In IRIX
4.0,
overlay plane support was
introduced. SGI's 4Dwm Motif-based window manager
and other SGI X clients such as the xwsh terminal emulator make use of the overlay planes to speed pop-up menus and support efficient rubber banding. Additionally, the IRIS Graphics Library allows rendering into the overlay planes. Sample code is provided demonstrating how to support overlay planes in Motif-based applications.
In the 5.0 release of IRIX, the SGI X product's overlay support was substantially re-engineered to improve upon the original implementation. Both overlay and underlay visuals are supported on some graphics configurations such as the Reality Engine.
Hewlett-Packard [3,9] also supports overlays on some of their workstations in particular those with CRX24 and CRX48Z graphics hardware. Hewlett-Packard implements overlays by making the default visual be in an 8-bit overlay (supporting a transparent pixel). This approach means most X clients automatically run in the overlay planes.
Many other vendors have also implemented overlays for X or are considering implementing them.
The code presented in the appendices works correctly on Silicon Graphics and Hewlett-Packard workstations which support X overlay windows.