<p>I want to use the mouse scrollwheel in my OpenGL GLUT program to zoom in and out of a scene? How do I do that?</p>

Note that venerable
Nate Robin's GLUT
library doesn't support the scrollwheel. But, later implementations of GLUT like
FreeGLUT
do.
Using the scroll wheel in FreeGLUT is dead simple. Here is how:
Declare a callback function that shall be called whenever the scroll wheel is scrolled. This is the prototype:
```
```
void mouseWheel(int, int, int, int);
```
```
Register the callback with the (Free)GLUT function
glutMouseWheelFunc()
.
```
```
glutMouseWheelFunc(mouseWheel);
```
```
Define the callback function. The second parameter gives the direction of the scroll. Values of +1 is forward, -1 is backward.
```
```
void mouseWheel(int button, int dir, int x, int y)
{
    if (dir > 0)
    {
        // Zoom in
    }
    else
    {
        // Zoom out
    }

    return;
}
```
```
That's it!