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!

Answer 3

Based on what @J. Calleja said, you have two choices

Method 1 - Random access

If you want to random access the element of Mat, just simply use

Mat.at<data_Type>(row_num, col_num) = value;

Method 2 - Continuous access

If you want to continuous access, OpenCV provides Mat iterator compatible with STL iterator and it's more C++ style

MatIterator_<double> it, end;
for( it = I.begin<double>(), end = I.end<double>(); it != end; ++it)
{
    //do something here
}

or

for(int row = 0; row < mat.rows; ++row) {
    float* p = mat.ptr(row); //pointer p points to the first place of each row
    for(int col = 0; col < mat.cols; ++col) {
         *p++;  // operation here
    }
}

If you have any difficulty to understand how Method 2 works, I borrow the picture from a blog post in the article Dynamic Two-dimensioned Arrays in C, which is much more intuitive and comprehensible.

See the picture below.

Answer 4

OCV goes out of its way to make sure you can't do this without knowing the element type, but if you want an easily codable but not-very-efficient way to read it type-agnostically, you can use something like

double val=mean(someMat(Rect(x,y,1,1)))[channel];

To do it well, you do have to know the type though. The at<> method is the safe way, but direct access to the data pointer is generally faster if you do it correctly.

Answer 5

For cv::Mat_<T> mat just use mat(row, col)

Accessing elements of a matrix with specified type cv::Mat_< _Tp > is more comfortable, as you can skip the template specification. This is pointed out in the documentation as well.

code:

cv::Mat1d mat0 = cv::Mat1d::zeros(3, 4);
std::cout << "mat0:\n" << mat0 << std::endl;
std::cout << "element: " << mat0(2, 0) << std::endl;
std::cout << std::endl;

cv::Mat1d mat1 = (cv::Mat1d(3, 4) <<
    1, NAN, 10.5, NAN,
    NAN, -99, .5, NAN,
    -70, NAN, -2, NAN);
std::cout << "mat1:\n" << mat1 << std::endl;
std::cout << "element: " << mat1(0, 2) << std::endl;
std::cout << std::endl;

cv::Mat mat2 = cv::Mat(3, 4, CV_32F, 0.0);
std::cout << "mat2:\n" << mat2 << std::endl;
std::cout << "element: " << mat2.at<float>(2, 0) << std::endl;
std::cout << std::endl;

output:

mat0:
[0, 0, 0, 0;
 0, 0, 0, 0;
 0, 0, 0, 0]
element: 0

mat1:
[1, nan, 10.5, nan;
 nan, -99, 0.5, nan;
 -70, nan, -2, nan]
element: 10.5

mat2:
[0, 0, 0, 0;
 0, 0, 0, 0;
 0, 0, 0, 0]
element: 0