Wednesday, December 24, 2008

Computer Vision on OS X with Python and OpenCV

After my macbook's sudden demise and spontaneous, inexplicable regeneration, I've decided to try and port some things I've worked on in GNU/Linux to OS X. Also, having a built-in microphone and camera on a portable computer is pretty amazing when you work primarily with audio and video and need to test things.

I was really happy with how my previous forays in pygame had been going, so over the holidays I thought I 'd try and make a version of my previous camLoops prototype, but for OS X.
Also, at Alexandre Quessy's invitation, I've added to the ToonLoop project. More on that in the coming weeks, so stay "tooned" (if not put off by that terrible pun).

*UPDATE The initial opencv version of camLoops is now available. I've only tested it on a MacBook running OS X so far, but it should also work in GNU/Linux.

I did some research online and there did not seem to be much in the way of camera- frame-grabbing modules for OS X with python bindings (granted it is a pretty niche area). The most complete are Apple's QTKit, the Cocoa framework (which has python bindings) for Quicktime and OpenCV, or Open Source Computer Vision, which is a cross-platform, BSD-licensed library written in C with Python bindings. Since I have no interest in wasting time using a Mac-only, proprietary framework like QTKit, the choice was pretty obvious. The installation, however, was not.

Given my heavy use of fink packages (when I should really just give in and roll GNU/Linux on my laptop), I am used to having to fight with cross-platform librairies/frameworks. I was even pleased to find an OS X specific build instruction page on the OpenCV wiki. It's quite likely that my approach is not the best solution for building but it's what worked for me.

I decided for better or worse to get the current production version of Python, which is 2.6.1 at the time of this writing. I grabbed the disk image and ran the installer (and breathed a sigh of relief). For pygame to work, I had to get PyObjC, which is used to build Cocoa apps for OS X in Python. This is not required for OpenCV, so skip ahead if you're not using pygame. This also required an svn checkout of the 1.4 branch of pyobjc which works on Tiger, as no slick disk-images with installers are available from the website at present. To grab the branch, do:

svn co pyobjc

Fortunately, the PyObjC people know their target audience and all it took once in the pyobjc directory to make a nice installer was:

python bdist_mpkg --open

*UPDATE Previously OpenCV was using CVS for version control, they have since migrated to Subversion. Check out a clean version with:

svn co opencvlibrary
In the INSTALL file, it is suggested that you do autoreconf -i --force
This failed for me (even though my autotools are up to date) so i used the pre-existing configuration files and left autoconf alone.

In the opencv directory, create a build directory and enter it with mkdir build; cd build

From the build directory, run configure with a few flags set (replace sw with /opt/local if you are using darwinports instead of fink):

../configure CPPFLAGS="-I/sw/include" LDFLAGS="-L/sw/lib" --with-python

Now compile and install with make && sudo make install. You will be prompted for your password.

Edit your ~/.profile to include the following two lines, which will be different if you set the --prefix option on your configure script to something other than the default /usr:


To test that everything worked, run your python interpreter and try import opencv. Provided that there is no errors such as 'no module opencv', you can start trying the python examples in the samples directory.

I ran python 0, where 0 is the index of the camera you want the program to use. The program then draws a red square around what it thinks is your face. Extra points for being beard proof, and hats off to Mark Asbach for his work on OS X support for OpenCV.

Monday, December 15, 2008

Live animation with pygame and video4linux

 EDIT: I've uploaded the code (and merged it into one file) on gitorious.

Last Friday, the SAT saw the premiere of a new work by Alexandre Quessy and Isabelle Caron called Motifs Urbains. The work was part of the experimentation phase of the propulse[art] project. The demo involved Alexandre generating live, multichannel audio and looping it, while at the same time in a separate room Isabelle did live stop-motion animation and looped it using Alexandre's Processing-based software, ToonLoop. Both rooms featured simultaneous playback of the audio and animation.
Inspired by the demo and my recent tinkering with pygame, I decided to try and write a prototype for a simple stop motion animation looper like ToonLoop. The result is Thanks to Nirav Patel's 2008 Google Summer of Code project, pygame (subversion revision 1744 or later) now supports video4linux camera input. The camLoop program shows the camera playback in the left hand region of the window, and the animation in the right hand of the video. The user can then grab frames live, and the accumulated frames will be played back in the right hand side of the window:

If you run the program during the winter months, you'll be subjected to a nauseating holiday theme overlayed on the incoming video. This example was simply to demonstrate how it's possible to easily draw arbitrary objects on live video. From Nirav's blog, it would appear that CoreVideo support is also forthcoming.