From 93fe5f44c4df5d18911cca936e9a8a6313540c20 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Fredrik=20Nor=C3=A9n?= <fredrik.jw.noren@gmail.com>
Date: Sun, 11 Sep 2016 13:07:04 -0400
Subject: [PATCH] Add flip_y_up_to_z_up method to Matrix4

This is for converting between models which often come in a y-up coordinate system (maya) and game which often use a z-up system.

Math from http://stackoverflow.com/questions/22571364/threejs-matrix-from-z-up-coordinate-system-to-y-up-coordinate-system
---
 src/matrix.rs | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/src/matrix.rs b/src/matrix.rs
index 9d3acdf8..bf0a959c 100644
--- a/src/matrix.rs
+++ b/src/matrix.rs
@@ -299,6 +299,18 @@ impl<S: BaseFloat> Matrix4<S> {
                      S::zero(),
                      S::one())
     }
+    
+    /// Create a matrix which flips the y and z coordnates.
+    ///
+    /// This is used to transform from a y-up coordinate system to
+    /// a z-up coordinate systems.
+    #[inline]
+    pub fn flip_y_up_to_z_up() -> Matrix4<S> {
+        Matrix4::new(S::one(), S::zero(), S::zero(), S::zero(),
+                     S::zero(), S::zero(), S::one(), S::zero(),
+                     S::zero(), -S::one(), S::zero(), S::zero(),
+                     S::zero(), S::zero(), S::zero(), S::one())
+    }
 }
 
 impl<S: BaseFloat> Zero for Matrix2<S> {