It looks like Peter's function assumes a parallel projection where the camera is facing -z. I wonder what would be useful in a 3 dimensional space. I can imagine the following scenarios with mapping mouse coordinates:

Intersection of the ray of that pixel's position/direction in 3D coordinates with the XY, YZ, or XZ planes depending on the camera position and type (parallel or perspective).
Intersection of the ray of that pixel's position/direction and an object that it touches. If it doesn't intersect an object it could either return a point at infinity or an intersection with a plane.

Implementation would probably involve inspection of the z buffer at that pixel along with Peter's reverse projection. What would a nice interface look like?

David

2008/11/29 Peter Verswyvelen <bugfact@gmail.com>:
> The windowPoint function in Reactive/GLUT/SimpleGL.hs has the following
> comment:
> -- | Convert a window position to a logical X,Y. Logical zero is at the
> -- origin, and [size??].
> --
> -- TODO: this def is completely broken. What do I even want? Probably to
> -- go backward through the viewing transform to the XY plane.
> The function I made in the past for doing this is:
>
> windowPoint :: GL.Position -> IO (Double,Double)
> windowPoint pos@(GL.Position x y) =
> do
> viewport@(_,GL.Size _ height) <- get GL.viewport
> modelviewMatrix <- getMatrix $ GL.Modelview 0
> projectionMatrix <- getMatrix $ GL.Projection
> let pos3 = GL.Vertex3 (fromIntegral x) (fromIntegral (height-y)) 0
> GL.Vertex3 vx vy _ <- GLU.unProject pos3 modelviewMatrix
> projectionMatrix viewport
> return (vx,vy)
> where
> getMatrix :: GL.MatrixMode -> IO(GL.GLmatrix Scalar)
> getMatrix mode = get $ GL.matrix $ Just mode
>
>
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--
David Sankel