2012 0011 0024

pythonbrew+opencv+debian

There are a number of ways to go about building a modern development environment for scientific computing and computer vision in python. If you’re used to developing on bleeding-edge, however, the latest debian stable makes it a chore to get started with the latest and greatest. It ships with python2.6 instead of 2.7, and opencv is notoriously out of date in a number of distributions, debian included. I typically use Arch, but the server-class machines I have access to were running debian, so I had to bootstrap my setup into this environment.

Challenge accepted.

Thankfully, pythonbrew (or pythonz) comes to the rescue by making it easy to handle multiple pythons for a single account (without having to install them system-wide) as well as providing wrappers around virtualenv. However, not everything is rosy. The python you choose has to be built with shared libraries if you want to install opencv later:

pythonbrew install --configure="--enable-shared" 2.7.3 

After this, you can bootstrap a virtualenv as usual

pythonbrew venv init
pythonbrew venv create debian
pythonbrew venv use debian

and install any requisite stuff you might need (minimum numpy/scipy)

pip install numpy
pip install scipy
pip install pymorph
pip install matplotlib
pip install distutils

Unfortunately, there’s no such pip package for opencv. Thankfully, the debian installation guide isn’t too far out of date, and many of the listed packages to apt-get are still relevant.

wget http://downloads.sourceforge.net/project/opencvlibrary/opencv-unix/2.4.3/OpenCV-2.4.3.tar.bz2
tar xjvf OpenCV-2.4.3.tar.bz2
cd OpenCV-2.4.3
mkdir {build,release}
cd release

At this point, we need to delve into where pythonbrew puts all its related files to configure opencv correctly. First, your installed python will be available in one of two places (here python 2.7.3 is used as an example):

~/.pythonbrew/venvs/Python-2.7.3/{venv-name}/bin/python
~/.pythonbrew/pythons/Python-2.7.3/bin/python

All virtualenvs based on a particular version of python will have a copy of that python binary for use in their own isolated environment. In addition, the virtualenv has an include directory that you should use, since all your additional packages installed into the virtualenv will place their headers in this directory:

~/.pythonbrew/venvs/Python-2.7.3/{venv-name}/include/python2.7

The hitch, however, is that the virtualenv does not have a copy/symlink of the shared library we specifically built when first compiling python using pythonbrew, unlike a typical native python install. This means that cmake’s approach to locate this library will fail. Thus we must point opencv to this

~/.pythonbrew/pythons/Python-2.7.3/lib/libpython2.7.so

for it to build corectly.

Speaking of cmake, there is a bug in the cmake included in debian that prevents it from building opencv correctly. I was lazy and simply grabbed a binary of the latest cmake,

wget http://www.cmake.org/files/v2.8/cmake-2.8.9-Linux-i386.tar.gz

which worked on my debian build, but it’s better to compile it if you plan to continue using it for more than a one-off build.

Finally, understanding opencv’s cmake flags is important for getting everything stitched together:

PYTHON_EXECUTABLE=~/.pythonbrew/venvs/Python-2.7.3/{venv-name}/bin/python
PYTHON_INCLUDE_DIR=~/.pythonbrew/venvs/Python-2.7.3/debian/include/python2.7
PYTHON_LIBRARY=~/.pythonbrew/pythons/Python-2.7.3/lib/libpython2.7.so

Additionally, if you find that numpy isn’t autodetected, you can specify

PYTHON_NUMPY_INCLUDE_DIR=~/.pythonbrew/venvs/Python-2.7.3/debian/lib/python2.7/site-packages/numpy/core/include

You can also specify your virtualenv path to install the python libraries

PYTHON_PACKAGES_PATH=~/.pythonbrew/venvs/Python-2.7.3/{venv-name}/lib/python2.7/site-packages

or just symlink/copy the resulting cv2.so and cv.py files there later.

Putting it all together, I used this command to generate the makefile which compiles correctly against pythonbrew’s python (where debian is my virtualenv name):

~/cmake-2.8.9-Linux-i386/bin/cmake \
-D CMAKE_INSTALL_PREFIX=../build \
-D BUILD_NEW_PYTHON_SUPPORT=ON \
-D BUILD_PYTHON_SUPPORT=ON \
-D BUILD_EXAMPLES=OFF \
-D PYTHON_EXECUTABLE=~/.pythonbrew/venvs/Python-2.7.3/debian/bin/python \
-D PYTHON_INCLUDE_DIR=~/.pythonbrew/venvs/Python-2.7.3/debian/include/python2.7 \
-D PYTHON_LIBRARY=~/.pythonbrew/pythons/Python-2.7.3/lib/libpython2.7.so \
-D PYTHON_NUMPY_INCLUDE_DIR=~/.pythonbrew/venvs/Python-2.7.3/debian/lib/python2.7/site-packages/numpy/core/include \
-D PYTHON_PACKAGES_PATH=~/.pythonbrew/venvs/Python-2.7.3/debian/lib/python2.7/site-packages \
../
make
make install

Depending on what you’re doing, there may be other tricks with LD_LIBRARY_PATH to make specific things work, but your pythonbrewed python should be primed to access opencv from here.