Trying to find Magnitude of Mat image - c++

Question

I'm trying to learn OpenCV (Using version 3.0.0).
Right now I'm trying to see what the point operatioins do to various images, everything is going fine until I tried to do the magnitude operation, which requires inputs be in the form of
magnitude(InputArray x, InputArray y, OutputArray magnitude)
It also describes that x and y should be floating-point arrays of x/y-coordinates of the vectors and also the same size.
I've tried making a Vector of Mat's and splitting up the input image into these vectors and then doing the magnitude operator on them, but this didn't work. So I think I need to pass the arguments as columns and rows, but now I'm getting the error
OpenCV Error: Assertion failed (src1.size() == src2.size() && type == src2.type() && (depth == CV_32F || depth == CV_64F)) in magnitude, file /home/<user>/opencv-3.0.0-beta/modules/core/src/mathfuncs.cpp, line 521
terminate called after throwing an instance of 'cv::Exception'
what(): /home/<user>/opencv-3.0.0-beta/modules/core/src/mathfuncs.cpp:521: error: (-215) src1.size() == src2.size() && type == src2.type() && (depth == CV_32F || depth == CV_64F) in function magnitude
Aborted (core dumped)
And I'm not sure why, because I am clearly converting the input Mats to CV_64F types.
Am I using the magnitude function wrong? Or just passing it the wrong data?
void Magnitude(Mat img, Mat out)
{
img.convertTo(img, CV_64F);
out.convertTo(out, CV_64F);
for(int i = 0 ; i < img.rows ; i ++)
for(int j = 0 ; j < img.cols ; j++)
cv::magnitude(img.row(i), img.col(j), out.at<cv::Vec2f>(i,j));
cv::normalize(out,out,0,255,cv::NORM_MINMAX);
cv::convertScaleAbs(out,out);
cv::imshow("Magnitude", out);
waitKey();
}

Answer 1

void magnitude(InputArray x, InputArray y, OutputArray magnitude)
where x, y and magnitude must have the same size. In your case it means that your image have to be quadratic. Is it right?
A sample usage:
cv::Sobel(img, gx, img.depth(), 1, 0, 3);
cv::Sobel(img, gy, img.depth(), 0, 1, 3);
cv::Mat mag(gx.size(), gx.type());
cv::magnitude(gx, gy, mag);