[FFmpeg-devel] Status and Plans for Subtitle Filters

[Marton Balint]

On Mon, 24 Feb 2020, Nicolas George wrote:

> Michael Niedermayer (12020-02-24):
>>> No, they can't: being the same subtitle or not is part of the semantic.
>
>> Does anyone else share this oppinion ?
>>
>> iam asking because we need to resolve such differences of oppinion to
>> move forward.
>> Theres no way to design an API if such relativly fundamental things
>> have disagreements on them
>
> It's not a matter of opinion, it is actually quite obvious:
>
> # 1
> # 00:00:10,000 --> 00:00:11,000
> # Hello.
> #
> # 2
> # 00:00:11,000 --> 00:00:12,000
> # Hello.
>
> … means that two people said Hello in quick succession while:

That is not the real issue (although the normally used techniques to 
signal different speakers is coloring, alignment or simply putting both 
sentences in a single subtitle).

The real issue is that for animations like \move{} the rendering cannot be 
splitted. So it seems if we want to support animations, hard splitting is 
not an option.

>
> Some subtitles have overlap all over the place. I am thinking in
> particular of some animé fansub, with on-screen signs and onomatopoeia
> translated and cultural notes, all along with dialogue. De-overlapping
> would increase their size considerably, and cause actual dialogue to be
> split, which results in the problems I have explained above.
>
> But I don't know why you are so focussed on this. Overlapping is not a
> problem, it's just something to keep in mind while designing the API,
> like the fact that bitmap subtitles have several rectangles. It's
> actually quite easy to handle.

My problem with overlapping is that in order to render subtitles 
at a given time you need more than one AVSubtitle. That is a 
fundamental difference to audio or video AVFrames where a single object 
fully represents the media at a given time.

Maybe we should deal with collections of AVSubtitles which affect time 
durations, this way you don't need to hard-merge the subtitle rectangles 
but still can reference objects which fully describe subtitles for a 
time period.

Regards,
Marton