OpenCV and Matlab (and Visual Studio C++)

Matlab and OpenCV both have advantages for image processing work. I wanted to implement some code written in OpenCV (in MSVSC++) but run from a Matlab command window. After digging around on the web for help, I managed to get this to work. What makes this challenging is the OpenCV library linking. Simple C/C++/Fortran code can be implemented as Matlab functions by creating MEX files. To gain access to the OpenCV library, a MEX file can be written that has another file linked in at compile time. This second file can use the OpenCV library as usual. Without getting into the details, here are the steps I took. Use your head and change the directory paths and what not to match your setup.

Setting up Matlab

• From a Matlab command window, run the command mex -setup. Follow the dialog and change the compiler to use the MSVSC++ compiler.
• Find mexopts.bat (see below for locating this file) and open for editing. 
• Add set OPENCVDIR=C:\Program Files\OpenCV2.1. 
• Add %OPENCVDIR%\include\opencv to the set INCLUDE statement. 
• Add OPENCVDIR%\lib;%OPENCVDIR%\bin to the set LIB statement. 
• Add cv210.lib highgui210.lib cvaux210.lib cxcore210.lib to the end of the set LINKFLAGS statement. 

The location for mexopts.bat on my system is C:\Documents and Settings\Default\Application Data\MathWorks\MATLAB\R2008b. You can find where Matlab is looking for this file by trying to compile a MEX file with the verbose option enabled (e.g. mex -v program.cpp).

Setting up Visual Studio
Edit project properties as follows to allow compilation of MEX files.

• Add the following folders to the C/C++ additional include directories:
• [MATLAB_DIR]\extern\include
• [MATLAB_DIR]\simulink\include
• Add the following folder to the Linker additional directories:
• [MATLAB_DIR]\extern\lib\win32\microsoft
• Add the following Linker input additional dependencies (one per line):
• libmx.lib
• libmex.lib
• libmat.lib
• Add #include "mex.h" to your source code to allow the use and compilation of mx* functions.

Note: For the instructions above, replace [MATLAB_DIR] with the actual path on your system.

Creating Files

• OpenCV File
• Assuming you already have a working project that uses OpenCV, modify the source file to contain only a set of functions you would like to call from a MEX file.
• MEX File
• The MEX file needs to be written. See MathWorks' online documentation for how to do this. I used an example from here. 
• Include extern references to functions that you want to use from your OpenCV file.
• Use the functions in your MEX file.

Compiling

• Compile the source that uses OpenCV to create an object file (e.g. opencv_file.obj)
• Run the following command in a Matlab command window (with correct paths to your files):
• mex mex_file.cpp opencv_file.obj
• Fix any errors to get this to compile.

You will now have a file similar to mex_file.mexw32. This is the MEX file that acts as a normal Matlab function does (e.g. you can call mex_file()).