OpenCV changing Mat inside a function (Mat scope) - opencv

Question

I am passing a Mat to another function and changing it inside the called function. I had expected that being a more complex type it was automatically passed by reference so that the matrix would have changed in the calling function, but it doesn't. Could someone point me at the explanation of how to correctly return a changed Mat from a function?
Here's the code snippet:
void callingFunction(Mat img)
{
Mat tst(100,500,CV_8UC3, Scalar(0,255,0));
saveImg(tst, "Original image", true);
testImg(tst);
saveImg(tst, "Want it to be same as inside testImg but is same as Original", true);
}
void testImg(Mat img)
{
int rs = 50; // rows
int cs = 100; // columns
img = Mat(rs, cs, CV_8UC3, Scalar(255,0,0));
Mat roi(img, Rect(0, 0, cs, rs/2));
roi = Scalar(0,0,255); // change a subsection to a different color
saveImg(img, "inside testImg", true);
}
Thanks!

Answer 1

You have to define Mat as parameter-reference (&). Here's edited code:
void callingFunction(Mat& img)
{
Mat tst(100,500,CV_8UC3, Scalar(0,255,0));
saveImg(tst, "Original image", true);
testImg(tst);
saveImg(tst, "Want it to be same as inside testImg but is same as Original", true);
}
void testImg(Mat& img)
{
int rs = 50; // rows
int cs = 100; // columns
img = Mat(rs, cs, CV_8UC3, Scalar(255,0,0));
Mat roi(img, Rect(0, 0, cs, rs/2));
roi = Scalar(0,0,255); // change a subsection to a different color
saveImg(img, "inside testImg", true);
}

Answer 2

I wondered about the same question myself, so I would like to further clarify the answer given by #ArtemStorozhuk (which is correct).
The OpenCV documentation is misleading here, because it appears you're passing the matrix by value, but in fact the constructor of cv::OutputArray is defined as follows:
_OutputArray::_OutputArray(Mat& m)
so it gets the matrix by reference!
Since operations like cv::Mat::create create a new matrix, the operation releases the reference and set the couter to 1. Thus, in order to keep the result in the calling function, you have to pass the matrix by reference.

Answer 3

If its true that you have to explicitly pass by reference, then how do all the OpenCV functions work? None of them pass values by reference, yet they somehow seem to write to the passed in Mat just fine. For example, here is the declaration for the Sobel function in imgproc.hpp:
//! applies generalized Sobel operator to the image
CV_EXPORTS_W void Sobel( InputArray src, OutputArray dst, int ddepth,
int dx, int dy, int ksize=3,
double scale=1, double delta=0,
int borderType=BORDER_DEFAULT );
as you can see, it passes in src and dst without a &. And yet I know that after I call the Sobel with an empty dst, it will end up filled. No '&' involved.