Made random_cropper's API a little more convenient for cropping out only

background crops.

dlib/image_transforms/random_cropper.h

@@ -50,7 +50,7 @@ namespace dlib
             double value
         )
         {
-            DLIB_CASSERT(0 <= value && value < 1);
+            DLIB_CASSERT(0 <= value && value <= 1);
             background_crops_fraction = value;
         }
 
@@ -162,6 +162,19 @@ namespace dlib
             (*this)(images[idx], rects[idx], crop, crop_rects);
         }
 
+        template <
+            typename image_type1
+            >
+        image_type1 operator() (
+            const image_type1& img
+        )
+        {
+            image_type1 crop;
+            std::vector<mmod_rect> junk1, junk2;
+            (*this)(img, junk1, crop, junk2);
+            return crop;
+        }
+
         template <
             typename image_type1,
             typename image_type2

dlib/image_transforms/random_cropper_abstract.h

@@ -83,7 +83,7 @@ namespace dlib
         );
         /*!
             requires
-                - 0 <= value < 1
+                - 0 <= value <= 1
             ensures
                 - #get_background_crops_fraction() == value
         !*/
@@ -307,6 +307,23 @@ namespace dlib
                   inside the crop.
                 - #crop_rects.size() <= rects.size()
         !*/
+
+        template <
+            typename image_type1
+            >
+        image_type1 operator() (
+            const image_type1& img
+        );
+        /*!
+            requires
+                - img.size() != 0
+                - image_type1 == an image object that implements the interface defined in
+                  dlib/image_processing/generic_image.h 
+            ensures
+                - This function simply calls (*this)(img, junk1, crop, junk2) and returns
+                  crop.  Therefore it is simply a convenience function for extracting a
+                  random background patch.
+        !*/
     };
 
 // ----------------------------------------------------------------------------------------