Accessing a matrix element in the "Mat" object (not the CvMat object) in OpenCV C++

Question

How to access elements by row, col in OpenCV 2.0's new "Mat" class? The documentation is linked below, but I have not been able to make any sense of it. http://opencv.willowgarage.com/documentation/cpp/basic_structures.html#mat

Answer 1

On the documentation:

http://docs.opencv.org/2.4/modules/core/doc/basic_structures.html#mat

It says:

(...) if you know the matrix element type, e.g. it is float, then you can use at<>() method

That is, you can use:

Mat M(100, 100, CV_64F); cout << M.at<double>(0,0); 

Maybe it is easier to use the Mat_ class. It is a template wrapper for Mat. Mat_ has the operator() overloaded in order to access the elements.

Answer 2

The ideas provided above are good. For fast access (in case you would like to make a real time application) you could try the following:

//suppose you read an image from a file that is gray scale Mat image = imread("Your path", CV_8UC1); //...do some processing uint8_t *myData = image.data; int width = image.cols; int height = image.rows; int _stride = image.step;//in case cols != strides for(int i = 0; i < height; i++) {     for(int j = 0; j < width; j++)     {         uint8_t val = myData[ i * _stride + j];         //do whatever you want with your value     } } 

Pointer access is much faster than the Mat.at<> accessing. Hope it helps!