1 files changed, 44 insertions, 0 deletions

diff --git a/Spear/Math/MatrixUtils.hs b/Spear/Math/MatrixUtils.hs
index 88ad3b1..e6e498f 100644
--- a/Spear/Math/MatrixUtils.hs
+++ b/Spear/Math/MatrixUtils.hs
@@ -7,8 +7,11 @@ where
 
 import Spear.Math.Matrix3 as M3
 import Spear.Math.Matrix4 as M4
+import Spear.Math.Vector2 as V2
+import Spear.Math.Vector3 as V3
 
 
+-- | Compute the normal matrix of the given matrix.
 fastNormalMatrix :: Matrix4 -> Matrix3
 fastNormalMatrix m =
     let m' = M4.transpose . M4.inverseTransform $ m
@@ -16,3 +19,44 @@ fastNormalMatrix m =
         (M4.m00 m') (M4.m10 m') (M4.m20 m')
         (M4.m01 m') (M4.m11 m') (M4.m21 m')
         (M4.m02 m') (M4.m12 m') (M4.m22 m')
+
+
+-- | Compute the inverse transform of the given transformation matrix.
+--
+-- This function maps an object's transform in 2D to the object's inverse in 3D.
+-- 
+-- The XY plane in 2D translates to the X(-Z) plane in 3D.
+--
+-- Use this in games such as RPGs and RTSs.
+rpgInverse :: Float -- ^ Height above the ground.
+           -> Matrix3 -> Matrix4
+rpgInverse h mat =
+    let r = let r' = M3.right mat in vec3 (V2.x r') (V2.y r') 0
+        u = V3.unity
+        f = let f' = M3.forward mat in vec3 (V2.x f') 0 (-V2.y f')
+        t = let t' = M3.position mat in -(vec3 (V2.x t') 0 (-V2.y t'))
+    in mat4
+        (V3.x r) (V3.y r) (V3.z r) (t `V3.dot` r)
+        (V3.x u) (V3.y u) (V3.z u) (t `V3.dot` u)
+        (V3.x f) (V3.y f) (V3.z f) (t `V3.dot` f)
+        0     0     0     1
+
+
+-- | Compute the inverse transform of the given transformation matrix.
+--
+-- This function maps an object's transform in 2D to the object's inverse in 3D.
+-- 
+-- The XY plane in 2D translates to the XY plane in 3D.
+-- 
+-- Use this in games like platformers and space invaders style games.
+pltInverse :: Matrix3 -> Matrix4
+pltInverse mat =
+    let r = let r' = M3.right mat in vec3 (V2.x r') (V2.y r') 0
+        u = let u' = M3.up mat in vec3 (V2.x u') (V2.y u') 0
+        f = V3.unitz
+        t = let t' = M3.position mat in vec3 (V2.x t') (V2.y t') 0
+    in mat4
+        (V3.x r) (V3.y r) (V3.z r) (t `V3.dot` r)
+        (V3.x u) (V3.y u) (V3.z u) (t `V3.dot` u)
+        (V3.x f) (V3.y f) (V3.z f) (t `V3.dot` f)
+        0     0     0     1