[FFmpeg-devel] GSoC 2025 - OggKate subtitles support

[FFmpeg-devel] GSoC 2025 - OggKate subtitles support

[Gianluca Cannata]

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
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Re: [FFmpeg-devel] GSoC 2025 - OggKate subtitles support

[Nicolas George]

Gianluca Cannata (HE12025-03-04):
> 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 ?

I think I have never encountered an Ogg Kate file in the wild and I do
not know of any specific software that supports them. That makes me
think Ogg Kate is one of these dead Xiph projects and supporting it
would be a waste of time.

Which features do you require for your project?

Regards,

-- 
  Nicolas George
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Re: [FFmpeg-devel] GSoC 2025 - OggKate subtitles support

[Gianluca Cannata]

Hi Nicolas,

I would like to add motion to subtitles like fading, scrolling, changing
colors and font size, eccetera.

Is it something that can be of public interest ?

If not, which mailing list is the most appropriate to ask support for
implementing such a thing ?

Sincerely

Gianluca

Il giorno mar 4 mar 2025 alle ore 11:46 Nicolas George <george@nsup.org> ha
scritto:

> Gianluca Cannata (HE12025-03-04):
> > 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 ?
>
> I think I have never encountered an Ogg Kate file in the wild and I do
> not know of any specific software that supports them. That makes me
> think Ogg Kate is one of these dead Xiph projects and supporting it
> would be a waste of time.
>
> Which features do you require for your project?
>
> Regards,
>
> --
>   Nicolas George
> _______________________________________________
> ffmpeg-devel mailing list
> ffmpeg-devel@ffmpeg.org
> https://ffmpeg.org/mailman/listinfo/ffmpeg-devel
>
> To unsubscribe, visit link above, or email
> ffmpeg-devel-request@ffmpeg.org with subject "unsubscribe".
>
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Re: [FFmpeg-devel] GSoC 2025 - OggKate subtitles support

[Nicolas George]

Gianluca Cannata (HE12025-03-04):
> I would like to add motion to subtitles like fading, scrolling, changing
> colors and font size, eccetera.

ASS already supports all this. Have you tried using it for your project?

> Il giorno mar 4 mar 2025 alle ore 11:46 Nicolas George <george@nsup.org> ha
> scritto:

Top-posting is forbidden on this mailing-list. If you do not know what
it means look it up.

Regards,

-- 
  Nicolas George
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Re: [FFmpeg-devel] GSoC 2025 - OggKate subtitles support

[Gianluca Cannata]

> ASS already supports all this. Have you tried using it for your project?

I have not tried yet. I will have.

> Top-posting is forbidden on this mailing-list. If you do not know what
it means look it up.

Sorry.

Thank you again for your support.

Sincerely

Gianluca
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Re: [FFmpeg-devel] GSoC 2025 - OggKate subtitles support

[Soft Works]

> -----Original Message-----
> From: ffmpeg-devel <ffmpeg-devel-bounces@ffmpeg.org> On Behalf Of
> Gianluca Cannata
> Sent: Dienstag, 4. März 2025 09:40
> To: ffmpeg-devel@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.

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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Re: [FFmpeg-devel] GSoC 2025 - OggKate subtitles support

[epirat07]

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

>> -----Original Message-----
>> From: ffmpeg-devel <ffmpeg-devel-bounces@ffmpeg.org> On Behalf Of
>> Gianluca Cannata
>> Sent: Dienstag, 4. März 2025 09:40
>> To: ffmpeg-devel@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
>
>
>
>
>
>
>
>
> _______________________________________________
> ffmpeg-devel mailing list
> ffmpeg-devel@ffmpeg.org
> https://ffmpeg.org/mailman/listinfo/ffmpeg-devel
>
> To unsubscribe, visit link above, or email
> ffmpeg-devel-request@ffmpeg.org with subject "unsubscribe".
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Re: [FFmpeg-devel] GSoC 2025 - OggKate subtitles support

[Soft Works]

> -----Original Message-----
> From: ffmpeg-devel <ffmpeg-devel-bounces@ffmpeg.org> On Behalf Of
> epirat07@gmail.com
> Sent: Dienstag, 4. März 2025 22:06
> To: FFmpeg development discussions and patches <ffmpeg-devel@ffmpeg.org>
> Subject: Re: [FFmpeg-devel] GSoC 2025 - OggKate subtitles support
> 
> 
> 
> On 4 Mar 2025, at 20:44, Soft Works wrote:
> 
> >> -----Original Message-----
> >> From: ffmpeg-devel <ffmpeg-devel-bounces@ffmpeg.org> On Behalf Of
> >> Gianluca Cannata
> >> Sent: Dienstag, 4. März 2025 09:40
> >> To: ffmpeg-devel@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


Thanks, I've been there but it was slow in loading, now I looked again and realized that there's new activity and new versions since Feb 2025 - after a 14 years break. That's a nice coincidence.
Weirdly it's the second resurrection I discovered today, there's also a new Aegisub version after 10 years of silence 😊

sw



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