[FFmpeg-devel] Subtitle filters in libavfilter

Chris Burel chrisburel at gmail.com
Tue Jan 29 22:42:38 CET 2013

A while ago, I wrote about trying to add support for rendering text-based
subtitles.  What I'm trying to achieve is to support transcoding .ass
subtitles to xsub, to stream to the PS3.

I saw recently that some new video filters had been added, one called "ass"
and one called "subtitle", for hard-coding the subtitles into the video
stream.  These filters were implemented as video filters.  I started
looking at the code, and it seems like most of the code could be reused if
there were a bit more division of labor.  I'd like to see one filter for
rendering text subtitles to images, that would output a subtitle stream,
and a separate filter that would hard-code those subtitles on top of a
video stream.  That way, all the code for rendering the subtitles could be
used for transcoding them to any image-based subtitle format.

I started looking at adding subtitle support to libavfilter, but I got
bogged down in bits that I don't see how they apply to a subtitle stream,
like the query_formats() function.  Is there someone that could help me
write a null subtitle filter, to see what all the necessary parts are?


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