Converting a std::vector<float> to mxArray using ocvMxArrayFromVector

Hello,

I'm currently working with mex matlab and OpenCV libraries. I'm using the results of the computation of HOG features, which is of type std::vector<float>, and need it as an output of the mex function.

I'm therefore trying to use the ocvMxArrayFromVector function, which is supposed to conver a vector of a given type to the required mxArray data type output.

The part of code I'm using is:

vector<float> descriptorValues(outDim);
hog->compute(croppedImg, descriptorValues, Size(0,0), Size(0,0), locations);
plhs[0] = ocvMxArrayFromVector(descriptorValues);

But when compiling it returns the following error:

Error using mexOpenCV (line 122)
/tmp/mex_1394440023880_2588/HOGDescriptorOCV.o: In function `computeHOGFeatures(int, mxArray_tag**, mxArray_tag
const**)':
HOGDescriptorOCV.cpp:(.text+0xb24): undefined reference to `ocvMxArrayFromVector(std::vector<float,
std::allocator<float> > const&)'
collect2: error: ld returned 1 exit status

I'm compiling using g++ on an Ubuntu, matlab version R2016b.

I would be very thankful for your help.

David

Answer 1

[James Tursa]

I don't know about the ocvMxArrayFromVector routine, or how to advise you to link everything in properly, but here is a short example that takes a C++ vector of floats and copies it into an mxArray for output:

#include <vector>
using namespace std;
#include <string.h>
#include "mex.h"

void mexFunction(int nlhs, mxArray *plhs[], int nrhs, const mxArray *prhs[])
{
    int i, n=5;
    vector<float> descriptorValues(n);

      // Load up some sample data
      for( i=0; i<n; i++ ) {
          descriptorValues[i] = (float) (i + 1);
      }

      // Pretend we don't know the size and get it dynamically
      n = descriptorValues.size();

      // Create the output mxArray and copy the data
      plhs[0] = mxCreateNumericMatrix( 1, n, mxSINGLE_CLASS, mxREAL );
      memcpy( mxGetData(plhs[0]), &descriptorValues[0], n*sizeof(float) );
}

[David Moreno]

This solved my problem, thank you :)

Answer 2

[xingxingcui]

preference here, perhas helps

https://stackoverflow.com/questions/2867340/how-to-create-a-string-array-in-matlab/55369548#55369548