Error using v4l2sink when combining 2 camera streams

[Jim Ruxton]

If I want to send a video stream on /dev/video4 to a loopback device in 
Ubuntu I can use :

./gst-launch-1.0 -v -m  v4l2src device=/dev/video4 ! v4l2sink 
device=/dev/video0

And the video on /dev/video4 can be seen with :

ffplay /dev/video0

I can merge 2 videos with:

*./gst-launch-1.0 v4l2src device=/dev/video4 ! videoscale ! queue ! 
videoconvert ! video/x-raw, format=YUY2, framerate=30/1, width=640, 
height=480! alpha alpha=1 ! videobox border-alpha=0 left=-640 ! 
compositor name=Mix  ! videoconvert ! autovideosink v4l2src 
device=/dev/video2 ! videoscale ! queue ! videoconvert ! video/x-raw, 
format=YUY2, framerate=30/1, width=640, height=480! alpha alpha=1 ! 
videobox border-alpha=0 right=-640 ! Mix. And see the merged camera 
videos on my monitor but I can't seem to send those merged videos to a 
loopback device. For example if I try changing autovideosink to v4l2sink 
device=/dev/video0: ***./gst-launch-1.0 v4l2src device=/dev/video4 ! videoscale ! 
videoconvert ! video/x-raw, format=YUY2, framerate=30/1, width=640, 
height=480! alpha alpha=1 ! videobox border-alpha=0 left=-640 ! 
videomixer name=Mix ! videoconvert ! **v***4l2sink 
device=/dev/video0***sync=false v4l2src device=/dev/video2 ! videoscale 
! videoconvert ! video/x-raw, format=YUY2, framerate=30/1, width=640, 
height=480! alpha alpha=1 ! videobox border-alpha=0 right=-640 ! Mix. I 
get the following result with no video output: Setting pipeline to 
PAUSED ... Pipeline is live and does not need PREROLL ... Setting 
pipeline to PLAYING ... New clock: GstSystemClock ERROR: from element 
/GstPipeline:pipeline0/GstV4l2Src:v4l2src1: Internal data stream error. 
Additional debug info: gstbasesrc.c(3072): gst_base_src_loop (): 
/GstPipeline:pipeline0/GstV4l2Src:v4l2src1: streaming stopped, reason 
not-negotiated (-4) Execution ended after 0:00:00.381495593 Setting 
pipeline to PAUSED ... Setting pipeline to READY ... Setting pipeline to 
NULL ... Freeing pipeline ... I have also tried not specifying a video 
format as I have in my first example above but get the same error. Does 
anyone know how I can send my merged stream to the loopback device? 
Thanks Jim *

*

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