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In OpenGL, an object is made up of geometric primitives such as triangle, quad, line segment and point. A primitive is made up of one or more vertices. OpenGL supports the following primitives:

When double buffering is enabled, glutSwapBuffers synchronizes with the screen refresh interval (VSync). That is, the buffers will be swapped at the same time when the monitor is putting up a new frame. As the result, idle() function, at best, refreshes the animation at the same rate as the refresh rate of the monitor (60Hz for LCD/LED monitor). It may operates at half the monitor refresh rate (if the computations takes more than 1 refresh interval), one-third, one-fourth, and so on, because it need to wait for the VSync.