[FFmpeg-devel] [PATCH v8 12/13] avfilter/split_cc: Add split_cc filter for closed caption handling

[Soft Works]

> -----Original Message-----
> From: ffmpeg-devel <ffmpeg-devel-bounces at ffmpeg.org> On Behalf Of Andreas
> Rheinhardt
> Sent: Wednesday, 22 September 2021 04:43
> To: ffmpeg-devel at ffmpeg.org
> Subject: Re: [FFmpeg-devel] [PATCH v8 12/13] avfilter/split_cc: Add split_cc
> filter for closed caption handling
> 
> Soft Works:
> >
> >
> >> -----Original Message-----
> >> From: ffmpeg-devel <ffmpeg-devel-bounces at ffmpeg.org> On Behalf Of Andreas
> >> Rheinhardt
> >> Sent: Wednesday, 22 September 2021 04:18
> >> To: ffmpeg-devel at ffmpeg.org
> >> Subject: Re: [FFmpeg-devel] [PATCH v8 12/13] avfilter/split_cc: Add
> split_cc
> >> filter for closed caption handling
> >>
> >>> This is a good question, as to whether it is safe to assume that the
> >>> filter's lifetime doesn't end before the last frame has been processed.
> >>> I thought it would be?
> >>
> >> So the frames are supposed to be never forwarded to the user? Or is the
> >> user supposed to stop using these frames after the filter's lifetime
> ended?
> >
> >
> > The subtitle_header is global and is constant across all frames.
> > A resulting media file has only a (= the) single "subtitle header".
> > (it's not like a "header for each subtitle line or event").
> >
> 
> Who ensures that it is constant across all frames? What happens if one
> mixes frames from two different sources? And who owns the subtitle header?

In cases when the subtitle stream originates from a decoder and frames 
are submitted into the filtergraph, ffmpeg.c makes a copy of it (field 
of AVCodecContext) and stores it in InputStream->subtitle_heartbeat->subtitle header 
and stores the pointer to this in each subtitle frame.
It gets freed during ffmpeg uninit.

In cases when the subtitle stream originates from a filter like split_cc
or graphicsubs2text, the filter creates the header and owns it, and stores 
the reference in each subtitle frame it produces.
It gets freed during filter uninit.

Relevant is the subtitle_header of the first frame that arrives at a target.
A target can be an encoder at the end of the filtergraph or other filters
like overlay_textsubs. 

Any filters in-between may use the subtitle_header for reference when 
performing certain manipulations, but are not allowed (meant to) modify it.

softworkz


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