added Vector::refract()

git-svn-id: svn://anubis/fart/trunk@372 7f9b0f55-74a9-4bce-be96-3c2cd072584d

util/Vector.h

@@ -63,6 +63,23 @@ class Vector
             return (*this) - target * (2 * dot(target));
         }
 
+        /*
+         * from http://www.flipcode.com/archives/
+         *   Reflections_and_Refraction_in_Raytracing.shtml
+         * target: normal vector of surface
+         * n1: refraction index of object we're coming from
+         * n2: refraction index of object we're going into
+         */
+        Vector refract(const Vector & target, double n1, double n2) const
+        {
+            const double n = n1 / n2;
+            const double cosI = dot(target);
+            const double sinT2 = n * n * (1.0 - cosI * cosI);
+            if (sinT2 > 1.0)
+                return Vector(0.0, 0.0, 0.0);
+            return (*this) * n - target * (n + sqrt(1.0 - sinT2));
+        }
+
         Vector getPerpendicular() const
         {
             Vector t = *this;