Changed the random_cropper's set_min_object_size() routine to take min box

dimensions in the same format as the mmod_options object (i.e. two lengths
measured in pixels).  This should make defining random_cropping strategies that
are consistent with MMOD settings much more straightforward since you can just
take the mmod_options settings and give them to the random_cropper and it will
do the right thing.

dlib/image_transforms/random_cropper.h

@@ -18,7 +18,8 @@ namespace dlib
         chip_dims dims = chip_dims(300,300);
         bool randomly_flip = true;
         double max_rotation_degrees = 30;
-        double min_object_size = 0.25; // cropped object will be at least this fraction of the size of the image.
+        long min_object_length_long_dim = 75; // cropped object will be at least this many pixels along its longest edge.
+        long min_object_length_short_dim = 30; // cropped object will be at least this many pixels along its shortest edge.
         double max_object_size = 0.7; // cropped object will be at most this fraction of the size of the image.
         double background_crops_fraction = 0.5;
         double translate_amount = 0.10;
@@ -78,14 +79,19 @@ namespace dlib
             double value
         ) { max_rotation_degrees = std::abs(value); }
 
-        double get_min_object_size (
-        ) const { return min_object_size; }
+        long get_min_object_length_long_dim (
+        ) const { return min_object_length_long_dim; }
+        long get_min_object_length_short_dim (
+        ) const { return min_object_length_short_dim; }
+
         void set_min_object_size (
-            double value
+            long long_dim,
+            long short_dim
         ) 
         { 
-            DLIB_CASSERT(0 < value);
-            min_object_size = value; 
+            DLIB_CASSERT(0 < short_dim && short_dim <= long_dim);
+            min_object_length_long_dim = long_dim; 
+            min_object_length_short_dim = short_dim; 
         }
 
         double get_max_object_size (
@@ -175,9 +181,6 @@ namespace dlib
             extract_image_chip(img, crop_plan, crop);
             const rectangle_transform tform = get_mapping_to_chip(crop_plan);
 
-            const unsigned long min_object_size_absolute_rows = std::round(min_object_size*crop_plan.rows);
-            const unsigned long min_object_size_absolute_cols = std::round(min_object_size*crop_plan.cols);
-
             // copy rects into crop_rects and set ones that are outside the crop to ignore or
             // drop entirely as appropriate.
             crop_rects.clear();
@@ -190,8 +193,12 @@ namespace dlib
                 if (get_rect(crop).intersect(rect.rect).area() != 0)
                 {
                     // set to ignore if not totally in the crop or if too small.
-                    if (!get_rect(crop).contains(rect.rect) || (rect.rect.height() < min_object_size_absolute_rows && rect.rect.width() < min_object_size_absolute_cols))
+                    if (!get_rect(crop).contains(rect.rect) || 
+                        (rect.rect.height() < min_object_length_long_dim  && rect.rect.width() < min_object_length_long_dim) || 
+                        (rect.rect.height() < min_object_length_short_dim || rect.rect.width() < min_object_length_short_dim))
+                    {
                         rect.ignore = true;
+                    }
 
                     crop_rects.push_back(rect);
                 }
@@ -223,15 +230,39 @@ namespace dlib
             if (has_non_ignored_box(rects) && rnd.get_random_double() >= background_crops_fraction)
             {
                 auto rect = rects[randomly_pick_rect(rects)].rect;
+
                 // perturb the location of the crop by a small fraction of the object's size.
-                const point rand_translate = dpoint(rnd.get_double_in_range(-translate_amount,translate_amount)*rect.width(), 
+                const point rand_translate = dpoint(rnd.get_double_in_range(-translate_amount,translate_amount)*std::max(rect.height(),rect.width()), 
                                                     rnd.get_double_in_range(-translate_amount,translate_amount)*std::max(rect.height(),rect.width()));
 
-                // perturb the scale of the crop by a fraction of the object's size
-                const double rand_scale_perturb = rnd.get_double_in_range(min_object_size, max_object_size); 
+                // We are going to grow rect into the cropping rect.  First, we grow it a
+                // little so that it has the desired minimum border around it.  
+                drectangle drect = centered_drect(center(rect)+rand_translate, rect.width()/max_object_size, rect.height()/max_object_size);
+
+                // Now make rect have the same aspect ratio as dims so that there won't be
+                // any funny stretching when we crop it.  We do this by growing it along
+                // whichever dimension is too short.
+                const double target_aspect = dims.cols/(double)dims.rows;
+                if (drect.width()/drect.height() < target_aspect)
+                    drect = centered_drect(drect, target_aspect*drect.height(), drect.height());
+                else 
+                    drect = centered_drect(drect, drect.width(), drect.width()/target_aspect);
+
+                // Now perturb the scale of the crop.  We do this by shrinking it, but not
+                // so much that it gets smaller than the min object sizes require. 
+                double current_width = dims.cols*rect.width()/drect.width();
+                double current_height = dims.rows*rect.height()/drect.height();
+
+                // never make any dimension smaller than the short dim.
+                double min_scale1 = std::max(min_object_length_short_dim/current_width, min_object_length_short_dim/current_height);
+                // at least one dimension needs to be longer than the long dim.
+                double min_scale2 = std::min(min_object_length_long_dim/current_width, min_object_length_long_dim/current_height);
+                double min_scale = std::max(min_scale1, min_scale2); 
+
+                const double rand_scale_perturb = 1.0/rnd.get_double_in_range(min_scale, 1); 
+                crop_rect = centered_drect(drect, drect.width()*rand_scale_perturb, drect.height()*rand_scale_perturb);
+                DLIB_CASSERT(crop_rect.width() == crop_rect.height());
 
