Made pyramid_down<1> equivalent to pyramid_disable.

dlib/image_transforms/image_pyramid.h

@@ -782,7 +782,7 @@ namespace dlib
     {
     public:
 
-        COMPILE_TIME_ASSERT(N > 1);
+        COMPILE_TIME_ASSERT(N > 0);
 
         template <typename T>
         vector<double,2> point_down (
@@ -888,6 +888,9 @@ namespace dlib
         }
     };
 
+    template <>
+    class pyramid_down<1> : public pyramid_disable {};
+
     template <>
     class pyramid_down<2> : public dlib::impl::pyramid_down_2_1 {};
 

dlib/image_transforms/image_pyramid_abstract.h

@@ -17,12 +17,14 @@ namespace dlib
     {
         /*!
             REQUIREMENTS ON N
-                N > 1
+                N > 0
 
             WHAT THIS OBJECT REPRESENTS
                 This is a simple functor to help create image pyramids.  In particular, it
                 downsamples images at a ratio of N to N-1.
 
+                Note that setting N to 1 means that this object functions like
+                pyramid_disable (defined at the bottom of this file).  
 
                 WARNING, when mapping rectangles from one layer of a pyramid
                 to another you might end up with rectangles which extend slightly