glutVisibilityFunc sets the visibility callback for the current window. The visibility callback for a window is called when the visibility of a window changes. The state callback parameter is either GLUT_NOT_VISIBLE or GLUT_VISIBLE depending on the current visibility of the window. GLUT_VISIBLE does not distinguish a window being totally versus partially visible. GLUT_NOT_VISIBLE means no part of the window is visible, i.e., until the window's visibility changes, all further rendering to the window is discarded.
GLUT considers a window visible if any pixel of the window is visible or any pixel of any descendant window is visible on the screen.
Passing NULL to glutVisibilityFunc disables the generation of the visibility callback.
If the visibility callback for a window is disabled and later re-enabled, the visibility status of the window is undefined; any change in window visibility will be reported, that is if you disable a visibility callback and re-enable the callback, you are guaranteed the next visibility change will be reported.
Setting the visibility callback for a window disables the window status callback set for the window (and vice versa). The window status callback is set with glutWindowStatusFunc. glutVisibilityFunc is deprecated in favor of the more informative glutWindowStatusFunc.