glutMouseWheelFunc man page
glutMouseWheelFunc — Sets the mouse wheel callback for the current window.
OpenGLUT - input
glutMouseWheelFunc(void( *callback )( int wheel, int direction, int x, int y ));
Client hook for mouse wheel events.
If the mouse wheel is spun over your (sub)window, OpenGLUT will, in theory, report this via the MouseWheel callback.
is the wheel number,
is +/- 1, and
are the mouse coordinates.
If you do not register a wheel callback, wheel events will be reported as mouse buttons.
This callback is bound to the
Due to lack of information about the mouse, it is impossible to implement this correctly on X at this time. Use of this function limits the portability of your application. (This feature
work on X, just not reliably.) You are encouraged to use the standard, reliable mouse-button reporting, rather than wheel events.
Windows created via glutCreateMenuWindow() always cascade keyboard and mouse events to their parent.