Added sub_image() and sub_image_proxy.

dlib/image_transforms/interpolation.h

@@ -14,6 +14,80 @@
 namespace dlib
 {
 
+// ----------------------------------------------------------------------------------------
+
+    template <typename T>
+    struct sub_image_proxy
+    {
+        sub_image_proxy (
+            T& img_,
+            const rectangle& rect_
+        ) : img(img_), rect(rect_.intersect(get_rect(img_)))
+        {}
+
+        T& img;
+        const rectangle rect;
+    };
+
+    template <typename T>
+    struct image_traits<sub_image_proxy<T> >
+    {
+        typedef typename image_traits<T>::pixel_type pixel_type;
+    };
+    template <typename T>
+    struct image_traits<const sub_image_proxy<T> >
+    {
+        typedef typename image_traits<T>::pixel_type pixel_type;
+    };
+
+    template <typename T>
+    inline long num_rows( const sub_image_proxy<T>& img) { return img.rect.height(); }
+    template <typename T>
+    inline long num_columns( const sub_image_proxy<T>& img) { return img.rect.width(); }
+
+    template <typename T>
+    inline void* image_data( sub_image_proxy<T>& img) 
+    { 
+        typedef typename image_traits<T>::pixel_type pixel_type;
+        return static_cast<pixel_type*>(image_data(img.img)) + img.rect.left() + img.rect.top()*num_columns(img.img); 
+    } 
+    template <typename T>
+    inline const void* image_data( const sub_image_proxy<T>& img) 
+    {
+        typedef typename image_traits<T>::pixel_type pixel_type;
+        return static_cast<const pixel_type*>(image_data(img.img)) + img.rect.left() + img.rect.top()*num_columns(img.img); 
+    }
+
+    template <typename T>
+    inline long width_step(
+        const sub_image_proxy<T>& img
+    ) 
+    { 
+        return width_step(img.img);
+    }
+
+    template <
+        typename image_type
+        >
+    sub_image_proxy<image_type> sub_image (
+        image_type& img,
+        const rectangle& rect
+    )
+    {
+        return sub_image_proxy<image_type>(img,rect);
+    }
+
+    template <
+        typename image_type
+        >
+    const sub_image_proxy<const image_type> sub_image (
+        const image_type& img,
+        const rectangle& rect
+    )
+    {
+        return sub_image_proxy<const image_type>(img,rect);
+    }
+
 // ----------------------------------------------------------------------------------------
 
     class interpolate_nearest_neighbor

dlib/image_transforms/interpolation_abstract.h

@@ -1125,6 +1125,71 @@ namespace dlib
               and stores the single output chip into #chip.
     !*/
 
+// ----------------------------------------------------------------------------------------
+
+    template <
+        typename image_type
+        >
+    struct sub_image_proxy
+    {
+        /*!
+            REQUIREMENTS ON image_type
+                - image_type == an image object that implements the interface defined in
+                  dlib/image_processing/generic_image.h 
+
+            WHAT THIS OBJECT REPRESENTS
+                This is a lightweight image object for referencing a subwindow of an image.
+                It implements the generic image interface and can therefore be used with
+                any function that expects a generic image, excepting that you cannot change
+                the size of a sub_image_proxy.  
+                
+                Note that it only stores a pointer to the image given to its constructor
+                and therefore does not perform a copy.  Moreover, this means that an
+                instance of this object becomes invalid after the image it references is
+                destroyed.
+        !*/
+        sub_image_proxy (
+            T& img,
+            const rectangle& rect
+        );
+        /*!
+            ensures
+                - This object is an image that represents the part of img contained within
+                  rect.  If rect is larger than img then rect is cropped so that it does
+                  not go outside img.
+        !*/
+    };
+
+    template <
+        typename image_type
+        >
+    sub_image_proxy<image_type> sub_image (
+        image_type& img,
+        const rectangle& rect
+    );
+    /*!
+        requires
+            - image_type == an image object that implements the interface defined in
+              dlib/image_processing/generic_image.h 
+        ensures
+            - returns sub_image_proxy<image_type>(img,rect)
+    !*/
+
+    template <
+        typename image_type
+        >
+    const sub_image_proxy<const image_type> sub_image (
+        const image_type& img,
+        const rectangle& rect
+    );
+    /*!
+        requires
+            - image_type == an image object that implements the interface defined in
+              dlib/image_processing/generic_image.h 
+        ensures
+            - returns sub_image_proxy<const image_type>(img,rect)
+    !*/
+
 // ----------------------------------------------------------------------------------------
 
     chip_details get_face_chip_details (