-                const long box_size = std::max(rect.height(),rect.width())/rand_scale_perturb;
-                crop_rect = centered_rect(center(rect)+rand_translate, box_size, box_size);
             }
             else
             {
@@ -302,7 +333,8 @@ namespace dlib
         out << "  chip_dims.cols:              " << item.get_chip_dims().cols << endl;
         out << "  randomly_flip:               " << std::boolalpha << item.get_randomly_flip() << endl;
         out << "  max_rotation_degrees:        " << item.get_max_rotation_degrees() << endl;
-        out << "  min_object_size:           " << item.get_min_object_size() << endl;
+        out << "  min_object_length_long_dim:  " << item.get_min_object_length_long_dim() << endl;
+        out << "  min_object_length_short_dim: " << item.get_min_object_length_short_dim() << endl;
         out << "  max_object_size:             " << item.get_max_object_size() << endl;
         out << "  background_crops_fraction:   " << item.get_background_crops_fraction() << endl;
         out << "  translate_amount:            " << item.get_translate_amount() << endl;

dlib/image_transforms/random_cropper_abstract.h

@@ -35,7 +35,8 @@ namespace dlib
                 - #get_chip_dims() == chip_dims(300,300)
                 - #get_randomly_flip() == true
                 - #get_max_rotation_degrees() == 30
-                - #get_min_object_size() == 0.25
+                - #get_min_object_length_long_dim() == 70 
+                - #get_min_object_length_short_dim() == 30
                 - #get_max_object_size() == 0.7
                 - #get_background_crops_fraction() == 0.5
                 - #get_translate_amount() == 0.1
@@ -143,27 +144,38 @@ namespace dlib
                 - #get_max_rotation_degrees() == std::abs(value)
         !*/
 
-        double get_min_object_size (
+        long get_min_object_length_long_dim (
         ) const; 
         /*!
             ensures
                 - When a chip is extracted around an object, the chip will be sized so that
-                  at least one of the object's height or width are >= get_min_object_size() *
-                  the chip's height and width, respectively.  E.g. if the chip is 640x480
-                  pixels in size then the object will be at least 480*get_min_object_size()
-                  pixels tall or 640*get_min_object_size() pixels wide.  This also means
-                  that if get_min_object_size() >1 then the object will only be partially
-                  visible in the crop since it will be too big to fit.  
+                  the longest edge of the object (i.e. either its height or width,
+                  whichever is longer) is at least #get_min_object_length_long_dim() pixels
+                  in length.  When we say "object" here we are referring specifically to
+                  the rectangle in the mmod_rect output by the cropper.
+        !*/
+
+        long get_min_object_length_short_dim (
+        ) const;
+        /*!
+            ensures
+                - When a chip is extracted around an object, the chip will be sized so that
+                  the shortest edge of the object (i.e. either its height or width,
+                  whichever is shorter) is at least #get_min_object_length_short_dim()
+                  pixels in length.  When we say "object" here we are referring
+                  specifically to the rectangle in the mmod_rect output by the cropper.
         !*/
 
         void set_min_object_size (
-            double value
+            long long_dim,
+            long short_dim
         ); 
         /*!
             requires
-                - 0 < value 
+                - 0 < short_dim <= long_dim
             ensures
-                - #get_min_object_size() == value
+                - #get_min_object_length_short_dim() == short_dim
+                - #get_min_object_length_long_dim() == long_dim
         !*/
 
         double get_max_object_size (

examples/dnn_mmod_ex.cpp

@@ -159,7 +159,9 @@ int main(int argc, char** argv) try
     std::vector<std::vector<mmod_rect>> mini_batch_labels; 
     random_cropper cropper;
     cropper.set_chip_dims(200, 200);
-    cropper.set_min_object_size(0.2);
+    // Usually you want to give the cropper whatever min sizes you passed to the
+    // mmod_options constructor, which is what we do here.
+    cropper.set_min_object_size(40,40);
     dlib::rand rnd;
     // Run the trainer until the learning rate gets small.  This will probably take several
     // hours.

examples/dnn_mmod_train_find_cars_ex.cpp

@@ -310,7 +310,9 @@ int main(int argc, char** argv) try
     random_cropper cropper;
     cropper.set_seed(time(0));
     cropper.set_chip_dims(350, 350);
-    cropper.set_min_object_size(0.20); 
+    // Usually you want to give the cropper whatever min sizes you passed to the
+    // mmod_options constructor, or very slightly smaller sizes, which is what we do here.
+    cropper.set_min_object_size(69,28); 
     cropper.set_max_rotation_degrees(2);
     dlib::rand rnd;
 

examples/random_cropper_ex.cpp

@@ -44,9 +44,13 @@ int main(int argc, char** argv) try
     // You can tell it how much scale jittering you would like by saying "please
     // make the objects in the crops have a min and max size of such and such".
     // You do that by calling these two functions.  Here we are saying we want the
-    // objects in our crops to be between 0.2*400 and 0.8*400 pixels in height.
-    cropper.set_min_object_size(0.2);
+    // objects in our crops to be no more than 0.8*400 pixels in height and width.
     cropper.set_max_object_size(0.8);
+    // And also that they shouldn't be too small. Specifically, each object's smallest
+    // dimension (i.e. height or width) should be at least 60 pixels and at least one of
+    // the dimensions must be at least 80 pixels.  So the smallest objects the cropper will
+    // output will be either 80x60 or 60x80.
+    cropper.set_min_object_size(80,60);
     // The cropper can also randomly mirror and rotate crops, which we ask it to
     // perform as well.
     cropper.set_randomly_flip(true);