1. Prepare you machine
You need to have puredata and Gem installed on your system.
And you need to know where they are…

You also need a compiling toolchain, e.g. GCC on Linux/Mac or Microsoft Visual C++ on Windows.

2. Get the sources :
Download the pix_opencv sources from Puredata external repository :
svn co svn://svn.code.sf.net/p/pure-data/svn/trunk/externals/pix_opencv
or if you don't have svnl, can get a "snapshot",i.e. a zip file with the current code, here :
https://sourceforge.net/p/pure-data/svn/HEAD/tree/trunk/externals/pix_opencv/

3. Get opencv :
Ubuntu/Linux
The easiest way to get it is to install it from the repository :
sudo apt-get install libopencv-*

Mac OSX :
The easiest way is to get from some packaging repository like Macports, fink or homebrew.
with fink :
sudo fink install opencv-dev
with macport :
sudo port install opencv

Windows : 
Download the binary release on opencv.org
You can follow the quickstart guide on opencv.org to setup environment variables.

4. Build the Sources :

Ubuntu/Linux & MacOSX
Go to the fresh created folder, i.e. « pix_opencv » and build the things :
cd pix_opencv
make

If you see something like « can’t find m_pd.h », you need to tweak pd’s path with this command :
make CFLAGS=-I/path/to/pd
If you see something like « can’t find Base/GemPixObj.h », you need to tweak Gem’s path with this command :
make CFLAGS=-I/path/to/gem
You can combine options like :
make CFLAGS=-I/path/to/pd CFLAGS=-I/path/to/gem

Windows
You need Visual C++ (at least the free express edition) to build pix_opencv on Windows.
There is a solution in build/vs2010 folder.
It was made with Visual C++ Express 2010 and surely need some tweaks before producing any DLL.
You need to change the include and the library paths.
The include must contain the following :
  - puredata include (pd-extended) or src (vanilla) folder : path\to\pd\src or path\to\pd-extended\include
  - Gem include folder : path\to\Gem
  - OpenCV include path, wich is ususally $(OPENCV_DIR)\..\..\include if you setup OPENCV_DIR according to Windows quickstart guide on opencv.org.