[Bug 678146] Closed Caption support

[GStreamer (bugzilla.gnome.org)]

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

Steve Maynard <steve> changed:

           What    |Removed                     |Added
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                 CC|                            |steve at secondstryke.com

--- Comment #4 from Steve Maynard <steve at secondstryke.com> 2013-07-19 17:10:18 UTC ---
I created a branch of get-plugins-bad and added elements mpeguserdatademux and
ccfilter. I modified mpegvparse to add a boolean user-data to its src caps
(much like system-stream etc).  Autoplug finds mpeguserdatademux based on this
new cap and plugs after mpegvparse.  mpeguserdatademux has an always src pad
for video data passthrough which ends up connecting to the decoder via
multiqueue. It also has a sometimes src pad for MPEG user data (current caps:
video/x-user-data).  The new ccfilter element autoplugs based on caps
video/x-user-data and has a sometimes src pad with caps text/vtt.  Updated
versions of uridecodebin connect to these pads with the assistance of WebKit
courtesy of Brendan Long.

I have been trying to complete this work on closed captions support and am
having some difficulties getting the text cue timing correct.  In my original
test implementation I had been running in parallel with tsdemux and the element
received PES headers where I extracted the 90KHz PTS clock information and used
that for the CC text cue timing.  In my refactored implementation the
mpeguserdatademux element is plugged after mpegvparse and I no longer receive
PES headers.

I tried using GST_BUFFER_PTS(buf) and the resulting CC cue times appeared not
to be linear with the time stamps in the logs and cues rendered at the wrong
time.  I have tried using the element clock which is valid after the NewClock
event but that time requires an offset based on user interaction with WebKit
(or not when running with gst-launch).  Any comments, thoughts, or insights
would be appreciated.  I eventually hope to submit this branch
(text_track_refactor) for your approval to begin to address this issue.

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