3 MEX-files

Table of Contents:

• 3.1 OpenGL/Visual C++ Specifics
• 3.2 Matlab Specifics

MEX files are used to allow subroutines implemented in C or C++ to be ran in Matlab as if they were built-in functions or M-files. Therefore, compilation of a C++ program as a MEX-file will generate dynamically linkable subrountines (i.e., a .dll file). In the spider application, the .dll generated is called 'cmex.dll'. For in-depth reading on how MEX-files are generated, see discussion in Appendix 2.

There are functions and semantics that make MEX-files unique from C++ programs. This page outlines the differences and discusses the interface between Matlab and the OpenGL/Visual C++ program.

3.1 Opengl/Visual C++ Specifics

The following additions need to be made to the opengl/Visual C++ program in order to be able to compile the program into a MEX-file:

	#include "mex.h"
	...
	// instead of a main() function, mexFunction() is used as the gateway
	void mexFunction(int nlhs, mxArray *plhs[], int nrhs, const mxArray *prhs[]) {
	...
	}

nlhs - # of left hand side arguments

plhs - pointer to left hand side arguments

nrhs - # of right hand side arguments

prhs - pointer to right hand side arguments

Since the mxArray is a special structure that contains Matlab data, I extract only the numerical value of the mxArray elements (by using the mxGetScalar() function). I then proceed to assign global parameters in the OpenGL program to the settings found in the prhs mxArray:

void mexFunction(int nlhs, mxArray *plhs[], int nrhs, const mxArray *prhs[]) {
	...
	int		i;
	
	for (i = 0; i < nrhs; i++) {
		inputParams[i] = mxGetScalar(prhs[i]);
	}
	numParams = nrhs;

	// Set all the parameters and dimensions of the spider geometry.
	setColors();
	setAmbienceColor();
	setDiffuseColor();
	setSpecularColor();
	setBackgroundColor();
	setAbdomenParams();
	setThoraxParams();
	setHeadParams();
	setLegParams();
	setTimeStep();
	...
}

Upon GUI interaction from the Matlab end, these values are constantly changed. Every user interaction that occurs from the Matlab GUI resets all these global variables to the values specified in the prhs mxArray[].

3.2 Matlab Specifics

I created a variable called 'vars' that is essentially global since it is always updated and contained in the 'userdata' section of the 'Spider GUI' window:

vars = 

            myname: 'spider_gui'
     spline1button: 29.0006
     spline2button: 30.0006
     spline3button: 31.0006
     spline4button: 32.0006
          filename: 33.0006
       splineIndex: 1
              tmax: 120
        NumControl: 120
               FPS: 30
    ForceZeroSlope: 1
        AngleRange: 60
                 t: [1x120 double]
                tt: [1x3600 double]
        numSplines: 4
                 y: [120x4 double]
                yy: [3600x4 double]
                sp: 90.0005
             line1: 39.0012
         mousedown: 0
            values: {1x633 cell}
             names: {1x153 cell}