[Bug 726194] v4l2src does not cope well when a capture card is sometimes interlaced, sometimes progressive at the same resolution

[GStreamer (bugzilla.gnome.org)]

https://bugzilla.gnome.org/show_bug.cgi?id=726194
  GStreamer | gst-plugins-good | git

--- Comment #17 from Simon Farnsworth <simon at farnz.org.uk> 2014-11-03 10:30:53 UTC ---
(In reply to comment #16)
> Need some opinion on getting this in upcoming 1.4.4. It is conceptually a bug
> fix, even though this has been broken for years in fact. I'm mostly incline to
> include it in stable, but would like some more opinions / point of view.

FWIW, I'm running with this patch applied against 1.4.3; it's a considerable
improvement from my point of view.

However, there is a slight chance of regression. Previously, the caps from
v4l2src would always contain either interlace-mode=mixed or
interlace-mode=progressive. Now, they will contain one of
interlace-mode=progressive, interlace-mode=interleaved, or interlace-mode={
progressive, interleaved }. Pipelines that depended on being able to ask for
interlace-mode=mixed and get interlaced video from v4l2src will now fail.

For master, I don't see this as an issue - we're simply reporting our caps
accurately. For 1.4, however, there is the risk that a user depended on the old
buggy caps and is now caught out by what should have been a safe upgrade.

As background, interlace-mode=mixed would normally be appropriate for things
like MPEG-2 and H.264 3:2 pulldown signalling; when you're encoding film
material at 60 fields per second, the encoded stream contains 24 frames per
second of video, and uses the RFF flag to indicate the 3:2 pulldown cadence.
You can then switch back to providing a true 60 fields per second stream when
you're in video, and return to 24 frames per second with 3:2 cadence signalling
for film sourced material. interlace-mode=mixed has semantics that precisely
match this and enable it to correctly signal the move from 24 frames per second
with 3:2 pulldown back to 60 fields per second then back to 24 frames per
second with 3:2 pulldown.

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