Matrix4::getIdentity() implemented by Eigen

Also moved the method into the .h file and changed it to return by value rather
than const reference. Although I haven't profiled, I doubt that there is much
(if any) performance benefit of returning by reference rather than simply
inlining the method body + RVO.

libs/math/Matrix4.cpp

@@ -48,18 +48,6 @@ Matrix4::Matrix4(double xx_, double xy_, double xz_, double xw_,
 
 // Named constructors
 
-// Identity matrix
-const Matrix4& Matrix4::getIdentity()
-{
-    static const Matrix4 _identity(
-        1, 0, 0, 0,
-        0, 1, 0, 0,
-        0, 0, 1, 0,
-        0, 0, 0, 1
-    );
-    return _identity;
-}
-
 // Get a translation matrix for the given vector
 Matrix4 Matrix4::getTranslation(const Vector3& translation)
 {

libs/math/Matrix4.h

@@ -72,7 +72,10 @@ class Matrix4
     const Eigen::Projective3d& eigen() const { return _transform; }
 
     /// Obtain the identity matrix.
-    static const Matrix4& getIdentity();
+    static Matrix4 getIdentity()
+    {
+        return Matrix4(Eigen::Projective3d::Identity());
+    }
 
     /// Get a matrix representing the given 3D translation.
     static Matrix4 getTranslation(const Vector3& translation);

libs/transformlib.h

@@ -13,7 +13,8 @@ class IdentityTransform :
 	/// \brief Returns the identity matrix.
 	const Matrix4& localToParent() const
 	{
-		return Matrix4::getIdentity();
+		static const Matrix4 ID = Matrix4::getIdentity();
+        return ID;
 	}
 };