Overlays- what are they?

I just came across a bit of code that implemented rubberbanding in openGL using overlays.
It doesnt work on my PC. Yeah i know it shouldnt. Overlays are meant for workstations or so as i heard. The code author mentioned that SGI machines probably worked fine , and SPARC might have some probs with execution.
Now the real problem - going through the GLUT specification didn’t yield any idea about this (the code used GLUT for implmenting overlays).
Any other book on Opengl (including redbook) doesn’t provide a clue either.
And damn you guys give better answers than anyone else that I couldn’t fire up GOOGLE for finding the answer myself.
So, what are we waiting for?

Last time I looked (around 1999), GLUT only supported overlay planes on SGI machine with some special sauce code.
Right, OpenGL overlay are only supported on workstation class boards.
On windows check out the documentation of wglCreateLayerContext and the LAYERPLANEDESCRIPTOR struct. http://msdn.microsoft.com/library/default.asp?url=/library/en-us/opengl/ntopnglo_5a5v.asp
It’s Windows specific code anyway, grab a Win32 API OpenGL example and select a pixelformat with overlay.
Futile for a consumer graphics board. You can use a buffer region extension instead to save and restore your “mainplane”.

[This message has been edited by Relic (edited 03-03-2004).]

You can emulate layers, which was my one suggestion on your question about writing a paint program.