Made the disabled version of pyramid_down support the new image interface.

Also added an overload of operator() for pyramid_down that takes just
a single image and downsamples it.

dlib/image_transforms/image_pyramid.h

@@ -124,7 +124,19 @@ namespace dlib
             COMPILE_TIME_ASSERT( pixel_traits<in_pixel_type>::has_alpha == false );
             COMPILE_TIME_ASSERT( pixel_traits<out_pixel_type>::has_alpha == false );
 
-            down.clear();
+            set_image_size(down, 0, 0);
+        }
+
+        template <
+            typename image_type
+            >
+        void operator() (
+            image_type& img
+        ) const
+        {
+            typedef typename image_traits<image_type>::pixel_type pixel_type;
+            COMPILE_TIME_ASSERT( pixel_traits<pixel_type>::has_alpha == false );
+            set_image_size(img, 0, 0);
         }
     };
 
@@ -454,6 +466,18 @@ namespace dlib
 
             }
 
+            template <
+                typename image_type
+                >
+            void operator() (
+                image_type& img
+            ) const
+            {
+                image_type temp;
+                (*this)(img, temp);
+                swap(temp, img);
+            }
+
         private:
 
 
@@ -788,6 +812,17 @@ namespace dlib
                 }
             }
 
+            template <
+                typename image_type
+                >
+            void operator() (
+                image_type& img
+            ) const
+            {
+                image_type temp;
+                (*this)(img, temp);
+                swap(temp, img);
+            }
         private:
 
 
@@ -911,6 +946,18 @@ namespace dlib
             set_image_size(down, ((N-1)*num_rows(original))/N, ((N-1)*num_columns(original))/N);
             resize_image(original, down);
         }
+
+        template <
+            typename image_type
+            >
+        void operator() (
+            image_type& img
+        ) const
+        {
+            image_type temp;
+            (*this)(img, temp);
+            swap(temp, img);
+        }
     };
 
     template <>

dlib/image_transforms/image_pyramid_abstract.h

@@ -64,6 +64,24 @@ namespace dlib
                   points outside the #down image.  
         !*/
 
+        template <
+            typename image_type
+            >
+        void operator() (
+            image_type& img
+        ) const;
+        /*!
+            requires
+                - image_type == an image object that implements the interface defined in
+                  dlib/image_processing/generic_image.h 
+                - pixel_traits<typename image_traits<image_type>::pixel_type>::has_alpha == false
+            ensures
+                - This function downsamples the given image and stores the results in #img.
+                  In particular, it is equivalent to performing: 
+                    (*this)(img, temp); 
+                    swap(img, temp);
+        !*/
+
     // -------------------------------
 
         template <typename T>