Physics & Astronomy

Capturing VPython Animations

One of the Frequently Asked Questions on the VPython documenation website is: "Is there a way to capture VPython graphical output as a movie?" The answer is "yes," and this webpage is a description of a simple and free method for Linux users that requires no new software not already included in many standard Linux distributions.

Vpython output

Requirements:
  • Working VPython installation
  • ImageMagick suite of graphics tools, in particular the utilities import and convert (part of most distributions).
  • The standard linux utility xwininfo.

The basic idea is to capture the contents of the VPython window once each animation loop, and write a set of graphics files with an ordered set of names -- something like vp0001.gif, vp0002.gif, ... This requires system call to the import command. This set of frames can then be turned into an animated gif that can be viewed on just about any browser using convert.

Details
  • For convenience I create a directory under the working directory in which to store the images, e.g, "/frames."
  • In the VPython code, import the os module.
  • In the VPython code, create an iterator, say i, that is incremented every time through the animation loop.

  • Within the animation loop add a line something like the following:
    os.open(import -window 0x3a00003 frames/vp'+str(i).zfill(4)+'.gif)
    This is a system call to the linux import utility, once every time through the loop, that captures the VPython window and writes it as a gif in the "frames" directory, with the iterator i as part of the filename, resulting in the ordered files vp0001.gif, vp0002.gif, etc.

    You can determine the window identifier for the VPython display, in this case, 0x3a00003, by running xwininfo while a VPython animation is running. After executing xwininfo, click on the animation window to see all the information about the window.

    NOTE: You may have slow down the VPyhthon image to allow time for each import command to complete. (The actual speed of the captured animation will depend on the delay chosen in the next step.)

  • Create an animated gif from the captured frames:
    convert -delay 50 -loop 0 frames/vp*.gif output.gif
    This can be displayed in just about any browser, as above.
  • Movies in other formats can be generated from the captured images using ffmpeg.


This page maintained by
Marty Ligare, mligare@bucknell.edu

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