Added an overload of zero_border_pixels() that lets you give a rectangle as

input to define the non-border area.

dlib/image_transforms/assign_image.h

@@ -274,6 +274,39 @@ namespace dlib
         assign_border_pixels(img, x_border_size, y_border_size, zero_pixel);
     }
 
+// ----------------------------------------------------------------------------------------
+
+    template <
+        typename image_type
+        >
+    void zero_border_pixels (
+        image_type& img,
+        rectangle inside
+    )
+    {
+        inside = inside.intersect(get_rect(img));
+        if (inside.is_empty())
+            return;
+
+        for (long r = 0; r < inside.top(); ++r)
+        {
+            for (long c = 0; c < img.nc(); ++c)
+                assign_pixel(img[r][c], 0);
+        }
+        for (long r = inside.top(); r <= inside.bottom(); ++r)
+        {
+            for (long c = 0; c < inside.left(); ++c)
+                assign_pixel(img[r][c], 0);
+            for (long c = inside.right()+1; c < img.nc(); ++c)
+                assign_pixel(img[r][c], 0);
+        }
+        for (long r = inside.bottom()+1; r < img.nr(); ++r)
+        {
+            for (long c = 0; c < img.nc(); ++c)
+                assign_pixel(img[r][c], 0);
+        }
+    }
+
 // ----------------------------------------------------------------------------------------
 
 }

dlib/image_transforms/assign_image_abstract.h

@@ -165,6 +165,28 @@ namespace dlib
                       (i.e. assigns 0 to every pixel in the border of img)
     !*/
 
+// ----------------------------------------------------------------------------------------
+
+    template <
+        typename image_type
+        >
+    void zero_border_pixels (
+        image_type& img,
+        rectangle inside
+    );
+    /*!
+        requires
+            - image_type == is an implementation of array2d/array2d_kernel_abstract.h
+            - pixel_traits<typename image_type::type> is defined  
+        ensures
+            - #img.nc() == img.nc()
+            - #img.nr() == img.nr()
+              (i.e. the size of img isn't changed by this function)
+            - All the pixels in img that are not contained inside the inside rectangle
+              given to this function are set to 0.  That is, anything not "inside" is on
+              the border and set to 0.
+    !*/
+
 // ----------------------------------------------------------------------------------------
 
 }