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 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 :

...

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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