinfo/ffmpeg-devel>>   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.
>> In addition to reencoding, if I were to write, for example, a program which allows one to open and
>> analyze or edit subtitles, it would be useful to decode all subtitles while having access to the forced
>> flag. I'm sure there are other use cases as well. I find having to duplicate the stream and compare which
>> subtitles are present in both the forced-only stream and the all subtitles stream a very hackish
>> approach to access a piece of information that is so readily available. I chose AVSubtitleRect because
>> it seemed the most logical location to look for such a piece of information. I don't quite understand
>> how one would add an array to AVSubtitle without encountering similar breakage.
> I missed that we do not use an allocation function for AVSubtitle, so
> that would probably leave the only option to add it without breaking
> API to add it to the AVCodecContext, which is a bit ugly.

We don't use an allocation function for AVSubtitle, but the 
AVSubtitleRect structures are allocated within the decoders (for example 
pgssubdec.c:405), and the function avsubtitle_free() is used to 
deallocate the rects. So decoding-wise, I don't believe there should be 
any problem with adding a field. But now that you mention it, it may be 
a problem if AVSubtitleRect was allocated externally for for encoding. 
Perhaps we could include a subtitle_rect_version/spec/size field in 
AVCodecContext structure or another location which must be set and which 
an encoder should check before it attempts to access the "forced" field 
(or any future fields).

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