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.wi

Answer 1

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(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_ it, end;
for( it = I.begin(), end = I.end(); 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.