OpenCV Return value from mouse callback function

Question

In OpenCV I want to return the point position like Point(x,y) to the main() function that I　click on the image in the mouse callback function . Is there anyway other than se

Answer 1

to expand Safirs idea there, apart from a class or such, you could just pass in the point itself:

void on_mouse( int e, int x, int y, int d, void *ptr )
{
    Point*p = (Point*)ptr;
    p->x = x;
    p->y = y;
}

Point p;
namedWindow("win");
setMouseCallback("win",on_mouse, (void*)(&p) );

// changed value of p will be accessible here 

Answer 2

No, this isn't possible, since the on_mouse() is a callback function. Here is the opencv documentation of it.

So, "global" variables are the only way to solve this problem. Alternatively, if you're looking for a nicer solution, you can create a wrapper class in which you have the namedWindow and the MouseCallback and a private member variable, which is manipulated when mouse callback function is called.

Answer 3

You can avoid using global variables by passing a pointer to your data as a parameter to setMouseCallback(). Agree with @berek, just wanted to show a full example below to avoid confusion about global variables.

using namespace cv; 

void on_mouse( int e, int x, int y, int d, void *ptr )
{
    Point*p = (Point*)ptr;
    p->x = x;
    p->y = y;
}

in main() {
    Point p;
    namedWindow("window");
    Mat image = imread("someimage.jpg");
    imshow(image);


    //pass a pointer to `p` as parameter
    setMouseCallback("window",on_mouse, &p ); 

    // p will update with new mouse-click image coordinates 
    // whenever user clicks on the image window 
}