+// calculate the projection of u along the direction of d.
+template<typename T>
+inline
+SGVec4<T>
+projection(const SGVec4<T>& u, const SGVec4<T>& d)
+{
+ T denom = dot(d, d);
+ T ud = dot(u, d);
+ if (SGLimits<T>::min() < denom) return u;
+ else return d * (dot(u, d) / denom);
+}
+