[FFmpeg-devel] GSoC 2025 - OggKate subtitles support

[epirat07 at gmail.com]

On 4 Mar 2025, at 20:44, Soft Works wrote:

>> -----Original Message-----
>> From: ffmpeg-devel <ffmpeg-devel-bounces at ffmpeg.org> On Behalf Of
>> Gianluca Cannata
>> Sent: Dienstag, 4. März 2025 09:40
>> To: ffmpeg-devel at ffmpeg.org
>> Subject: [FFmpeg-devel] GSoC 2025 - OggKate subtitles support
>>
>> Good morning,
>>
>> I have to write a karaoke solution with FFmpeg libav libraries for a
>> project of mine and I have found OggKate subtitles a good choice to
>> implement such solution and I am thinking of writing a Kate demuxer and
>> decoder along with a filter that will use OpenGL to render Kate
>> subtitles
>> capabilities on frame.
>>
>> What do you think ?
>>
>> There is someone interested in mentoring this proposal project ?
>>
>> Or is it too overkill ?
>>
>> Sincerely
>>
>> Gianluca
>> _______________________________________________
>
>
> Hi Gianluca,
>
> while it's true that "OggKate" doesn't appear to be well-known (I've never heard of it before), but it's also the only officially spec-ed subtitle format in Ogg containers (besides raw text).
> I think that fact makes it interesting to implement it - one could also argue that the reason why it isn't widespread yet is that ffmpeg doesn't support it 😊
>
> The license of the reference implementation source code appears to be permissive (https://github.com/Distrotech/libkate?tab=License-1-ov-file#readme), so an adaption might be possible as well - yet, IANAL.

The official upstream repo for kate is here btw https://gitlab.xiph.org/xiph/kate

>
> Regarding the effort, I think that the implementation of a renderer is too steep, especially when aiming to support fancy things like animation. Anyway, a specific renderer would not quite fit into the ffmpeg architecture, because:
>
> Every (text) subtitle decoder decodes to ASS format and every (text) subtitle encoder encodes from ASS to its target format. This means that implementing a decoder for OggKate would require to translate the Kate format to ASS.
> Since we do have an ASS renderer (via libass), it would be still possible to achieve the goal of rendering those subtitles, even with animations.
>
>
> I have seen that there are two representations of Kate subtitles: A text format and a bitstream format. What gets muxed into an Ogg container is always the bitstream format, the text/file format is just an extra and not needed, so it can be skipped, which means in turn that there's no need for developing a demuxer.
>
> As mentioned above, for the format to have a chance to become more widespread, there would also need to be an encoder.
>
> Making this a project about developing an encoder+decoder pair for Ogg Kate would appear to be a nice project IMO.
>
> sw
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