MosaicExampleSetup

This page contains example code for mosaic.

Example set-up for a conference: Plans

Problem description

For a conference we needed a web stream for participants who were not able to come. The set-up needed to be flexible as well transportable, since the conference took place at two venues which were several hundred meters apart. VLC was chosen because there was already a somehow working set-up available which was modified for this conference.

Hardware

We were using a dual-core AMD box (Athlon X2 4600+) with 1 GB of memory and only two PCI slots (Fujitsu Siemens PC). So nothing out of the ordinary. We had two PolyCom remote controllable cameras which can output SVIDEO signals which we feed into to BT878 based grabber cards.

Software

We used the vlc version coming with Ubuntu Gutsy 0.8.6c along with xawtv for direct control of the cameras (the encoding process and buffering of the clients usually introduce a lag of a few seconds which makes it hard to steer the camera efficiently.

Network distribution

One of our problems was that at one of the conference venues the network bandwidth was somehow limited and we feared to overload that network. We thus decided to stream only to a single dedicated box which is connected to a network with a much larger bandwidth. This box was restreaming the content to anyone interested.

Details

Needed devices

Video

Grabber cards registered their devices where the camera output can be viewed from:

/dev/video0
/dev/video1

The very first step is to check, if these are working. For this, start

xawtv -c /dev/video0

and we saw a blue window, because xawtv sets the input mode to composite not SVIDEO. To change this, right-click on the blue window, and change the input to SVIDEO. I have not found a possibility to change this via the command line.

If video0 is working, try the same with video1 it should work as well.

Audio

The grabber cards I used, do have audio input but I only got very bad quality. With the on-board audio I got a much better quality so this was used. If you are a fan of a graphical interface try to record input from /dev/dsp (need the OSS kernel modules) with audacity. Don't simply connect the output of the sound card to loudspeakers and the input of your microphone to the microphone input of your computer. The sound card may directly connect those two channels without the software having the possibility to record anything. If you find the input of the grabber cards are good enough, they are usually labeled /dev/dsp0 or something like this.

Setup

For a simple set-up with two equally sized images next to each other I use the following config file for vlc:

# first camera uses /dev/video0 and the on-board sound /dev/audio or /dev/dsp
new cam1 broadcast enabled
setup cam1 input v4l:/dev/video0:norm=PAL:channel=1:adev=/dev/audio
setup cam1 option rtsp-tcp
setup cam1 output #transcode{vcodec=mp4v,vb=512,scale=1}:duplicate{dst=mosaic-bridge{id=1,height=400,width=300},
select=video,dst={transcode{acodec=mp3,ab=64,channels=1}:bridge-out{id=0}},select=audio}

# second camera. Important: adev=/dev/null otherwise I couldn't get it to run
new cam2 broadcast enabled
setup cam2 input v4l:/dev/video1:norm=PAL:channel=1:adev=/dev/null
setup cam2 output #transcode{vcodec=mp4v,vb=512,scale=1}:duplicate{dst=mosaic-bridge{id=2,height=400,width=300}}

# set up for the background
new background broadcast enabled
setup background input fake:
setup background option mosaic-width=800
setup background option mosaic-height=300
setup background option mosaic-rows=1
setup background option mosaic-cols=2
setup background option mosaic-position=1
setup background option mosaic-order="1,2"
setup background option fake-file="back.gif"
setup background option fake-width=800
setup background option fake-height=300
setup background option fake-fps="8"
setup background output #transcode{sfilter=mosaic,soverlay,vcodec=mp4v,acodec=mp3,ab=64,channels=1,scale=1}:bridge-in{id-offset=100}:std{access=http{mime=video/x-asf-stream},mux=asf,dst=/}

# put it all together
control background play
control cam1 play
control cam2 play

For us it was not so important to have both images the same. The first camera just showed the audience while the second one looked at the projections made which needed to be of higher quality. We used therefore a special background image with a logo in the bottom left corner and this configuration:

new cam1 broadcast enabled
setup cam1 input v4l:/dev/video0:norm=PAL:channel=1:adev=/dev/audio
setup cam1 option rtsp-tcp
setup cam1 output #transcode{vcodec=mp4v,vb=384,scale=1}:duplicate{dst=mosaic-bridge{id=1,height=300,width=400},select=video,dst={transcode{acodec=mp3,ab=64,channels=1}:bridge-out{id=0}},select=audio}

new cam2 broadcast enabled
setup cam2 input v4l:/dev/video1:norm=PAL:channel=1:adev=/dev/null
setup cam2 output #transcode{vcodec=mp4v,vb=1500,scale=1}:duplicate{dst=mosaic-bridge{id=2,height=600,width=800},select=video}

new background broadcast enabled
setup background input fake:
setup background option mosaic-width=1200
setup background option mosaic-height=600
setup background option mosaic-rows=2
setup background option mosaic-cols=3
setup background option mosaic-position=1
setup background option mosaic-order="1,2,0,0,0,0"
setup background option fake-file="aei3.gif"
setup background option fake-width=1200
setup background option fake-height=600
setup background option fake-fps="8"
setup background option mosaic-keep-picture=1
setup background option mosaic-keep-aspect-ratio=1
setup background output #transcode{sfilter=mosaic,soverlay,vcodec=mp4v,vb=2000,acodec=mp3,ab=64,channels=1,scale=1}:bridge-in{id-offset=100}:std{access=http{mime=video/x-asf-stream},mux=asf,dst=/}

control background play
control cam1 play
control cam2 play

Either of these set-ups will start a streaming server listening on port 8080 for incoming http connections.

Out next problem was, how shall we prevent our small internet connection from the conference venue from being saturated?

Internet setup

We used a very simple but effective way. For this we not only needed our streaming server (from now on named origin), but also another server on a much faster network (named broadcast). We started the server on origin with

vlc -vvv --color -I telnet --vlm-conf conference.conf --ttl 10 --upd-caching 2000

Then we start a ssh tunnel from origin to broadcast (best placed into a screen environment):

ssh -N -c blowfish -R8000:localhost:8080 account@broadcast

This will forward the local port 8080 (on origin) to port 8000 on broadcast.

Finally, we start another vlc instance on broadcast to restream the original stream:

vlc -vvv http://localhost:8000/ --udp-caching 1500 --sout '#standard{access=http{mime=video/x-ms-asf},mux=asf,dst=/}' --http-port=7070

Final tips

Port multiplication

If you need to "multiply" a port easily, you can use iptables on Linux:

#possibly not all of these modules are strictly needed:
modprobe xt_tcpup
modprobe ip_tables
modprobe iptable_filter
modprobe ip_conntraqck
modprobe iptable_nat
modprobe ipt_state
modprobe ipt_MASQUERADE
modprobe ipt_REDIRECT

echo "1" > /proc/sys/net/ipv4/ip_forward

# I'm flushing everything here, this may not be good for you
iptables -F
iptables -F -t nat
iptables -A INPUT -i lo -j ACCEPT
iptables -t nat -A PREROUTING -i eth0 -p tcp --dport 8080 -j REDIRECT --to-ports 7070

This will forward all requests to port 8080 transparently to port 7070 (where our broadcast server listens). Note: Port 8080 might be better because some firewalls allow only http related ports out and 8080 is a typical proxy port.

Credits

Thanks a lot for the folks on #videolan, especially ILEoo for helping me a lot when need was great! Without your help I don't think I would have finished this set-up in time!