diff --git a/geom/ray.cpp b/geom/ray.cpp
index 5956ac69..433d1a7f 100644
--- a/geom/ray.cpp
+++ b/geom/ray.cpp
@@ -44,6 +44,17 @@ ray<S,T>::at (T t) const
 }
 
 
+///////////////////////////////////////////////////////////////////////////////
+util::geom::ray3f
+util::geom::operator* (util::matrix4f lhs, ray3f rhs)
+{
+    return {
+        (lhs * rhs.origin.homog<4>    ()).redim<3> (),
+        (lhs * rhs.direction.homog<4> ()).redim<3> ()
+    };
+}
+
+
 ///////////////////////////////////////////////////////////////////////////////
 template <size_t S, typename T>
 std::ostream&
diff --git a/geom/ray.hpp b/geom/ray.hpp
index 376a74d5..dc782d93 100644
--- a/geom/ray.hpp
+++ b/geom/ray.hpp
@@ -66,6 +66,11 @@ namespace util::geom {
     typedef ray<3,float> ray3f;
 
 
+    ///////////////////////////////////////////////////////////////////////////
+    ray3f
+    operator* (matrix4f, ray3f);
+
+
     ///////////////////////////////////////////////////////////////////////////
     /// returns the distance along the ray in a ray-plane intersection
     ///