[Bug 678146] Closed Caption support

[GStreamer (bugzilla.gnome.org)]

https://bugzilla.gnome.org/show_bug.cgi?id=678146
  GStreamer | gst-plugins-bad | git

--- Comment #2 from Sebastian Dröge <slomo at circular-chaos.org> 2013-07-05 08:45:56 UTC ---
For extract the CEA 608/708 from the h264 anciliary streams the idea would be
to do that in h264parse, as there we already have the information to be able to
extract it.
This would then need to be either exposed as a separate stream via a pad, or as
GstMeta on the h264 buffers.

For subtitles, exposing as a separate stream has the big problem that we can't
now beforehand if there ever will be such a subtitle stream or not. So we could
add it far too late and decodebin would ignore it. In decodebin's current
design.

So I see the following options here:
1) Expose as a separate stream and have CC stream decoder elements, fix
decodebin to accept streams after it already exposed streams. This would have
to be done very carefully to not cause any problems with the no-more-pads logic
in decodebin and detecting if something is "just" a new stream, or something is
the start of a complete new stream group (think: HLS, chained oggs, etc).

2) Parse the CC subtitles into a GstMeta, put them on the h264 buffers in the
parsers. Then inside playbin redirect these subtitle metas as a new subtitle
stream from the video stream to the subtitle input selector. For this to be
meaningful we would need a generic way of specifying if something is a subtitle
meta and also identify different subtitle meta streams (to allow playbin to
expose multiple of them...).

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