Liquidsoap svn : Video streams

Basically streaming videos does not change anything compared to streaming audio: you just have to use video files instead of sound files! For instance, if you want to stream a single file to an icecast server in ogg format (with theora and vorbis as codecs for audio and video) you can simply type:

source = single("video.avi")


And of course you could have used a playlist instead of single to have multiple files, or used other formats for the stream.

In order to test a video stream, it is often convenient to use the output.sdl operator (or which will open a window and display the video stream inside. These can handle streams with video only, you can use the drop_audio operator to remove the sound part of a stream if needed.

You should be expecting much higher resource needs (in cpu time in particular) for video than for audio. So, be prepared to hear the fan of your computer! The size of videos have a great impact on computations; if your machine cannot handle a stream (i.e. it's always catching up) you can try to encode to smaller videos for a start.

Useful tips & tricks

Video is a really exciting world where there are lots of cool stuff to do.


Transitions at the beginning or at the end of video can be achieved using and video.fade.out. For instance, fading at the beginning of videos is done by

source ="fade",duration=3.,source)

Adding a logo

You can add a logo (any image) using the video.add_image operator, as follows:

source = video.add_image(

Inputting from a webcam

If your computer has a webcam, it can be used as a source thanks to the input.v4l2 operator. For instance:


Video in video

Suppose that you have two video sources source and source2 and you want to display a small copy of source2 on top of source. This can be achieved by

source2 = video.scale(scale=0.2,x=10,y=10,source2)
source = add([source,source2])

Scrolling text

Adding scrolling text at the bottom of your video is as easy as

source = video.add_text.sdl(
       "Hello world!", source)

You might need to change the font parameter so that it matches a font file present on your system.


There are many of effects that you can use to add some fun to your videos: video.greyscale, video.sepia, video.lomo, etc. Read the documentation to find out about them. If you have compiled Liquidsoap with frei0r support, and have installed frei0r plugins, they will be named video.frei0r.*. You can have a list of those supported on your installation as usual, using liquidsoap --list-plugins.

Presenting weather forecast

You can say that a specific color should be transparent using video.transparent. For instance, you can put yourself in front of a blue screen (whose RGB color should be around 0x0000ff) and replace the blue screen by an image of the weather using

img = single("weather.jpg")
cam = input.v4l2()
cam = video.transparent(color=0x0000ff,precision=0.2,cam)
source = add([img,cam])

Detailed examples

The anonymizer

Let's design an “anonymizer” effect: I want to blur my face and change my voice so that nobody will recognise me in the street after seeing the youtube video. Here is what we are going to achieve:

This video was produced thanks to the following script:

# Input from webcam
cam = input.v4l2()

# Detect faces (this generates a white disk over faces)
mask = video.frei0r.opencvfacedetect(cam)
# Pixellize the video
censored = video.frei0r.pixeliz0r(blocksizex=0.1,blocksizey=0.1,cam)
# Generate a mask for video without the face
unmask = video.frei0r.invert0r(mask)
# Put the pixellized face over the video
s = video.frei0r.addition(
# We have to bufferize the source because its clock it GStreamer's clock
s = buffer(buffer=0.1,mksafe(s))

# Input audio from microphone
mic = input.pulseaudio(clock_safe=false)
# Transpose sound to generate a funny voice
mic = soundtouch(pitch=1.5,mic)
# Add sound to video
s = mux_audio(audio=mic,s)

# Let's hear the sound
# Let's see the video

s = mksafe(s)
# Output the video/sound into a file in theora/vorbis format
output.file(%ogg(%theora(quality=63),%vorbis), "anonymous.ogv", s)

Controlling with OSC

In this example we are going to use OSC integration in order to modify the parameters in realtime. There are many OSC clients around, for instance I used TouchOSC :

Here is how the video was made:

