PixelFormat with layer? or not?


First a summary of informations :

With the PIXELFORMATDESCRIPTOR, I use always iLayerType as PFD_MAIN_PLANE.
If I look at the documentation of PIXELFORMATDESCRIPTOR, I see it’s possible to set other value to this member

Value			Meaning

PFD_MAIN_PLANE The layer is the main plane.
PFD_OVERLAY_PLANE The layer is the overlay plane.
PFD_UNDERLAY_PLANE The layer is the underlay plane.

The documentation with Visual C++ add the comment : “iLayerType Ignored. Earlier implementations of OpenGL used this member, but it is no longer used.”
The documentation with C++ Builder add the comment :“the current version supports only the main plane (there is no support for overlay or underlay planes)”

In the red book, I see some functions with specific use for layers
ex: wglDescribeLayerPlane(), wglCreateLayerContext, wglSwapLayerBuffers(), …

So the Questions :

Where is the truth ?
Is it possible to define a PIXELFORMATDESCRIPTOR with iLayerType different of PFD_MAIN_PLANE?
What are the advantages and disadvantages of working with layers?
Is there some codes examples of use opengl with layers?



Until now hardware did not support this so even if you did try…
Not even the software renderer could do it.
I’m not sure, but maybe the geforce can now. Can someone confirm?