Author: Phil Birch

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Finding webcams

List all devices on windows using ffmpeg

ffmpeg -list_devices true -f dshow -i dummy

and on Linux

v4l2-ctl –list-devices

To get the device capabilites

ffmpeg -f dshow -list_options true -i video="Mobius"

where “Mobius” is the name of the camera.

On the Mac use

ffmpeg -f avfoundation -list_devices true -i ""
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How to write lossless video in Python

OpenCV does a reasonable job of reading videos from file or webcams. It’s simple and mostly works. When it comes to writing videos,
it however leaves a lot to be desired. There is little control over the codecs and it is almost impossible to know which codecs are installed. It also wants to know things like the frame size at intailisation. This isn’t always a problem, but if you don’t know it yet it means you have to set up the video writer inside your main processing loop.

To make something as cross-platform compatible as possible it would be nice to use FFmpeg. There are a few python wrappers around, but as far as I can tell they are mainly used for transcoding type applications. One solution is run FFmpeg as a subprocess and set its input to accept a pipe. Then every video frame is passed through the pipe. You write this yourself, in fact it’s only a few lines of code. However, the scikit-video package will do this for us, with some nice boilerplate to make life easier.

The steps are:

  1. install FFmpeg — if you running on Linux use your system’s package manager if it’s not already installed. If you’re unlucky enough to be using Windows you need to download the zip file from here, and add the bin directory to your system’s path.
  2. install scikit-video –I tried installing scikit-video via pip on my Anaconda distro but the version was too old. Instead, I cloned the github version and installed that. Instructions are provided on github.

Below is a simple example the grabs from your webcam and records lossless video.

#test recording of video
import cv2
import skvideo.io


capture=cv2.VideoCapture(0) #open the default webcam
outputfile = "test.mp4"   #our output filename
writer = skvideo.io.FFmpegWriter(outputfile, outputdict={
  '-vcodec': 'libx264',  #use the h.264 codec
  '-crf': '0',           #set the constant rate factor to 0, which is lossless
  '-preset':'veryslow'   #the slower the better compression, in princple, try 
                         #other options see https://trac.ffmpeg.org/wiki/Encode/H.264
}) 
while True:
    ret,frame=capture.read()
    if ret==False:
        print("Bad frame")
        break
    cv2.imshow('display',frame)
    writer.writeFrame(frame[:,:,::-1])  #write the frame as RGB not BGR
    ret=cv2.waitKey(10)
    if ret==27: #esc
        break

writer.close() #close the writer
capture.release()
cv2.destroyAllWindows()

view raw

pythonVideoCapture.md

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Reskinning Max2play

Reskinning Max2play

Max2play is a great way to have an out of the box music server running on a Raspberry Pi with a touch screen. I’ve installed Max2play on a Raspberry Pi 3 and 7 inch touch screen. The only problem with it is that I would like to run other software on the Pi as well (such as control my Phillips Hue lighting). Out the box, the Pi now boots up and runs the full screen Jivelite app which controls the music.  There are two options I could think of: 1. Write a plugin for Jivelite to control the Hue lights, or 2, control the lighting with a separate app and have the ability to launch jivelite manually. I went for 2 since Jivelite is written in Lua which I don’t know.

The first problem is how to launch Jivelite manually. For this I added a Jivelite icon to the desktop.  I’m assuming Max2play is installed with Jivelite options and everything is working.

In the Jivelite plugin settings, disable autostart. In the Settings/Reboot tab, enable Autostart desktop (the desktop is normally started with the jivelite plugin so we need to enable it here).

In the Pi’s file manager go to Edit/Preferences – enable ‘Open files with Single click’ – it’s not easy double clicking with the touch screen. You might also want to increase the icon sizes whilst your there. These setting affect the desktop as well.

SSH into the Raspberry pi – either from the terminal or via the web plugin you can install. Go to the desktop folder and create a desktop launch shortcut. We will start by making one to launch jivelite. So create a text file with nano jivelite.desktop and add

[Desktop Entry]

Type=Application

Icon=/home/pi/music.png  <edit this to point to your own icon>

Name=Jivelite

Comment=Start the Jivelite music player

Exec=/opt/jivelite/jivelite/bin/jivelite

Save the file and that’s it. You should be now be able to launch jivelite from the desktop icon. The quit button does not quit the application (it seems to stop the music), so you have to quit from the menu options on Jivelite. (There appears to be a patch file that creates this behavior but I’ll investigate that at a latter date.)

To autostart at login, make a symbolic link to the .desktop file and place it in ~/.config/autostart

You may want to disable the screensaver which blanks the screen after 10 minutes as is. Remove the @xscreensaver -no-splash line from /etc/xdg/lxsession/LXDE/autostart and from ~/.config/lxsession/LXDE/autostart Stopping the screensaver server has no effect. I’m not sure what causes the screen to blank when running the GUI. It might be power saving in X. To disable the screensaver, the easiest method is to install a screensaver client and configure that not to run.

sudo apt-get install xscreensaver

After that’s completed there will be a screensaver option in the LXDE GUI menu. Run that and disable the screensaver from there.