How to access pixel values of CV_32F/CV_64F Mat?

Question

I was working on homography and whenever I try to check the values of H matrix (type CV_64F) using H.at<float>(i, j) I get random numbers(sometimes garbage value). I want to access pixel values of float matrix. Is there any way to do it?

Mat A = Mat::eye(3, 3, CV_64F);
float B;
for(int i=0; i<A.rows; i++)
{
    for(int j=0; j<A.cols; j++)
    {
        printf("%f\n", A.at<float>(i, j));
    }
}

imshow("identity", A);
waitKey(0);

This shows correct image of an identity matrix but while trying to access pixel values, I get

0.000000 1.875000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000

Why is this so?

Silverman

Answer 1

You should try this:

A.at<double>(i, j);

because your matrix is of "type" CV_64F which in turn means it contains elements of type double, not float.

By the way, I'm not sure whether you are aware of this, but you can use cout to print the matrix like so:

std::cout << A << std::endl;

I found this to be useful for inspecting a small matrix or a slice of a matrix.

Inulin

Answer 2

The example below initializes a Hilbert matrix:

Mat H(100, 100, CV_64F);
for(int i = 0; i < H.rows; i++)
    for(int j = 0; j < H.cols; j++)
        H.at<double>(i,j)=1./(i+j+1);

Keep in mind that the size identifier used in the at operator cannot be chosen at random. It depends on the image from which you are trying to retrieve the data. The table below gives a better insight in this:

If matrix is of type CV_8U then use Mat.at<uchar>(y,x).

If matrix is of type CV_8S then use Mat.at<schar>(y,x).

If matrix is of type CV_16U then use Mat.at<ushort>(y,x).

If matrix is of type CV_16S then use Mat.at<short>(y,x).

If matrix is of type CV_32S then use Mat.at<int>(y,x).

If matrix is of type CV_32F then use Mat.at<float>(y,x).

If matrix is of type CV_64F then use Mat.at<double>(y,x).

(Taken from OpenCV docs)

Cleric