Parsing problems with Matricies in C++ Mex Files

Hello~!

I am trying to create a .mex64 file which takes in an Grey Image (or later RGB) with a few other parameters, does a few computations and outputs a vector.

I am stuck with trying to read the image. My output for each point is always 0, and have no idea what I'm doing wrong.

I am using Matlab 2013a with the compiler from Visual Studio 2008.

This is my code:

void mexFunction(int nlhs, mxArray *plhs[], int nrhs, const mxArray *prhs[])
{
  size_t m, n;
  double *Img;
  int cx, cy, a, b, theta, numPoints;
  
  /* Check for proper number of arguments */
    if (nrhs != 7) { 
      mexErrMsgIdAndTxt( "MATLAB:SampleEllipse:invalidNumInputs",
                "Seven input arguments required."); 
    } else if (nlhs > 1) {
      mexErrMsgIdAndTxt( "MATLAB:SampleEllipse:maxlhs",
                "Too many output arguments."); 
    } 
  
  m = mxGetM(prhs[5]);
  n = mxGetN(prhs[5]);

  cx = mxGetScalar(prhs[0]);
  cy = mxGetScalar(prhs[1]);
  a = mxGetScalar(prhs[2]);
  b = mxGetScalar(prhs[3]);
  theta = mxGetScalar(prhs[4]);
  Img = mxGetPr(prhs[5]);
  numPoints = mxGetScalar(prhs[6]);

  printf("%d %d %d %d %d %d | ptr: %u\r\n", cx, cy, a, b, theta, numPoints, Img);

  for (int i = 0; i < n; i++){
    for (int j = 0; j < m; j++){
      printf("[%d*%d]+%d=%d: %d\n", i, m, j, (i*m) + j, Img[(i*m) + j]);
    }
  }
}

In matlab, I call it like (mex function is called 'SampleEllipse'):

testArray = [1 2 3; 4 5 6; 7 8 9];
SampleEllipse(1,2,3,4,5,testArray, 12)

and get an output of:

[0*3]+0=0: 0
[0*3]+1=1: 0
[0*3]+2=2: 0
[1*3]+0=3: 0
[1*3]+1=4: 0
[1*3]+2=5: 0
[2*3]+0=6: 0
[2*3]+1=7: 0
[2*3]+2=8: 0

This has me rather confounded and would appreciate any assistance!

Thanks!

Answer 1

[James Tursa]

You are printing out a double floating point value using an integer format %d. Use a floating point format instead (%f, %g). E.g.,

printf("[%d*%d]+%d=%d: %f\n", i, m, j, (i*m) + j, Img[(i*m) + j]);

[John]

Ha doh! Thanks, that did not occur to me as the error (obviously, because I had to ask...).