[gstreamer-bugs] [Bug 541956] New: v4l2src only requests interlaced video

[GStreamer (bugzilla.gnome.org)]

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  GStreamer | gst-plugins-good | Ver: 0.10.x
           Summary: v4l2src only requests interlaced video
           Product: GStreamer
           Version: 0.10.x
          Platform: Other
        OS/Version: Linux
            Status: UNCONFIRMED
          Severity: normal
          Priority: Normal
         Component: gst-plugins-good
        AssignedTo: gstreamer-bugs at lists.sourceforge.net
        ReportedBy: dsd at gentoo.org
         QAContact: gstreamer-bugs at lists.sourceforge.net
     GNOME version: Unspecified
   GNOME milestone: Unspecified


v4l2src only uses V4L2_FIELD_INTERLACED which is a whole-frame capture mode (it
requests interlaced video where the two fields have already been merged
together).

However most webcams only offer progressive video (i.e. V4L2_FIELD_NONE) and
will reject any requests for interlaced video. Other drivers will only offer
progressive and will silently 'correct' gstreamer's request for interlaced
video.

I questioned this driver inconsistency here:
http://marc.info/?l=linux-video&m=121434022130546&w=2
And Mauro then told me on IRC that both behaviours are correct: it's fine for
drivers to reject unsupported fields with -EINVAL, and it's also fine for
simple drivers to silently fix up the user's request.

So in order to speak to a wider number of cameras, gstreamer should tolerate
the fact that drivers may reject a request for interlaced images with -EINVAL
and should then retry for progressive (Mauro confirmed this approach is the
best we have at the moment).

This patch also allows OLPC to drop a patch that we needed to get the XO webcam
working: http://dev.laptop.org/attachment/ticket/7294/v4l2-nointerlace.patch
http://dev.laptop.org/ticket/7294


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