Introduce point3 and color

Author: hollasch

books/RayTracingInOneWeekend.html

@@ -270,12 +270,23 @@
         public:
             double e[3];
     };
+
+    // Type aliases for vec3
+    using point3 = vec3;   // 3D point
+    using color = vec3;    // RGB color
     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
     [Listing [vec3-class]: <kbd>[vec3.h]</kbd> `vec3` class]
 </div>
 
 We use `double` here, but some ray tracers use `float`. Either one is fine -- follow your own
-tastes. The second part of the header file contains vector utility functions:
+tastes.
+
+You'll notice that we also create two type aliases for `vec3`: `point3` and `color` (you may be
+familiar with the older `typedef` keyword). In C++, these create weak type aliases — passing a
+`color` for a `vec3` parameter is legal, and will generate no warnings. We use them solely as a way
+to clarify intent and use.
+
+The second part of the header file contains vector utility functions:
 
     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
     // vec3 Utility Functions

src/common/vec3.h

@@ -86,6 +86,11 @@ class vec3 {
 };
 
 
+// Type aliases for vec3
+using point3 = vec3;   // 3D point
+using color = vec3;    // RGB color
+
+
 // vec3 Utility Functions
 
 inline std::ostream& operator<<(std::ostream &out, const vec3 &v) {