Better disk surface sampling.

disk.hpp

@@ -3,25 +3,30 @@
 #define DISK_HPP
 
 #include <glm/glm.hpp>
+#include <glm/gtc/constants.hpp>
 
 #include "plane.hpp"
 
 using glm::vec3;
 using glm::normalize;
+using glm::pi;
 
 class Disk : public Plane {
 public:
   float m_radius;
   
   Disk(Material * mat = NULL): Plane(mat), m_radius(1.0f) {
+    pi2 = 2.0f * pi<float>();
     calculate_inv_area();
   }
 
   Disk(float x, float y, float z, float nx, float ny, float nz, float _r, Material * mat = NULL): Plane(x, y, z, nx, ny, nz, mat), m_radius(_r) {
+    pi2 = 2.0f * pi<float>();
     calculate_inv_area();
   }
 
   Disk(vec3 _p, vec3 _n, float _r, Material * mat = NULL): Plane(_p, _n, mat), m_radius(_r) {
+    pi2 = 2.0f * pi<float>();
     calculate_inv_area();
   }
 
@@ -32,6 +37,9 @@ public:
 
 protected:
   virtual void calculate_inv_area();
+
+private:
+  float pi2;
 };