lib/gamnit: intro EulerCamera::camera_to_world

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>

diff --git a/lib/gamnit/cameras.nit b/lib/gamnit/cameras.nit
index a1eed2f..245d071 100644 (file)
--- a/lib/gamnit/cameras.nit
+++ b/lib/gamnit/cameras.nit
@@ -162,6 +162,35 @@ class EulerCamera
                yaw = 0.0
                roll = 0.0
        end
+
+       # Convert the position `x, y` on screen, to world coordinates on the plane at `target_z`
+       #
+       # `target_z` defaults to `0.0` and specifies the Z coordinates of the plane
+       # on which to project the screen position `x, y`.
+       #
+       # This method assumes that the camera is looking along the Z axis towards higher values.
+       # Using it in a different orientation can be useful, but won't result in valid
+       # world coordinates.
+       fun camera_to_world(x, y: Numeric, target_z: nullable Float): Point[Float]
+       do
+               # TODO, this method could be tweaked to support projecting the 2D point,
+               # on the near plane (x,y) onto a given distance no matter to orientation
+               # of the camera.
+
+               target_z = target_z or else 0.0
+
+               # Convert from pixel units / window resolution to
+               # units on the near clipping wall to
+               # units on the target wall at Z = 0
+               var near_height = (field_of_view_y/2.0).tan * near
+               var cross_screen_to_near = near_height / (display.height.to_f/2.0)
+               var cross_near_to_target = (position.z - target_z) / near
+               var mod = cross_screen_to_near * cross_near_to_target * 1.72 # FIXME drop the magic number
+
+               var wx = position.x + (x.to_f-display.width.to_f/2.0) * mod
+               var wy = position.y - (y.to_f-display.height.to_f/2.0) * mod
+               return new Point[Float](wx, wy)
+       end
 end
 
 # Orthogonal camera to draw UI objects with services to work with screens of different sizes