Swapping Buffers

Swapping Buffers

A double-buffered animation displays one buffer while drawing into another (undisplayed) buffer, then swaps the displayed buffer with the other. In OpenGL, the displayed buffer is called the front buffer, and the undisplayed buffer is called the back buffer. This sort of action is common in OpenGL programs; however, swapping buffers is a window-related function, not a rendering function, so you can't do it directly with OpenGL.

To swap buffers, use glXSwapBuffers() or (when using the widget) the convenience function GLwDrawingAreaSwapBuffers(). The glXSwapBuffers() function takes a display and a window as input--pixmaps don't support buffer swapping--and swaps the front and back buffers in the drawable. All renderers bound to the window in question continue to have the correct idea of which is the front buffer and which the back buffer. Note that once you call glXSwapBuffers(), any further drawing to the given window is suspended until after the buffers have been swapped.

Silicon Graphics systems support hardware double buffering; this means buffer swap is instantaneous during the vertical retrace of the monitor. As a result, there are no tearing artifacts; that is, you don't simultaneously see part of one buffer and part of the next.

Caution: If the window's visual allows only one color buffer, or if the GLX drawable is a pixmap, glXSwapBuffers() has no effect (and generates no error). There's no need to worry about which buffer the X server draws into if you're using X drawing functions as well as OpenGL; the X server draws only to the current front buffer, and prevents any program from swapping buffers while such drawing is going on. Using the X double buffering extension (DBE), it is possible to render X into the back buffer. DBE is not supported in releases preceding IRIX 6.2.

Note that users like uniform frame rates such as 60 Hz, 30 Hz, or 20 Hz. Animation may otherwise look jerky. A slower consistent rate is therefore preferable to a faster but inconsistent rate. For additional information about optimizing frame rates, see "Optimizing Frame Rate Performance". See "The Swap Control Extension" to learn how to set a minimum period of buffer swaps.