To get the 3d world coordinate of a screen point, use remixlab.proscene.Camera.unprojectedCoordinatesOf. That method takes a PVector, say src. The src.x and src.y input values are expressed in pixels ((0,0) being the upper left corner of the window) and src.z is a depth value ranging in [0..1] (near and far plane respectively). Have also a look at the API :)