[Papervision3D] 3D point from 2D scene co-ordinate?

[Nick Kuh]

I think this has been asked before but I haven't found it answered:

How would one go about retrieving the 3D co-ordinates from the 2D x and y
screen co-ordinates of my scene - ie the top left, right, and bottom left
and right 2D points?

Using the 3D y values as always equal to 0.  I need the z and x values from
those x and y values of the 2D scene projection.

Thanks for any suggestions in advance.

Nick