Putting together a complex gstreamer command line...

nicko nick at desmith.net
Sun Apr 3 12:30:44 UTC 2016

For the sake of completeness, here is the question - I would really
appreciate help on this as I'm completely stuck:

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 H.264 over RTP to a UDP port (read by another host). Somewhere
in the comand line, I'm going to need to include glshader to apply some
distortion to each image before the merge...

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=        # Receiving host
PORT=5200                       # UDP port
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

# Wait until everything has finished

exit 0
# end

What I'm getting is the rather useless (to me) 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


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