diff --git a/Makefile.am b/Makefile.am
index a39b977f..9e9b9196 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -214,6 +214,7 @@ UTIL_FILES   =			\
     ray.hpp			\
     region.cpp			\
     region.hpp			\
+    region.ipp			\
     si.cpp			\
     signal.cpp			\
     signal.hpp			\
diff --git a/region.hpp b/region.hpp
index 48512805..483357a4 100644
--- a/region.hpp
+++ b/region.hpp
@@ -67,6 +67,10 @@ namespace util {
         region (point_t, point_t);
         explicit region (std::array<T,S*2>);
 
+        //---------------------------------------------------------------------
+        template <typename U>
+        constexpr region<S,U> cast (void) const;
+
         //---------------------------------------------------------------------
         size_type area     (void) const;
         size_type diameter (void) const;
@@ -133,4 +137,6 @@ namespace util {
     std::ostream& operator<< (std::ostream&, const util::region<S,T>&);
 }
 
+#include "region.ipp"
+
 #endif
diff --git a/region.ipp b/region.ipp
new file mode 100644
index 00000000..38000f3e
--- /dev/null
+++ b/region.ipp
@@ -0,0 +1,33 @@
+/*
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ * Copyright 2010-2015 Danny Robson <danny@nerdcruft.net>
+ */
+
+#ifdef __UTIL_REGION_IPP
+#error
+#endif
+#define __UTIL_REGION_IPP
+
+namespace util {
+    template <size_t S, typename T>
+    template <typename U>
+    constexpr region<S,U>
+    region<S,T>::cast (void) const
+    {
+        return {
+            p.template cast<typename region<S,U>::position_type> (),
+            e.template cast<typename region<S,U>::size_type> ()
+        };
+    }
+}