improve diffuse approximation

src/main.cpp

@@ -21,9 +21,9 @@ colour ray_colour(const ray& r, const hittable& world, int depth)
         return colour(0,0,0);
     }
 
-    if (world.hit(r, 0, infinity, rec))
+    if (world.hit(r, 0.001, infinity, rec))
     {
-        point3 target = rec.p + rec.normal + random_in_unit_sphere();
+        point3 target = rec.p + rec.normal + random_in_hemisphere(rec.normal);
         return 0.5 * ray_colour(ray(rec.p, target - rec.p), world, depth-1);
     }
 

src/vec3.h

@@ -138,3 +138,24 @@ vec3 random_in_unit_sphere()
         return p;
     }
 }
+
+vec3 random_unit_vector()
+{
+    auto a = random_double(0, 2*pi);
+    auto z = random_double(-1,1);
+    auto r = sqrt(1 - z*z);
+    return vec3(r*cos(a), r*sin(a), z);
+}
+
+vec3 random_in_hemisphere(const vec3& normal)
+{
+    vec3 in_unit_sphere = random_in_unit_sphere();
+    if (dot(in_unit_sphere, normal) > 0.0)
+    {
+        return in_unit_sphere;
+    }
+    else
+    {
+        return -in_unit_sphere;
+    }
+}