Added missing requires clauses and asserts.

dlib/image_transforms/random_cropper.h

@@ -135,6 +135,7 @@ namespace dlib
             std::vector<mmod_rect>& crop_rects
         )
         {
+            DLIB_CASSERT(num_rows(img)*num_columns(img) != 0);
             chip_details crop_plan;
             bool should_flip_crop;
             make_crop_plan(img, rects, crop_plan, should_flip_crop);
@@ -233,9 +234,10 @@ namespace dlib
 
         template <typename image_type>
         rectangle make_random_cropping_rect_resnet(
-            const image_type& img
+            const image_type& img_
         )
         {
+            const_image_view<image_type> img(img_);
             // figure out what rectangle we want to crop from the image
             double mins = 0.1, maxs = 0.95;
             auto scale = rnd.get_double_in_range(mins, maxs);

dlib/image_transforms/random_cropper_abstract.h

@@ -174,6 +174,11 @@ namespace dlib
         /*!
             requires
                 - images.size() == rects.size()
+                - for all valid i:
+                    - images[i].size() != 0
+                - array_type is a type with an interface compatible with dlib::array or
+                  std::vector and it must in turn contain image objects that implement the
+                  interface defined in dlib/image_processing/generic_image.h 
             ensures
                 - Randomly extracts num_crops chips from images.  We also copy the object
                   metadata for each extracted crop and store it into #crop_rects.  In
@@ -195,6 +200,13 @@ namespace dlib
         /*!
             requires
                 - images.size() == rects.size()
+                - for all valid i:
+                    - images[i].size() != 0
+                - image_type == an image object that implements the interface defined in
+                  dlib/image_processing/generic_image.h 
+                - array_type is a type with an interface compatible with dlib::array or
+                  std::vector and it must in turn contain image objects that implement the
+                  interface defined in dlib/image_processing/generic_image.h 
             ensures
                 - Selects a random image and creates a random crop from it.  Specifically,
                   we pick a random index IDX < images.size() and then execute 
@@ -212,6 +224,12 @@ namespace dlib
             std::vector<mmod_rect>& crop_rects
         );
         /*!
+            requires
+                - img.size() != 0
+                - image_type1 == an image object that implements the interface defined in
+                  dlib/image_processing/generic_image.h 
+                - image_type2 == an image object that implements the interface defined in
+                  dlib/image_processing/generic_image.h 
             ensures
                 - Extracts a random crop from img and copies over the mmod_rect objects in
                   rects to #crop_rects if they are contained inside the crop.  Moreover,