How to access data from a cv::Mat converted from mxArray

asked 2013-06-29 10:38:09 -0500

Simon gravatar image

updated 2013-06-29 12:50:36 -0500

I have a function which converts an mxArray mat to cv::Mat. The body of the method is listed bellow :

   cv::Mat Utils::mxArray2cvMat( mxArray* p_)
   {
   // Create cv::Mat object.
   mwSize ndims_= mxGetNumberOfDimensions(p_);
   const mwSize* dims=mxGetDimensions(p_);
   std::vector<int> d(dims, dims+ndims_);
   int ndims = (d.size()>2) ? d.size()-1 : d.size();
   int nchannels = (d.size()>2) ? *(d.end()-1) : 1;
   //depth = (depth==CV_USRTYPE1) ? DepthOf[classID()] : depth;
   int depth=CV_64F;
   std::swap(d[0], d[1]);
   cv::Mat mat(ndims, &d[0], CV_MAKETYPE(depth, nchannels));
   // Copy each channel.
   std::vector<cv::Mat> channels(nchannels);
   std::vector<mwSize> si(d.size(), 0); // subscript index
   int type = CV_MAKETYPE(depth, 1); // Source type
   for (int i = 0; i<nchannels; ++i)
   {
   si[d.size()-1] = i;
   void *pd = reinterpret_cast<void*>(
           reinterpret_cast<size_t>(mxGetData(p_))+
           mxGetElementSize(p_)*mxCalcSingleSubscript(p_, si.size(), &si[0]));
   cv::Mat m(ndims, &d[0], type, pd);
   // Read from mxArray through m
   m.convertTo(channels[i], CV_MAKETYPE(depth, 1));
   }
   cv::merge(channels, mat);
   return  mat;
   }

Now I want to access the values within the returned array. I have an array whose dimensions are 300 x 500 x 32. How can I access the values within this array?

I have the following code:

   cv::Mat f=Utils::mxArray2cvMat(features);
   cout<<f.at<double>(1,1,1);

However, it crashes giving some assertion failed error at runtime. I have also run the following command in order to identify the data type within the f variable ( cout<<f.type() ) and it returned the value 254. What type is that? And how can I access the values within this type of Mat?

Can you please give me any help regarding this matter?

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