Capturing jpegs from an h264 stream

[Alex Hewson]

Hi gstreamer folks.  This is my first post on the list so if I've come 
to the wrong place just point me to the right forum for questions about 
CLI usage.

I have one of the new camera add-ons for a Raspberry Pi. It doesn't yet 
have video4linux support but comes with a small program that spits out a 
1080p h264 stream. I have verified this works and got it pushing the 
video to stdout with:

raspivid -n -t 1000000 -vf -b 2000000 -fps 25 -o -

I would like to process this stream such that I end up with a snapshot 
of the video taken once a second.

Since it's 1080p I will need to use the rpi's hardware support for H264 
encoding. I believe gstreamer is the only app to support this so 
solutions using ffmpeg or avconv won't work. I've used the build script 
at http://www.trans-omni.co.uk/pi/GStreamer-1.0/build_gstreamer to make 
gstreamer and the plugin for hardware H264 encoding and it appears to work:

root at raspberrypi:~/streamtest# GST_OMX_CONFIG_DIR=/etc/gst 
gst-inspect-1.0 | grep 264
...
omx:  omxh264enc: OpenMAX H.264 Video Encoder
omx:  omxh264dec: OpenMAX H.264 Video Decoder

So I need to construct a gst-launch pipeline that takes video on stdin 
and spits out a fresh jpeg once a second. I know I can use gstreamer's 
'multifilesink' sink to do this so have come up with the following short 
script to launch it:

root at raspberrypi:~/streamtest# cat test.sh
#!/bin/bash

export GST_OMX_CONFIG_DIR=/etc/gst

raspivid -n -t 1000000 -vf -b 2000000 -fps 25 -o - |  \
gst-launch-1.0 fdsrc fd=0 ! decodebin ! videorate ! 
video/x-raw,framerate=1/1 ! jpegenc ! multifilesink location=img_%03d.jpeg

Trouble is it doesn't work: gstreamer just sits forever in the 
prerolling state and never spits out my precious jpegs.

root at raspberrypi:~/streamtest# ./test.sh
Setting pipeline to PAUSED ...
Pipeline is PREROLLING ...
[waits forever]

In case it's helpful output with gstreamer's -v flag set is at 
http://pastebin.com/q4WySu4L

Can anyone explain what I'm doing wrong?

-- 
Alex Hewson
m: +44 7895 265219 | e: mock at mocko.org.uk | Skype: alex.hewson