# Set the OSC port to match TouchOSC's default port

# Input from the webcam
s = input.v4l2_with_audio()
s = mksafe(s)

# We get the angle from fader 3
angle = osc.float("/1/fader3", 0.)
# we rescale the position of fader 3 so that it corresponds to a 2π rotation
angle = fun() -> angle() * 3.1416 * 2.
# ...and we rotate the video according to the angle
s = video.rotate(speed=0.,angle=angle,s)
# Change brightness according to fader 1
s = video.frei0r.brightness(brightness=osc.float("/1/fader1",0.5),s)
# Change contrast according to fader 2
s = video.frei0r.contrast0r(contrast=osc.float("/1/fader2",0.5),s)

# We have to buffer here otherwise we get clocks problems
s = buffer(s)

# Output sound and video

Blue screen

You want to show yourself in front of a video of a bunny, as in

The idea is to film yourself in front of a blue screen, make this blue screen transparent and put the resulting video in front of the bunny video (actually, I don't have a blue screen at home, only a white wall but it still kinda works).

# The video of the bunny
s = single("big_buck_bunny_720p_stereo.ogg")
# Input from the webcam
cam = input.v4l2()
# Flip the video around a vertical axis so that it is easier
# to position yourself
cam = video.frei0r.flippo(x_axis=true,cam)
# Make the white background transparent
# I had to tweak the precision parameter so that I will be seen
# but not the wall
cam = video.transparent(color=0xffffff,precision=0.64,cam)
# Superpose the two videos
s = add([s,cam])
# Output to SDL

Encoding with GStreamer codecs

Gstreamer codecs can be used to encode videos and audio as any natively supported format. For instance, suppose that you want to stream using harbor in x264 / mp3. This can be achieved as follows:

# Set the values for video size and fps.
# On my standard computer, higher values means
# that we cannot encode in realtime.
set("", 320)

# The video we want to stream.
s = single("big_buck_bunny_720p_stereo.ogg")
# This hack is necessary (for now) in order
# to leave the synchronization to GStreamer.

  %gstreamer(video="x264enc speed-preset=1",audio="lamemp3enc"),

The video can be read after that at http://localhost:8000/test and of course an output.icecast or output.file could have been used instead of output.harbor depending on your needs.

Streaming with GStreamer

The usual way to stream a video is using icecast, as for audio. However, it can happen that you want to use weired formats or ways to to stream. In this case, using GStreamer as output (as opposed to simply a codec as above) might be a good idea. For instance, suppose that you want to stream mp4 video using RTP. This can be done as follows:

s = single("test.mp4")"videoconvert ! avenc_mpeg4 ! rtpmp4vpay config-interval=2 ! udpsink host= port=5000", s)

The stream can then be read with vlc for instance using vlc test.sdp. Here, the contents of the file test.sdp is

m=video 5000 RTP/AVP 96
c=IN IP4
a=rtpmap:96 MP4V-ES/90000

Frequently asked questions


When I try

s = input.v4l2_with_audio()

I get the error

At line 2, char 13:
  this value has type
    active_source(audio=1+_,...) (inferred at ../scripts/gstreamer.liq, line 20, char 30-121)
  but it should be a subtype of

This error means that the stream s has an audio channel (as indicated by audio=1+_) whereas output.sdl wants no audio channel. Namely, it's type is

$ liquidsoap -h output.sdl

Display a video using SDL.

Type: (?id:string,?fallible:bool,?on_start:(()->unit),

which means that it wants 0 audio channel, 1 video channel and 0 midi channel. The solution to correct the script is simply to remove the audio channel using the drop_audio operator:

s = input.v4l2_with_audio()

Advanced parameters

Default size for videos

Internally, Liquidsoap uses a video format which is the same for all frames. You can change it by doing


Using higher values result in higher quality videos produced, but this also means more computations to perform!


Most videos need to be rescaled to the Liquidsoap internal format. The default converter is the GAVL library but you can choose other (such as native or ffmpeg) by

set("video.converter.preferred", "ffmpeg")

If you are using gavl, you can change the scaling mode by

set("video.converter.gavl.scale_mode", "quadratic")

Several modes beside quadratic are available, use liquidsoap --conf-descr to discover them. Keep in mind that you should keep a good balance between performance and quality!