Screencasting with OpenBSD
Any USB microphone should appear as a new audio device. Here is the dmesg for my mic by ART:
uaudio0 at uhub0 port 2 configuration 1 interface 0 "M-One USB" rev 1.10/0.01 addr 2 uaudio0: audio rev 1.00, 8 mixer controls audio1 at uaudio0
audioctl can read off all of the specific characterisitcs of this device
$ audioctl -f /dev/audio1 | grep record mode=play,record record.rate=48000 record.channels=1 record.precision=16 record.bps=2 record.msb=1 record.encoding=slinear_le record.pause=0 record.active=0 record.block_size=1960 record.bytes=0 record.errors=0
Now test the recording from the second audio device using aucat(1)
aucat -f rsnd/1 -o file.wav
If the device also has a headset audio can be played through the same device.
aucat -f rsnd/1 -i file.wav
Screen Capture using Xvfb
The rate at which a framebuffer for your video card is a feature of the hardware and software your using, and it's often very slow. x11vnc will print an estimate of the banwidth for the system your running.
x11vnc ... 09/05/2012 22:23:45 fb read rate: 7 MB/sec
This is about 4fps. We can do much better by using a virtual framebuffer. Here I'm setting up a new screen, setting the background color, starting cwm and an instance of xterm
Xvfb :1 -screen 0 720x540x16 & DISPLAY=:1 xsetroot -solid steelblue & DISPLAY=:1 cwm & DISPLAY=:1 xterm +sb -fa Hermit -fs 14 &
Much better! Now we're up around 20fps.
x11vnc -display :1 & ... 11/05/2012 18:04:07 fb read rate: 168 MB/sec
Make a connection to this virtual screen using raw encoding to eliminate time wasted on compression.
vncviewer localhost -encodings raw
A test recording with sound then looks like this
ffmpeg -f sndio -i snd/1 -y -f x11grab -r 12 -s 800x600 -i :1.0 -vcodec ffv1 ~/out.avi
Note: always stop the recording and playback using q, not Ctrl-C so that audio inputs are shut down properly.
Screen Capture using Xephyr
Xephyr is perhaps the easiest way to run X with a shadow framebuffer. This solution also avoids reading from the video card's RAM, so it's reasonably fast.
Xephyr -ac -br -noreset -screen 800x600 :1 & DISPLAY=:1 xsetroot -solid steelblue & DISPLAY=:1 cwm & DISPLAY=:1 xrdb -load ~/.Xdefaults & DISPLAY=:1 xterm +sb -fa "Hermit" -fs 14 &
Capture works in exactally the same way. This command tries to maintain 12fps.
ffmpeg -f sndio -i snd/1 -y -f x11grab -r 12 -s 800x600 -i :1.0 -vcodec ffv1 -acodec copy ~/out.avi
To capture keyboard and mouse input press Ctrl then Shift. This is very handy for using navigating a window manager in the nested X session.
I have sometimes found it helpful to launch applications and arrange them in a specific way. This will open up a web browser listing the current directory and position windows using xdotool
DISPLAY=:1 midori "file:///`pwd`" & sleep 2 DISPLAY=:1 xdotool search --name "xterm" windowmove 0 0 DISPLAY=:1 xdotool search --class "midori" windowmove 400 0 DISPLAY=:1 xdotool search --class "midori" windowsize 400 576
This will position the window precisely so that it appears to be in a tmux window on the right.
If you find that the audio is way out of sync with the video, you can ajust the start using the -ss before the audio input to specify the number of seconds to delay. My final recording command line, that delays the audio by 0.5 seconds, writing 12fps
ffmpeg -ss 0.5 -f sndio -i snd/1 -y -f x11grab -r 12 -s 800x600 -i :1.0 -vcodec ffv1 -acodec copy ~/out.avi
Sharring a Terminal with tmux
If you're trying to record a terminal session, tmux is able to share a session. In this way a recording of an X framebuffer can be taken without even using the screen. Start by creating the session.
tmux -2 -S /tmp/tmux0
Then on the remote side connect on the same socket
tmux -2 -S /tmp/tmux0 attach
Grabbing a screenshots on Xvfb server is easily accomplished with ImageMagick's import command
DISPLAY=:1 import -window root screenshot.png
Audio Processing and Video Transcoding
The first step is to ensure that the clip begins and ends where you'd like it to. The following will make a copy of the recording starting at time 00:00 and ending at 09:45
ffmpeg -i interactive-sql.avi \ -vcodec copy -acodec copy -ss 00:00:00 -t 00:09:45 interactive-sql-trimmed.avi mv interactive-sql-trimmed.avi interactive-sql.avi
Setting the gain correctly is very important with an analog mixer, but if you're using a USB mic there may not be a gain option; simply record using it's built-in settings and then adjust the levels afterwards using a utility such as normalize. First extact the audio as a raw PCM file and then run normalize
ffmpeg -i interactive-sql.avi -c:a copy -vn audio.wav normalize audio.wav
Next merge the audio back in again
ffmpeg -i interactive-sql.avi -i audio.wav \ -map 0:0 -map 1:0 -c copy interactive-sql-normalized.avi
The final step is to compress the screencast for distribution. Encoding to VP8/Vorbis is easy:
ffmpeg -i interactive-sql-normalized.avi -c:v libvpx -b:v 1M -c:a libvorbis -q:a 6 interactive-sql.webm
H.264/AAC is tricky. For most video players the color space needs to be set to yuv420p. The -movflags puts the index data at the beginning of the file to enable streaming/partial content requests over HTTP:
ffmpeg -y -i interactive-sql-normalized.avi -c:v libx264 \ -preset slow -crf 14 -pix_fmt yuv420p -movflags +faststart \ -c:a aac -q:a 6 interactive-sql.mp4