[FFmpeg-devel] Status and Plans for Subtitle Filters

Matt Zagrabelny mzagrabe at d.umn.edu
Mon Feb 24 22:56:22 EET 2020

On Sat, Feb 22, 2020 at 2:47 AM Clément Bœsch <u at pkh.me> wrote:
> On Fri, Feb 14, 2020 at 03:26:30AM +0000, Soft Works wrote:
> > Hi,
> >
> Hi,
> > I am looking for some guidance regarding future plans about processing subtitle streams in filter graphs.
> >
> > Please correct me where I'm wrong - this is the situation as I've understood it so far:
> [...]
> Your analysis was pretty much on point. I've been away from FFmpeg development
> from around the time of that patchset. While I can't recommend a course of
> action, I can elaborate on what was blocking and missing. Beware that this is
> reconstructed from my unreliable memory and I may forget important points.
> Last state can be found at https://github.com/ubitux/FFmpeg/tree/subtitles-new-api
> The last WIP commit includes a TODO.txt which I'm sharing here for the
> record:
> > TODO:
> > - heartbeat mechanism
> > - drop sub2video (needs heartbeat)
> > - properly deal with -ss and -t (need strim filter?)
> > - sub_start_display/sub_end_display needs to be honored
> > - find a test case for dvbsub as it's likely broken (ffmpeg.c hack is
> >   removed and should be replaced by a EAGAIN logic in lavc/utils.c)
> > - make it pass FATE:
> >   * fix cc/subcc
> >   * broke various other stuff
> > - Changelog/APIchanges
> > - proper API doxy
> > - update lavfi/subtitles?
> > - merge [avs]null filters
> > - filters doc
> > - avcodec_default_get_buffer2?
> > - how to transfer subtitle header down to libavfilter?
> The biggest TODO entry right now is the heartbeat mechanism which is required
> for being able to drop the sub2video hack. You've seen that discussed in the
> thread.
> Thing is, that branch is already a relatively invasive and may include
> controversial API change. Typically, the way I decided to handle subtitle
> text/rectangle allocation within AVSubtitle is "different" but I couldn't come
> up with a better solution. Basically, we have to fit them in AVFrame for a
> clean integration within FFmpeg ecosystem, but subtitles are not simple buffers
> like audio and video can be: they have to be backed by more complex dynamic
> structures.
> Also unfortunately, addressing the problem through an iterative process is
> extremely difficult in the current situation due to historical technical debt.
> You may have noticed that the decode and encode subtitles API are a few
> generations behind the audio and video ones. The reason it wasn't modernized
> earlier was because it was already a pita in the past.
> The subtitles refactor requires to see the big picture and all the problems at
> once. Since the core change (subtitles in AVFrame) requires the introduction of
> a new subtitles structure and API, it also involve addressing the shortcomings
> of the original API (or maybe we could tolerate a new API that actually looks
> like the old?). So even if we ignore the subtitle-in-avframe thing, we don't
> have a clear answer for a sane API that handles everything. Here is a
> non-exhaustive list of stuff that we have to take into account while thinking
> about that:
> - text subtitles with and without markup
> - sparsity, overlapping
> - different semantics for duration (duration available, no known duration,
>   event-based clearing, ...)
> - closed captions / teletext
> - bitmap subtitles and their potential colorspaces (each rectangle as an
>   AVFrame is way overkill but technically that's exactly what it is)
> This should give you a hint on why the task has been quite overwhelming.
> Subtitles were the reason I initially came into the multimedia world, and they
> might have played a role in why I distanced myself from it.
> That said, I'd say the main reason it was put in stand by was because I was
> kind of alone in that struggle. While I got a lot of support from people, I
> think the main help I needed would have been formalizing the API we wanted.
> Like, code and API gymnastic is not that much of a problem, but deciding on
> what to do, and what path we take to reach that point is/was the core issue.
> And to be honest, I never really made up my mind on abandoning the work. So I'm
> calling it again: if someone is interested in addressing the problem once and
> for all, I can spend some time rebasing the current state and clarifying what has
> been said in this mail in the details so we can work together on an API
> contract we want between FFmpeg and our users. When we have this, I think
> progress can be made again.

Nicolas and Clément, et. al,

Is financial support at all blocking progress in subtitle filters?

I'm afraid I don't have much ffmpeg coding expertise to contribute,
but I am interested in seeing better subtitle support in ffmpeg and am
looking to help where I can.

Let us know if there is anything else that non-coders could help with.



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