OpenCV 2.3 with VS 2008 - Mouse Events

Question

Obligatory - I\'m a newbie. Have a job that involves programming and I\'m teaching myself as I go. Needless to say as a teacher I get things wrong frequently and thoroughly.

Answer 1

You can also use the onMouse method as a static method.

class Graph
{
public:
     static void onMouse(int event, int x, int y, void* param)
     {
          //Your code here
     }

     //Everything else you may need
}

Now you should be able to call the onMouse method with:

cv::setMouseCallback(windowName, onMouse, (void*) param);

The param can be NULL or whatever you want to pass as parameter to the method, but you'll need to make a type-cast to the desired type.

Hope this was useful. Bye.

Answer 2

Yes callback functions in C++ are a joy, aren't they? You actually have to give OpenCV a function (not a class method) as you've already found out. However, you can hack around this awfulness using the following technique:

class MyClass
{
public:
     void realOnMouse(int event, int x, int y, int flags)
     {
         // Do your real processing here, "this" works fine.
     }
};

// This is a function, not a class method
void wrappedOnMouse(int event, int x, int y, int flags, void* ptr)
{
    MyClass* mcPtr = (MyClass*)ptr;
    if(mcPtr != NULL)
        mcPtr->realOnMouse(event, x, y, flags);
}

int main(int argv, char** argc)
{
    // OpenCV setup stuff...

    MyClass processor;
    cv::setMouseCallback(windowName, wrappedOnMouse, (void*)&processor);

    // Main program logic

    return 0;
}

That last parameter on setMouseCallback is quite useful for overcoming some of the problems you usually encounter like this.