diff --git a/image.cpp b/image.cpp
index 981de0fa..4ae8ff99 100644
--- a/image.cpp
+++ b/image.cpp
@@ -255,13 +255,22 @@ util::image::buffer<T>::downsample (float factor) const
 
 ///////////////////////////////////////////////////////////////////////////////
 template <typename T>
+size_t
+buffer<T>::offset (point<2,size_t> p) const
+{
+    return dot (stride (), p);
+}
+
+
+//-----------------------------------------------------------------------------
+template <typename T>
 T
 buffer<T>::operator[] (point<2,size_t> p) const
 {
     CHECK_LT (p.x, w);
     CHECK_LT (p.y, h);
 
-    return begin ()[p.y * s + p.x];
+    return begin ()[offset (p)];
 }
 
 
@@ -273,7 +282,7 @@ buffer<T>::operator[] (point<2,size_t> p)
     CHECK_LT (p.x, w);
     CHECK_LT (p.y, h);
 
-    return begin ()[p.y * s + p.x];
+    return begin ()[offset (p)];
 }
 
 
diff --git a/image.hpp b/image.hpp
index b63cafea..676b4c7f 100644
--- a/image.hpp
+++ b/image.hpp
@@ -61,6 +61,8 @@ namespace util { namespace image {
         size_t s;
 
         //---------------------------------------------------------------------
+        size_t offset (point<2,size_t>) const;
+
         T  operator[] (point<2,size_t>) const;
         T& operator[] (point<2,size_t>);