How to use X11 overlays with the Qt OpenGL extension

X11 overlays are a powerful mechanism for drawing annotations etc., on top of an image without destroying it, thus saving a great deal of image rendering time. For more information, consult the highly recommended book OpenGL Programming for the X Window System (Mark Kilgard, Addison Wesley Developers Press 1996).

Warning: From version 5.0 onwards, the Qt OpenGL Extension includes direct support for the use of OpenGL overlays. For many uses of overlays, this makes the technique described below redundant. See the overlay example program. The following is a discussion on how to use non-QGL widgets in overlay planes.

In the typical case, X11 overlays can easily be used together with the current version of Qt and the Qt OpenGL Extension. The following requirements apply:

1. Your X server and graphics card/hardware must support overlays. For many X servers, overlay support can be turned on with a configuration option; consult your X server installation documentation.

2. Your X server must (be configured to) use an overlay visual as the default visual. Most modern X servers do this, since this has the added advantage that pop-up menus, overlapping windows etc., will not destroy underlying images in the main plane, thereby avoiding expensive redraws.

3. The best (deepest) visual for OpenGL rendering is in the main plane. This is the normal case. Typically, X servers that support overlays provide a 24 bit deep TrueColor visual in the main plane, and an 8 bit PseudoColor (default) visual in the overlay plane.

The provided example program X11 overlay will check for these and report if anything is wrong. See About X11 Visuals, below for more information.

How it works