How to access data from cv::Mat

Question

I have a cv::Mat which hase to following size 480 x 640 x 32.

Can you please show me how can I access the data within this structure? Let's say that I want to access the element from position [2, 2, 2]. How can I do this?

EDIT 1:

I have tried using this template<typename T> const T& Mat::at(int i, int j, int k) const but I receive the following runtime error :

enter image description here

EDIT 2:

Here is how I am using the code :

cv::Mat f(382,510,32); f=Utils::toMat(features); cout<<f.at<double>(1,1,1);

where toMat is detailed bellow.

cv::Mat Utils::toMat( mxArray* p_) { 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; 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; }

In which why did you define/create your Mat? The way you have to call the at function strongly depend from that. Also, post the code in which you call the at function — Antonio
– Antonio, Commented Jun 25, 2013 at 19:59
cv::Mat f(382,510,32); f=Utils::toMat(features); cout<<f.at<double>(1,1,1); Basically, toMat, transform an mxArray to a cv::mat structure — Simon
– Simon, Commented Jun 25, 2013 at 21:24
Can you tell something more about this Utils::toMat? If you can post the code is even better. The problem is 100% located there. — Antonio
– Antonio, Commented Jun 25, 2013 at 21:28
Hmmmmm To start with, you got wrong the constructor of cv::Mat docs.opencv.org/modules/core/doc/basic_structures.html#mat-mat : the value 32 is used as type! — Antonio
– Antonio, Commented Jun 25, 2013 at 22:04

Antonio · Accepted Answer · 2013-06-25 22:21:21Z

> cv::Mat f(382,510,32);

To start with, you got wrong the constructor of cv::Mat http://docs.opencv.org/modules/core/doc/basic_structures.html#mat-mat : your value 32 is used as type, leading to some undefined behaviour.

You have to use this

 Mat::Mat(int ndims, const int* sizes, int type) const int[] mySizes = {382,510,32}; //I hope I have written this correctly... cv::Mat f(3,mySizes,CV_64F). // You find CV_64 in the same documentation page.

Then, your Utils::toMat functions looks really complicated to debug... I suggest that you read a little bit more about the documentation, and possibly that you re-implement the initialization (filling) of your matrix using the at method:

 f.at<double>(x,y,z) = ...

Why isn't this method working for a plain image? I am trying to read a simple image using cv::Mat image = imread("D:\\img.jpg") and I am trying to access a pixel value using the following instruction : cout<<image.at<double>(1,1,1)<<endl; Why does is crash? It returns the same error as the one showed in the print screen error from the body of the question.
@Simon The answer is very simple: when you use the cv::Mat::at() you have to be able to specify correctly the native type of the cv::Mat you are reading. For a color image, 8 bit depth, you have to use image.at<Vec3b>(x,y) by the way: jpg images have only 2 dimensions. This is the documentation of vec: docs.opencv.org/modules/core/doc/basic_structures.html#vec On the subject "understanding native type inside a cv::Mat", this coudl be interesting reading: stackoverflow.com/questions/10167534/…

fatihk · Accepted Answer · 2013-06-25 18:54:58Z

You can use .at<element_type>(i,j,k);

with reference:

template<typename T> const T& Mat::at(int i, int j, int k) const

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