Introduction to Overlays

Introduction to Overlays

An overlay plane is a set of bitplanes displayed preferentially to the normal planes. Non-transparent pixels in the overlay plane are displayed in preference to the underlying pixels in the normal planes. Windows in the overlay planes do not damage windows in the normal plane.

If you have something in the main window that's fairly expensive to draw into and want to have something else on top, such as an annotation, you can use a transparent overlay plane to avoid redrawing the more expensive main window. Overlays are well-suited for popup menus, dialog boxes, and "rubber-band" image resizing rectangles. You can also use overlay planes for text annotations floating "over" an image and for certain transparency effects.

Note: Transparency discussed here is distinct from alpha buffer blending transparency effects. See the section "Blending" in Chapter 7, "Blending, Anti-Aliasing, and Fog," in the OpenGL Programming Guide.

Figure 4-1 : Overlay Plane Used for Transient Information A special value in the overlay planes indicates transparency. On Silicon Graphics systems, it's always the value zero. Any pixel with the value zero in the overlay plane is not painted, allowing the color of the corresponding pixel in the normal planes to show.

The concepts discussed in this section apply more generally to any number of framebuffer layers, for example, underlay planes (which are covered up by anything in equivalent regions of higher-level planes).

You can use overlays in two ways:

To draw additional graphics in the overlay plane on top of your normal plane OpenGL widget, create a separate GLwMDrawingArea widget in the overlay plane and set the GLX_LEVEL resource to 1. Position the overlay widget on top of the normal plane widget.
Note that since the GLwMDrawingArea widget is not a manager widget, it is necessary to create both the normal and overlay widgets as children of some manager widget--for example, a form--and have that widget position the two on top of each other. Once the windows are realized, you must call XRaiseWindow() to guarantee that the overlay widget is on top of the normal widget. Code fragments in "How to Create Overlays" illustrate this. The whole program is included as overlay.c in the source tree.
To create menus, look at examples in /usr/src/X11/motif/overlay_demos. They are present if you have the motif_dev.sw.demo subsystem installed. Placing the menus in the overlay plane avoids the need for expensive redrawing of the OpenGL window underneath them. While the demos do not deal specifically with OpenGL, they do show how to place menus in the overlay plane.

A Note for IRIS GL Users

IRIS GL supports the concept of popup planes, which are one level higher than the default overlay plane. Drawing in the popup planes in IRIS GL doesn't necessarily require a window, but you can't count on avoiding damage to anything non-transient drawn in those planes (for example, objects drawn by other applications).

When working with OpenGL and the X Window System, the situation is different: You have to create a separate window for any overlay rendering. Currently, no OpenGL implementation on a Silicon Graphics system supports a level greater than one.