1 files changed, 19 insertions, 1 deletions

diff --git a/Spear/Math/Utils.hs b/Spear/Math/Utils.hs
index 28f012e..e86cacc 100644
--- a/Spear/Math/Utils.hs
+++ b/Spear/Math/Utils.hs
@@ -3,11 +3,14 @@ module Spear.Math.Utils
     Side(..)
 ,   Face(..)
 ,   orientation2d
+,   viewToWorld2d
 )
 where
 
 
-import Spear.Math.Vector2
+import Spear.Math.Matrix4 as M4
+import Spear.Math.Vector2 as V2
+import qualified Spear.Math.Vector3 as V3
 
 
 data Side = L | R deriving (Eq, Show)
@@ -19,3 +22,18 @@ data Face = F | B deriving (Eq, Show)
 -- | Return the signed area of the triangle defined by the given points.
 orientation2d :: Vector2 -> Vector2 -> Vector2 -> Float
 orientation2d p q r = (x q - x p) * (y r - y p) - (y q - y p) * (x r - x p)
+
+
+-- | Project the given point in view space onto the XZ plane in world space.
+viewToWorld2d :: Vector2
+              -> Matrix4
+              -> Vector2
+viewToWorld2d p viewI =
+    let
+        p1' = V3.vec3 (V2.x p) (V2.y p) 0
+        p1  = viewI `mulp` p1'
+        p2  = p1 - M4.forward viewI
+        lambda = (V3.y p1 / (V3.y p1 - V3.y p2))
+        p'  = p1 + V3.scale lambda (p2 - p1)
+    in
+        vec2 (V3.x p') (-V3.z p')