Causes of pipeline non-transitivity?

[Matthew Waters]

On 25/4/21 1:39 am, amindfv--- via gstreamer-devel wrote:
> I frequently find myself confused and not knowing where to look when I'm able to build a pipeline from A->B, and a pipeline from B->C, but not from A->B->C.
>
> For example, I can go from v4l2src->glupload:
>
>      $ gst-launch-1.0 v4l2src ! glupload ! glimagesink
>
> And I can go from glupload->glshader:
>
>      $ gst-launch-1.0 videotestsrc ! glupload ! glshader fragment="\"`cat myshader.frag`\"" ! glimagesink
>
> But when I try and combine the two, I get an error:
>
>      $ gst-launch-1.0 v4l2src ! glupload ! glshader fragment="\"`cat myshader.frag`\"" ! glimagesink
>
>      WARNING: erroneous pipeline: could not link gluploadelement0 to glfiltershader0
>
> Where should I look when I hit issues like this? Is it always the case, for example, that A and B have negotiated a format that's incompatible with C? Or does it mean I need to provide more specific properties to A (or B, or C)? I don't see anything in gst-inspect that indicates they wouldn't all work together.

Add -v to gst-launch-1.0 to see the negotiated caps for successful cases.

In this case, you are probably missing a glcolorconvert element 
(automatically contained within glimagesink) or a videoconvert element 
as OpenGL only deals with RGBA buffers byt v4l2src likely produces some 
YUV format..

Cheers
-Matt

> Thanks,
> Tom
>
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