[FFmpeg-devel] Rendering text-based subtitles
joolzg at btinternet.com
Sat Jun 4 15:13:03 CEST 2011
----- Original Message -----
> From: JULIAN GARDNER <joolzg at btinternet.com>
> To: FFmpeg development discussions and patches <ffmpeg-devel at ffmpeg.org>
> Sent: Saturday, 28 May 2011, 2:55
> Subject: Re: [FFmpeg-devel] Rendering text-based subtitles
>> From: Chris Burel <chrisburel at gmail.com>
>> To: ffmpeg-devel at ffmpeg.org
>> Sent: Saturday, 28 May 2011, 2:51
>> Subject: [FFmpeg-devel] Rendering text-based subtitles
>> I'm interested in getting support for rendering text based subtitles,
>> ass and srt. This would be useful for encoding text-based subtitles to
>> xsub. I've hacked in a patch to get this to work using libass, but I
> had to
>> do some things that seem wrong to get it to work. I'm writing this to
>> the process of cleaning up this patch so it can be merged in.
>> Here's some of the problems I faced:
>> Since I'm mainly concerned with playing the result of this process on
>> PS3, I was mainly concerned with getting the xsub encoder to work. On my
>> first pass, I put the rendering code into the xsub encoder. This proved to
>> be problematic for a couple of reasons. #1, it seemed wrong to have the
>> xsub encoder know about how to render ass subtitles. #2, ass allows
>> timecodes to overlap, but the PS3 doesn't. So for example, let's
> say two
>> ass events overlap. xsub encode needs to adjust the end time of the first
>> event to be just before the begin time of the next one. But the encoder
>> doesn't have access to the next event yet, so it can't do that. To
>> these issues, I moved the rendering code into the decoder of the ass
>> subtitles. The first problem I encountered here is that the decoder
>> know the output resolution (which libass needs to know). So right now I
>> have a resolution hard-coded in the decoder, which is terrible. But less
>> terrible than not having subtitles if they're timecodes overlap.
>> Another problem is that I had to make an addition to a struct in the libass
>> library that breaks its ABI. The author of that library didn't want to
>> include my patch because of this. The change is relatively small, but still
>> requires adding some fields to a struct. I created a bug/issue report for
>> this on the libass google code page:
>> A couple questions:
>> Is this a feature that the maintainers of ffmpeg would like to see merged
>> Where does the rendering code really belong? In the ass decoder? In the
>> ass encoder? Somewhere else entirely?
>> Is linking in libass a problem?
>> I wasn't planning on posting the patch yet, because this is my first
>> coding in ffmpeg and I don't want to embarrass myself :-). But if none
>> my questions make sense without seeing some code, then I'll post it.
>> ffmpeg-devel mailing list
>> ffmpeg-devel at ffmpeg.org
> I would be happy to help on this but im looking to render dvbsubs as hard
> I have a similar problem to you but in my case i get a start time and timeout,
> in case of a loss of stream or corruption.
> I fixed the decoder/encoder and now can make a ts with dvbsubs that work so was
> looking at now adding in hard subs,
> Let me know if your interested in joining forces to get this working for all
> To the ffmpeg guys is there a way or an idea how to do something along the lines
> of "decode ass/xsub/srt" subtitles and "encode dvbsubs", as
> we only have -scodec
> ffmpeg-devel mailing list
> ffmpeg-devel at ffmpeg.org
Any more info, code Chris?
Also can anyone tell me of a "good" way of stopping the teletext being passed on to the output system once i have rendered them to the video.
At the moment the code would put the subs on the screen AND then still be added to the outgoing stream, so how do i stop this from happening?
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