Well, I downloaded a tutorial about alpha volumes and it uses the function above. I don’t know what it does so I checked the GLUT specifications, but I didn’t find it.

Anyone knows how it works?

I think it is called when a keyboard button is relased, but I’m not sure.

Thanks in advice!

yes, glutSpecialFunc() registers keyboard presses that aren’t registered using glutKeyboardFunc. These include the Function keys, cursor keys etc. glutSpecialUpFunc() registers the release of these keys.