May 6, 2015 · Computer Vision

OpenCV and Python Project Setup

For my computer vision projects I like to setup an organized directory structure and keep all dependencies in one place. virtualenv is a great tool, which helps you accomplish this. I'll show you how I structure my projects, install OpenCV and install packages.

For my projects I use Python 2.7 and OpenCV 2.4.11 on Mac OS X. Many of these steps should be similar for other operating systems, except you will use a tool other than Homebrew to manage packages.

Install Python and OpenCV

If you haven't done this yet, you will want to install Python and OpenCV. I used Homebrew to do this, which is a package manager for OS X. I highly recommend installing Homebrew if you haven't done so using these instructions.

OS X comes with Python preinstalled, however, I occasionally had issues with stock Python, so I reinstalled it using Homebrew. On the command line type:

brew install python  

Next you want to install OpenCV. Before installing OpenCV, I found you have to install numpy system-wide.

pip install numpy  
brew tap homebrew/science  
brew install opencv  

OpenCV should now work with Python. You can test this out by opening a Python console by typing python on the command line and then running import cv2. If that works fine, you're ready to use OpenCV.

Set up virtualenv

virtualenv is a great tool for Python that manages project dependencies. Different versions of Python packages could potentially cause problems for your application. With virtualenv you can specify all the packages a project needs and which versions to use. They're isolated from system-wide packages and ones from other projects. As I'll show you later, it also helps make an application portable. To install virtualenv on the command line type:

pip install virtualenv  

Whenever you want to set up a virtualenv inside a project directory you run:

virtualenv --no-site-packages venv  

The environment is created inside the venv directory. To activate the environment type source venv/bin/activate and to deactivate type deactivate.

Having to source the virtual environment can get annoying really quickly, so I also use a tool called autoenv, which does this automatically. Install autoenv:

brew install autoenv  
echo 'source /usr/local/opt/autoenv/' >> ~/.bash_profile  

autoenv requires a .env file in the project directory. I create one using this example:

BASE_PATH=`dirname "${BASH_SOURCE}"`  

if [[ "${BASE_PATH}" == "${PWD}" ]]  
    if [[ -e venv/bin/activate ]]
        source venv/bin/activate

Install packages

To install packages for the virtual environment just run pip install <package>. Make sure the virtual environment is activated before running any pip commands. For an OpenCV project you'll need to install numpy. I would also recommend installing pytest to run tests.

To use OpenCV in a virtual environment you also need to copy OpenCV site-package files to the virtualenv site-package directory. You should be able to do this with:

cp /usr/local/lib/python2.7/site-packages/cv* ./venv/lib/python2.7/site-packages  

If this doesn't work you can find the location of the OpenCV files like this:

import cv2  
print cv2.__file__  

After you've finished installing packages you should generate a requirements file that lists the project's dependencies. In the project directory type:

pip freeze > requirements.txt  

Now whenever you set up this application on another system you only need to run this command to install all project dependencies:

pip install -r requirements.txt  

Project skeleton

I follow this guide to structure my project directory. There's a directory for all application files. There's a directory for test files. You also want a and file.

If you're using git (which I highly recommend) you should have a .gitignore file. I use this one and to the end I add:

# virtualenv

Create a new project

Up to this point you've installed Python, OpenCV and virtualenv. You've learned how to create a virtual environment, install packages and structure a project directory. To summarize, this is how I set up a new OpenCV project, assuming I have all the requirements installed:

mkdir new_project  
cd new_project  
mkdir project_name test  
touch project_name/ test/  
touch .gitignore  

Then I copy the previous example into .gitignore. The project directory is set up, so now set up virtualenv.

virtualenv --no-site-packages venv  
touch .env  

I copy the example into .env. Now you can install packages.

source venv/bin/activate  
pip install numpy  
pip install pytest  
cp /usr/local/lib/python2.7/site-packages/cv* ./venv/lib/python2.7/site-packages  
pip freeze > requirements.txt  

Next commit the project to git and now you're ready to start coding your OpenCV project! This project should also be easily portable to other systems.

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