Matlab MEX file - index correct, incorrect UINT32 values

I am getting incorrect answers from my mex function. I generate a test file with this code:

%Create a 4D array 2x,2y, rgb(3), framenumber(from 1 to 5)
%Framenumber from 1 to 5

testmatrix(1,1,1,1) = 0;
testmatrix(1,1,1,2) = 1;
testmatrix(1,1,1,3) = 2;
testmatrix(1,1,1,4) = 3;
testmatrix(1,1,1,5) = 4;

testmatrix(1,1,2,1) = 5;
testmatrix(1,1,2,2) = 6;
testmatrix(1,1,2,3) = 7;
testmatrix(1,1,2,4) = 8;
testmatrix(1,1,2,5) = 9;

testmatrix(1,1,3,1) = 10;
testmatrix(1,1,3,2) = 11;
testmatrix(1,1,3,3) = 12;
testmatrix(1,1,3,4) = 13;
testmatrix(1,1,3,5) = 14;

testmatrix(1,2,1,1) = 15;
testmatrix(1,2,1,2) = 16;
testmatrix(1,2,1,3) = 17;
testmatrix(1,2,1,4) = 18;
testmatrix(1,2,1,5) = 19;

testmatrix(1,2,2,1) = 20;
testmatrix(1,2,2,2) = 21;
testmatrix(1,2,2,3) = 22;
testmatrix(1,2,2,4) = 23;
testmatrix(1,2,2,5) = 24;

testmatrix(1,2,3,1) = 25;
testmatrix(1,2,3,2) = 26;
testmatrix(1,2,3,3) = 27;
testmatrix(1,2,3,4) = 28;
testmatrix(1,2,3,5) = 29;

testmatrix(2,1,1,1) = 30;
testmatrix(2,1,1,2) = 31;
testmatrix(2,1,1,3) = 32;
testmatrix(2,1,1,4) = 33;
testmatrix(2,1,1,5) = 34;

testmatrix(2,1,2,1) = 35;
testmatrix(2,1,2,2) = 36;
testmatrix(2,1,2,3) = 37;
testmatrix(2,1,2,4) = 38;
testmatrix(2,1,2,5) = 39;

testmatrix(2,1,3,1) = 40;
testmatrix(2,1,3,2) = 41;
testmatrix(2,1,3,3) = 42;
testmatrix(2,1,3,4) = 43;
testmatrix(2,1,3,5) = 44;

testmatrix(2,2,1,1) = 45;
testmatrix(2,2,1,2) = 46;
testmatrix(2,2,1,3) = 47;
testmatrix(2,2,1,4) = 48;
testmatrix(2,2,1,5) = 49;

testmatrix(2,2,2,1) = 50;
testmatrix(2,2,2,2) = 51;
testmatrix(2,2,2,3) = 52;
testmatrix(2,2,2,4) = 53;
testmatrix(2,2,2,5) = 54;

testmatrix(2,2,3,1) = 55;
testmatrix(2,2,3,2) = 56;
testmatrix(2,2,3,3) = 57;
testmatrix(2,2,3,4) = 58;
testmatrix(2,2,3,5) = 59;

testmatrix = uint8(testmatrix);

I created the following mex function:

#include <stdint.h>
#include "mex.h"

void
mexFunction(int nlhs,mxArray *plhs[],int nrhs,const mxArray *prhs[])
{
//Check - expect 1 input, expect 1 outputs
    if (nrhs != 1) {
        mexErrMsgIdAndTxt( "MATLAB:24bit_pixel_from_uint8_rgb:invalidNumInputs",
                "One input argument required.");
    }
    if (nlhs > 1) {
        mexErrMsgIdAndTxt( "MATLAB:24bit_pixel_from_uint8_rgb:maxlhs",
                "Too many output arguments.");
    }    

      //declare variables
      mwSize *dim_array, dims;
      mxArray *a_in_m, *b_out_m;
      uint32_t *a,b;
      int X,Y,RGB,FRAMENUMBER;
      int X_INDX,Y_INDX,RGB_INDX,FRAMENUMBER_INDX;

      dims             = mxGetNumberOfDimensions(prhs[0]);
      dim_array        = mxGetDimensions(prhs[0]);  
      X                = dim_array[0];
      Y                = dim_array[1];
      RGB              = dim_array[2];
      FRAMENUMBER      = dim_array[3];
      mexPrintf("dims: %d\n",dims);
      mexPrintf("X: %d\n",X);
      mexPrintf("Y: %d\n",Y);
      mexPrintf("RGB: %d\n",RGB);
      mexPrintf("FRAMENUMBER: %d\n",FRAMENUMBER);
      a_in_m = mxDuplicateArray(prhs[0]);

      a = mxGetData(a_in_m);
      mexPrintf("a is: %d\n",a[0]); //SHOULD BE 0!

      b_out_m = plhs[0] = mxCreateNumericArray(dims, dim_array, mxUINT32_CLASS, 0);
      b = mxGetPr(b_out_m);

      int linear_index = 0;
      for(X_INDX=0;X_INDX<X;X_INDX++)
      {
          for(Y_INDX=0;Y_INDX<Y;Y_INDX++)
          {
              for(RGB_INDX=0;RGB_INDX<RGB;RGB_INDX++)
              {
                  for(FRAMENUMBER_INDX=0;FRAMENUMBER_INDX<FRAMENUMBER;FRAMENUMBER_INDX++)
                  {
                     mexPrintf("Linear Index: %d\n",linear_index); 
                     mexPrintf("Value[%d]: %d\n",FRAMENUMBER_INDX + (RGB_INDX*FRAMENUMBER)+(Y_INDX*RGB*FRAMENUMBER)+(X_INDX*Y*RGB*FRAMENUMBER),a[FRAMENUMBER_INDX + (RGB_INDX*FRAMENUMBER)+(Y_INDX*RGB*FRAMENUMBER)+(X_INDX*Y*RGB*FRAMENUMBER)]); 
                     linear_index += 1;
                  }
              }
          }

      }
  }

When I call it I get the following:

>> rgb_to_uint8(testmatrix)
dims: 4
X: 2
Y: 2
RGB: 3
FRAMENUMBER: 5
a is: 755965440
Linear Index: 0
Value[0]: 755965440
Linear Index: 1
Value[1]: 840180485
Linear Index: 2
Value[2]: 924395530
Linear Index: 3
Value[3]: 772808449
...etc

Can somebody help me figure out what I've done wrong?

Answer 1

[James Tursa]

The input is a uint8 array, but you are using a uint32_t pointer to access the data. So there is a mismatch. Change this:

uint32_t *a,b;

To this:

unsigned char *a;

Also, your use of the variable b is all wrong. As you have it written, b it a uint32_t type ... it is NOT a pointer! And yet you assume it is a pointer in the following bit of code:

        b_out_m = plhs[0] = mxCreateNumericArray(dims, dim_array, mxUINT32_CLASS, 0);
        b = mxGetPr(b_out_m);

That bit of code is also screwy because you create a mxArray of class uint32, but then access the pointer with mxGetPr which returns a (double *) type. If you really want to use a uint32 class here, then so something like this instead:

uint32_t *b;
  :
      b_out_m = plhs[0] = mxCreateNumericArray(dims, dim_array, mxUINT32_CLASS, 0);
      b = (uint32_t *)mxGetData(b_out_m);