Gstreamer: Why do I need a videoconvert before displaying some filter?

Question

I am writing a very basic pipeline on GStreamer 1.0 and I would like to understand why I need a videoconvert before displaying the stream.

gst-launch-1.0 videotestsrc ! vertigotv ! videoconvert ! autovideosink

If I remove the videoconvert from the pipeline then I get an error and the stream does not play:

Setting pipeline to PAUSED ...
Pipeline is PREROLLING ...
ERROR: from element /GstPipeline:pipeline0/GstVideoTestSrc:videotestsrc0: Internal data flow error.
Additional debug info:
gstbasesrc.c(2933): gst_base_src_loop (): /GstPipeline:pipeline0/GstVideoTestSrc:videotestsrc0:
streaming task paused, reason not-negotiated (-4)
ERROR: pipeline doesn't want to preroll.
Setting pipeline to NULL ...
Freeing pipeline ...

I don't get why the caps can't be automatically negotiated.

Here are the capabilities template of the sink pad of vertigo:

Capabilities:
  video/x-raw
             format: { RGBx, BGRx }
              width: [ 1, 2147483647 ]
             height: [ 1, 2147483647 ]
          framerate: [ 0/1, 2147483647/1 ]

Here is the capabilities template of the src pad of autovideosink:

Capabilities:
  ANY

Can someone explain me why this element is necessary to play the pipeline. Why do I need to switch colorspace before displaying the pipeline? Is there a way not to use this element (maybe using caps)?

Answer 1

vertigotv element can only handle raw video frames in RGBx and BGRx formats. autovideosink can not guarantee support of RGBx / BGRx format. You need videoconvert element to ensure compatibility between vertigotv's source and autovideosink's sink pads.

autovideosink is a bin, which contains actual video sink inside. autovideosink's sink pad capabilities are determined by capabilities of sink pad of it's internal video sink.

It can use several types of video sink. glimagesink, for example, will work without videoconvert (as it can read RGBx / BGRx frames). osxvideosink, on other hand, will not work without videoconvert (because of lack support for RGBx / BGRx).

In case autovideosink will use video sink with RGBx / BGRx support, videoconvert element will just pass frames through, without performing any job. So, you can safely leave it for all the cases.