Using gstreamer with videomixer & 2 cameras streaming over UDP - raspberry-pi

Question

I have a Raspberry Pi Compute module with 2 cameras. I'm trying to use gstreamer with v4l2src selecting /dev/video0 & /dev/video1 to continually run at about 20FPS and use videomixer to combine the images side-by-side then output H264 over RTP to a UDP port (read by another host)/
The default (current) RPi v4l2src driver does not support two cameras, but as of today a beta is available that does, however it requires the beta 4.4.6 kernel.
The problem I'm having is in getting the mixer connected.
#!/bin/bash -x
#
# Script to start RPi Compute Module streaming over RTP (RFC3984)
# from both cameras
#
FPS=20 # Frames per second
WIDTH=640 # Image width
HEIGHT=480 # Image height
UPLINK_HOST=192.168.1.73 # Receiving host
PORT=5200 # UDP port
#
# TESTING WITH ONE CAMERA ONLY FOR THE MOMENT
#
function start_streaming
{
gst-launch-1.0 -ve videomixer name=mixer \
! x264enc \
! h264parse \
! rtph264pay config-interval=10 pt=96 \
! udpsink host=$UPLINK_HOST port=$PORT \
v4l2src device=/dev/video0 \
! video/x-raw,format=AYUV,width=$WIDTH,height=$HEIGHT,framerate=$FPS/1 \
! mixer.
}
# Start streaming on both cameras simultaneously
echo Image size: $WIDTH x $HEIGHT
echo Frame rate: $FPS
echo Starting cameras 0 and 1 streaming to $UPLINK_HOST:$PORT
start_streaming
# Wait until everything has finished
wait
exit 0
# end
What I'm getting is the rather useless message:
WARNING: erroneous pipeline: could not link v4l2src0 to mixer
I've fiddled about rather a lot and got nowhere - it's probably something trivial, but be blowed if I can see it !
Many thanks
Nick

Answer 1

I think the problem is the chosen format. You use the AYUV while your camera does not support it. Try to replace the AYUV by I420.