Ah yes, true, I only used this function for orthographic (aka parallel) projection.
With perspective projection, it makes more sense to return a ray, not a point. With this ray, one can do all kinds of intersections.
One could use the Z-buffer to figure out the closest object, but reading from the z-buffer with the CPU is rather slow, and might be rather inaccurate, but for testing purposes, that might work fine.
I guess it all comes down to what kind of "picking" interface one wants. It certainly would be nice to have an Event in reactive-fieldtrip for picking objects :) Since most (if not all?) geometry in fieldtrip is procedural, picking could be made exact?
On Mon, Dec 1, 2008 at 3:28 PM, David Sankel
<camior@gmail.com> wrote:
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