Add function to calculate sphere normal vector on given point on the sphere.

source/objects/object.cpp

@@ -24,6 +24,11 @@ Intersect Object::intersect(Ray r)
     return Intersect();
 };
 
+Tuple Object::normalAt(Tuple point)
+{
+    return Vector(0, 0, 0);
+}
+
 void Object::setTransform(Matrix transform)
 {
     this->transformMatrix = transform;

source/objects/sphere.cpp

@@ -35,4 +35,16 @@ Intersect Sphere::intersect(Ray r)
     }
 
     return ret;
+}
+
+Tuple Sphere::normalAt(Tuple point)
+{
+    Tuple object_point = this->inverseTransform * point;
+    Tuple object_normal = (object_point - Point(0, 0, 0)).normalise();
+    Tuple world_normal = this->inverseTransform.transpose() * object_normal;
+
+    /* W may get wrong, so hack it. This is perfectly normal as we are using a 4x4 matrix instead of a 3x3 */
+    world_normal.w = 0;
+
+    return world_normal.normalise();
 }