ray: add convenience point-point constructor

ray.cpp

@@ -32,6 +32,19 @@ ray<S,T>::ray (util::point<S,T> _origin,
 }
 
 
+//-----------------------------------------------------------------------------
+template <size_t S, typename T>
+ray<S,T>
+ray<S,T>::make (util::point<S,T> origin,
+                util::point<S,T> target)
+{
+    return {
+        origin,
+        (target - origin).normalised ()
+    };
+}
+
+
 ///////////////////////////////////////////////////////////////////////////////
 /// returns the distance along the ray in a ray-plane intersection
 ///

ray.hpp

@@ -32,6 +32,10 @@ namespace util {
         ray (point<S,T> origin,
              vector<S,T> direction);
 
+        static
+        ray<S,T> make (point<S,T> origin,
+                       point<S,T> target);
+
         // intersection tests
         T intersect (plane<S,T>) const;
         T intersect (AABB<S,T>) const;