Overlays are another frame buffer layer that can be displayed preferentially relative to the normal frame buffer. Overlays are common for OpenGL capable hardware. Overlays can be supported as part of the X Window System [6], independent of OpenGL. OpenGL's GLX support for X supports overlays in a manner consistent with X's overlay support.
OpenGL scenes are often intricate and take time and resources to redraw, so overlays are particularly useful in conjunction with OpenGL. Pop-up and pull-down menus using the overlays can avoid damaging OpenGL windows that are expensive to redraw. Overlays can also provide a way to perform rubber banding and annotation of OpenGL windows. Figure 2 shows how overlay planes can be used for annotation.