Better disk surface sampling.

disk.cpp

@@ -1,12 +1,16 @@
-#include <glm/gtc/constants.hpp>
+#include <iostream>
+#include <cassert>
 
 #include "disk.hpp"
 #include "sampling.hpp"
 
+using glm::vec2;
 using glm::cos;
 using glm::sin;
 using glm::dot;
-using glm::pi;
+using glm::distance;
+using glm::cross;
+using glm::abs;
 
 bool Disk::intersect(Ray & r, float & t) const {
   float _t;
@@ -23,14 +27,15 @@ bool Disk::intersect(Ray & r, float & t) const {
 }
 
 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;
+  float theta = random01() * pi2;
+  float r = random01() * m_radius;
+  vec3 nt, nb;
+  create_coords_system(m_normal, nt, nb);
+  float x = m_point.x + (r * cos(theta) * nt.x) + (r * sin(theta) * nb.x);
+  float y = m_point.y + (r * cos(theta) * nt.y) + (r * sin(theta) * nb.y);
+  float z = m_point.z + (r * cos(theta) * nt.z) + (r * sin(theta) * nb.z);
+
+  return vec3(x, y, z);
 }
 
 void Disk::calculate_inv_area() {