On Mon, Aug 04, 2014 at 08:19:38PM +0200, Hendrik Leppkes wrote: > Am 04.08.2014 19:59 schrieb "Gisle Sælensminde" <gi...@snirklasjon.no>: > > > > I'm trying to add support for closed captions in ffmpeg, namely cea608 and > > cea708. Unlike normal subtitles, these are embedded in the video frames > > themselves rather than a separate track or file. As a first step I try to > write > > bitstreams filters for extraction and inserting closed captions into > h.264 and > > mpeg2 videos. For cea608, I use .scc files to represent the cea608 data. > > Later an interpreter and producer of scc files can be made as a subtitle > > codec. > > > > I have already written a cea 608 tools for my employer, but that is in > > python, so not suitable for inclusion in ffmpeg, so this will be a > > new implementation. > > > > I'm currently writing the extraction filter and I had hoped that I could > use > > it as a filter with parameters as follows. > > > > ffmpeg -i vid_with_cc.ts -acodec copy -vcodec copy -bsf > cea680_extract?scc=out.scc -f null /dev/null > > > > This is not possible, since bitstream filters don't have parameters. The > main > > problems can be summarized as follows: > > > > - The lack of parameters means that I don't have a way to specify where > to store > > the .scc file. How can I do that? > > > > - I can't find a way to get out the timestamps (pts and dts) in a > bitstream > > filter. The cea 608 and 708 data is stored in pts order in the frames, > so > > I need to reorder the data before writing them to file. Also, the scc > files > > have timestamps, so I need timestamps for that too. I can only find the > > timebase in AVCodecContext, and the AVFormatContext or AVPacket is not > passed > > in to the bitstream filter. The AVPacket is a deprecated field, but it > is > > NULL, so of no use. Is there something I have overlooked here? > > > > I may be missing something, and it is of cause possible that the > bitstream filter > > approch is misguided, but as far as I can tell, that is the best option. > Does anyone > > have a clear idea of how this should be done? > > > Its probably a better idea to export the CC data during video decoding as > side data, like the mpeg2 decoder already does today. That way you get it > reordered and with timestamps, but of course it means you need to perform > decoding.
It's also fairly silly, since it means you have to decode the video just to get the subtitle stream! And as far as I can tell if you want to remux but with separate subtitle stream that would even mean that you have to re-encode the video for no good reason. A _good_ (but complex/quite some effort) solution would be for the AVParser to extract it (optionally even removing it from the video frames?) and create a proper subtitle stream. _______________________________________________ ffmpeg-devel mailing list ffmpeg-devel@ffmpeg.org http://ffmpeg.org/mailman/listinfo/ffmpeg-devel
