Made it so shift clicking on a selected rectangle in the image_display will add

a part with an automatically selected integer label if and only if you haven't
specified any string part labels and all the existing parts are already integer
labeled.  This makes it way easier to quickly add parts.

dlib/gui_widgets/widgets.cpp

@@ -6054,11 +6054,11 @@ namespace dlib
         point c1 = loc - origin;
         if (zoom_in_scale != 1)
         {
-            c1 = c1/(double)zoom_in_scale;
+            c1 = c1/zoom_in_scale;
         }
         else if (zoom_out_scale != 1)
         {
-            c1 = c1*(double)zoom_out_scale;
+            c1 = c1*zoom_out_scale;
         }
 
         overlay_rects[selected_rect].parts[part_name] = c1;
@@ -6486,6 +6486,56 @@ namespace dlib
 
 // ----------------------------------------------------------------------------------------
 
+    namespace 
+    {
+        
+        bool only_contains_equal_sized_int_strings(
+            const std::map<std::string,point>& parts
+        )
+        {
+            if (parts.size() == 0)
+                return true;
+
+            const size_t string_size = parts.begin()->first.size();
+            for (const auto& p : parts)
+            {
+                if (p.first.size() != string_size)
+                    return false;
+                for (auto ch : p.first)
+                    if (!std::isdigit(ch))
+                        return false;
+            }
+            return true;
+        }
+
+        void zero_pad_part_names (
+            std::map<std::string,point>& parts
+        )
+        {
+            if (parts.size() < 5)
+                return;
+
+            const size_t num_digits_required = 1+std::floor(std::log10(parts.size()-1));
+            // if the first thing in the map has the right number of digits then assume
+            // everything is fine.
+            if (parts.begin()->first.size() == num_digits_required)
+                return;
+
+            std::map<std::string,point> new_parts;
+
+            for (auto& p : parts)
+            {
+                auto key = p.first;
+                while (key.size() < num_digits_required)
+                    key = '0' + key;
+                new_parts[key] = p.second;
+            }
+
+            parts.swap(new_parts);
+        }
+
+    }
+
     void image_display::
     on_mouse_down (
         unsigned long btn,
@@ -6507,6 +6557,60 @@ namespace dlib
             parent.invalidate_rectangle(rect);
         }
 
+        // if the user shift left clicks when a rect is selected add a part annotation, but
+        // only if they haven't explicitly indicated part names.  If they have part names
+        // then make them pick them from the right click menu.  But if they haven't set
+        // part names then we will automatically create integer numbered parts starting from 0.
+        // We will also only allow auto part numbering if all the parts are already
+        // numbers.
+        if (btn == base_window::LEFT && (state&base_window::SHIFT) && rect_is_selected &&
+            part_names.size() == 0 && only_contains_equal_sized_int_strings(overlay_rects[selected_rect].parts))
+        {
+            // Find the next part name.  Just find the next unused integer starting from 0.
+            int part_num = 0;
+            size_t num_digits_required = 0;
+            for (const auto& p : overlay_rects[selected_rect].parts)
+            {
+                num_digits_required = p.first.size();
+                const int num = std::atoi(p.first.c_str());
+                if (num != part_num)
+                    break;
+                ++part_num;
+            }
+            std::string part_name = std::to_string(part_num);
+            // pad part name so it's the same length as the other parts.
+            while (part_name.size() < num_digits_required)
+                part_name = '0' + part_name;
+
+
+            const point loc(x,y);
+
+            // Transform loc from gui window space into the space used by the overlay
+            // rectangles (i.e. relative to the raw image)
+            const point origin(total_rect().tl_corner());
+            point c1 = loc - origin;
+            if (zoom_in_scale != 1)
+            {
+                c1 = c1/zoom_in_scale;
+            }
+            else if (zoom_out_scale != 1)
+            {
+                c1 = c1*zoom_out_scale;
+            }
+
+            overlay_rects[selected_rect].parts[part_name] = c1;
+
+            zero_pad_part_names(overlay_rects[selected_rect].parts);
+            parent.invalidate_rectangle(rect); 
+
+
+            if (event_handler.is_set())
+                event_handler();
+
+            return;
+        }
+
+
         if (rect.contains(x,y) == false || hidden || !enabled)
             return;
 
@@ -6530,7 +6634,6 @@ namespace dlib
         if (btn == base_window::RIGHT && (state&base_window::SHIFT))
         {
             const bool rect_was_selected = rect_is_selected;
-            rect_is_selected = false;
             parts_menu.disable();
 
             long best_dist = std::numeric_limits<long>::max();

dlib/gui_widgets/widgets_abstract.h

@@ -2349,7 +2349,12 @@ namespace dlib
                 can also add part labels (if they have been defined by calling add_labelable_part_name())
                 by selecting an overlay rectangle with the mouse and then right clicking
                 on the part.  If you want to move any rectangle or an object part then
-                shift+right click and drag it.
+                shift+right click and drag it.  Alternatively, if you haven't added any
+                part labels via add_labelable_part_name() you can add parts to a rectangle
+                by simply shift left clicking while it's selected.  This will add parts
+                with integer names and the integer names begin with 0 and increase. This
+                feature is only activated if the rectangle has no parts or all the parts
+                are already integer names.
                 
                 Finally, if you hold Ctrl and left click an overlay rectangle it will
                 change its label to get_default_overlay_rect_label() and color to