[gst-devel] about caps negotiation

Thomas Vander Stichele thomas at apestaart.org
Tue Aug 30 02:46:18 CEST 2005


> hi list,
> first thanks to ronald and thomas for the suggestion to switch from
> v4l2src to v4lsrc .....i don't understand why, but now i can capture
> from my webcam.

There's nothing to understand.  The v4lsrc element has had more work
done to it to work well with webcams, because I added that on top of
Ronald's code; while the v4l2src hasn't.  One day I will probably do
that, but as long as the v4l2 header doesn't get "officially" installed
on systems, I don't have much incentive to do so :)

> today i've tried to add some more fun to my pipeline:
> v4lsrc | ffmpegcolorspace | vertigoTV | ximagesink
> but the caps negotiation produces strange border line effect:
> my simple original pipeline was
> v4lsrc | ffmpegcolorspace | ximagesink
> and produced 320x160 at 5fps frame 
> with vertigoTV or any other effectv filter, i got 160x120 at 5fps
> trying to link with a filter doesn't risolve.
> can somebody explain the details of caps negotiation?

Is this in 0.8 or 0.9 ?

In 0.8, caps nego is something of a black art - depending on which links
the core chooses to negotiate first and the element's fixation
functions, anything can happen.  Typically if you have more than three
elements inbetween sources and sinks, you're just better off fixating
the links by filtering.  So tell us what the actual problem was when
doing that.  Remember that for the two most common types of webcams (pwc
and quickcam) your fps needs to be a multiple of 15/16 - so your 5 fps
wouldn't work.  7.5 should.


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