1. 30 Apr, 2013 1 commit
  2. 18 Apr, 2013 1 commit
    • Clément Bœsch's avatar
      subtitles: introduce ASS codec id and use it. · 7c1a002c
      Clément Bœsch authored
      Currently, we have a AV_CODEC_ID_SSA, which matches the way the ASS/SSA
      markup is muxed in a standalone .ass/.ssa file. This means the AVPacket
      data starts with a "Dialogue:" string, followed by a timing information
      (start and end of the event as string) and a trailing CRLF after each
      line. One packet can contain several lines. We'll refer to this layout
      as "SSA" or "SSA lines".
      
      In matroska, this markup is not stored as such: it has no "Dialogue:"
      prefix, it contains a ReadOrder field, the timing information is not in
      the payload, and it doesn't contain the trailing CRLF. See [1] for more
      info. We'll refer to this layout as "ASS".
      
      Since we have only one common codec for both formats, the matroska
      demuxer is constructing an AVPacket following the "SSA lines" format.
      This causes several problems, so it was decided to change this into
      clean ASS packets.
      
      Some insight about what is changed or unchanged in this commit:
      
        CODECS
        ------
      
        - the decoding process still writes "SSA lines" markup inside the ass
          fields of the subtitles rectangles (sub->rects[n]->ass), which is
          still the current common way of representing decoded subtitles
          markup. It is meant to change later.
      
        - new ASS codec id: AV_CODEC_ID_ASS (which is different from the
          legacy AV_CODEC_ID_SSA)
      
        - lavc/assdec: the "ass" decoder is renamed into "ssa" (instead of
          "ass") for consistency with the codec id and allows to add a real
          ass decoder. This ass decoder receives clean ASS lines (so it starts
          with a ReadOrder, is followed by the Layer, etc). We make sure this
          is decoded properly in a new ass-line rectangle of the decoded
          subtitles (the ssa decoder OTOH is doing a simple straightforward
          copy). Using the packet timing instead of data string makes sure the
          ass-line now contains the appropriate timing.
      
        - lavc/assenc: just like the ass decoder, the "ssa" encoder is renamed
          into "ssa" (instead of "ass") for consistency with the codec id, and
          allows to add a real "ass" encoder.
      
          One important thing about this encoder is that it only supports one
          ass rectangle: we could have put several dialogue events in the
          AVPacket (separated by a \0 for instance) but this would have cause
          trouble for the muxer which needs not only the start time, but also
          the duration: typically, you have merged events with the same start
          time (stored in the AVPacket->pts) but a different duration. At the
          moment, only the matroska do the merge with the SSA-line codec.
      
          We will need to make sure all the decoders in the future can't add
          more than one rectangle (and only one Dialogue line in it
          obviously).
      
        FORMATS
        -------
      
        - lavf/assenc: the .ass/.ssa muxer can take both SSA and ASS packets.
          In the case of ASS packets as input, it adds the timing based on the
          AVPacket pts and duration, and mux it with "Dialogue:", trailing
          CRLF, etc.
      
        - lavf/assdec: unchanged; it currently still only outputs SSA-lines
          packets.
      
        - lavf/mkv: the demuxer can now output ASS packets without the need of
          any "SSA-lines" reconstruction hack. It will become the default at
          next libavformat bump, and the SSA support will be dropped from the
          demuxer. The muxer can take ASS packets since it's muxed normally,
          and still supports the old SSA packets. All the SSA support and
          hacks in Matroska code will be dropped at next lavf bump.
      
      [1]: http://www.matroska.org/technical/specs/subtitles/ssa.html
      7c1a002c
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