diff --git a/extent.cpp b/extent.cpp
index d2061211..1d1214cb 100644
--- a/extent.cpp
+++ b/extent.cpp
@@ -29,6 +29,16 @@ template <typename T>
 util::extent<T>::extent (const T  _width, const T  _height):
         w (_width),
         h (_height)
+{
+    CHECK_GE (w, 0);
+    CHECK_GE (h, 0);
+}
+
+
+//-----------------------------------------------------------------------------
+template <typename T>
+util::extent<T>::extent (vector<2,T> _v):
+    extent (_v.x, _v.y)
 { ; }
 
 
diff --git a/extent.hpp b/extent.hpp
index 33aa7798..3dbbc4a2 100644
--- a/extent.hpp
+++ b/extent.hpp
@@ -24,6 +24,7 @@
 
 #include <iostream>
 
+
 namespace util {
     /**
      * A pure two-dimensional size, without positioning
@@ -33,6 +34,7 @@ namespace util {
         T w, h;
 
         extent (const T  _width, const T  _height);
+        extent (vector<2,T>);
         extent (const extent&);
         extent& operator= (const extent&);
         extent () = default;