sphere and disk area lights mostly ready (I think :S).

disk.cpp

@@ -1,6 +1,12 @@
-#include "disk.hpp"
+#include <glm/gtc/constants.hpp>
 
+#include "disk.hpp"
+#include "sampling.hpp"
+
+using glm::cos;
+using glm::sin;
 using glm::dot;
+using glm::pi;
 
 bool Disk::intersect(Ray & r, float & t) const {
   float _t;
@@ -15,3 +21,18 @@ bool Disk::intersect(Ray & r, float & t) const {
 
    return false;
 }
+
+vec3 Disk::sample_at_surface() const {
+  float theta = random01() * (2.0f * pi<float>());
+  float r = glm::sqrt(random01() * m_radius);
+  float x = r * cos(theta);
+  float y = r * sin(theta);
+  float z = 0.0f;
+  vec3 sample = vec3(x, y, z);
+  rotate_sample(sample, m_normal);
+  return sample;
+}
+
+void Disk::calculate_inv_area() {
+  m_inv_area = 1.0f / pi<float>() * (m_radius * m_radius);
+}