Up to now we have been creating static images and, in turn, it was sufficient to initialize the GLUT display mode with GLUT_RGB. This is our first example of an animation which, in turn, repeatedly updates the display durng runtime. We use double buffering to avoid flicker as the display changes. We establish double buffering by including the glut constant GLUT_DOUBLE in the call to glutInitDisplayMode. Including the constant in the call requires a bitwise OR of the two GLUT constants.