Using OpenGL C functions within Matlab using MEX

Hello everyone I have been working on a 3D model using Matlab functions and after finishing it I was instructed to "refine" it and make it more realistic in addition to extracting and displaying depth information using OpenGL (C) functions, I have been looking into this for the last few days and it seems possible but I just can't seem to successfully get the simplest Mex files to work because of header problems and missing files and even when I compile the Mex file Matlab crashes when I call it, I want to start by simply compiling a MEX application which creates an OpenGL window, then I assume I'll have to redo the model using OpenGL functions and attempt to display the depth buffer.

I would appreciate any help I can get. Thanks.

EDIT01: Here is an example of a code that I compiled without any problems but crashes MATLAB when I attempt to run it.

#include "mex.h"   

#define GL_VIEWPORT                       0x0BA2
#define GL_DEPTH_COMPONENT                0x1902
#define GL_FLOAT                          0x1406

void mexFunction(int nlhs, mxArray *plhs[], int nrhs, const mxArray *prhs[])
{
  int viewport[4], i, x, y;
  int colLen;
  float *data;
  double *matrix;

    glGetIntegerv(GL_VIEWPORT, viewport);
    data = (float*)malloc(viewport[2] * viewport[3] * sizeof(float));
    glReadPixels(0, 0, viewport[2], viewport[3], GL_DEPTH_COMPONENT, GL_FLOAT, data);

    plhs[0] = mxCreateNumericMatrix(viewport[3], viewport[2], mxDOUBLE_CLASS, mxREAL);
    matrix = mxGetPr(plhs[0]);
    colLen = mxGetM(plhs[0]);

    for(x = 0; x < viewport[2]; ++ x) {
        for(y = 0; y < viewport[3]; ++ y)
            matrix[x * colLen + y] = data[x + (viewport[3] - 1 - y) * viewport[2]];
    }

    free(data);
    return;
}

[Jan]

How do you compile this? In which line does this crash - you simply check this by inserting some return commands?

[Alla]

Thnaks for the reply Simon I compile it using MinGW-w64, it crashes when I call the mex file using these simple lines

peaks;
figure(1);
depthData=mexGetDepth;

This is not my code, it's an example I came across and thought I'd try in order to get my head around using Mex with Opengl so if you have any recommendations don't limit your self to this code.

[Jan]

Which command do you use to compile the Mex file? You need some OpenGL libraries, I assume. Which line of the C-Mex does crash? Insert the return commands inside the Mex function, compile and start again. This is the stoneage method of debugging, but for 10 lines this is sufficient.

[Walter Roberson]

A few minutes ago I happened to notice a Software Patch Renderer file exchange contribution that is written in C. Perhaps looking at that would help. Perhaps you can use some of the routines there to get the depth information

[Alla]

Simon, for compiling I use

mex filename.c "path to OpenGL32.lib"

and it seems like any gl call crashes matlab, I have had success compiling and running mex files before so maybe there is a problem with the OpenGL functions.

[Jan]

Perhaps this works from inside Matlab also: see https://www.mathworks.com/matlabcentral/fileexchange/26619-dvcrender

[Alla]

Thank you Simon, the submission you linked me does look very promising, too bad the link to the .NET assemblies is dead, I messaged the author but the submission is seven years old so I'm not holding my breath, I'll look into it more now though. Thank you for you time and valuable input Simon.

[li yang]

Hi Alaa,i am wondering if you have solved this problem

[Alla]

Helllo Li Yang, this was about a year ago, as I recall I could not use the MEX files because of the problems I described in the original question, I used this <https://uk.mathworks.com/matlabcentral/fileexchange/23792-opengl-net-examples?s_tid=prof_contriblnk%20OpenGL%20.NET%20wrapper OpenGL NI wrapper > OpenGL wrapper for a while and got decent results but it is quite restricted. I eventually moved on from using Matlab as a whole.

Answer 1

[Mark]

I found you need to run opengl in software mode and not hardware mode. Running the matlab command:

     opengl software

before plotting does this. So now mexGetDepth runs without crashing Matlab, but it just plain doesn't work. The viewport never gets set to anything. It's always what I initialize it to before calling glGetIntegerv. If I call glGetError after glGetIntegerv I get 1282 which translates to "invalid command."