Fixed a bug in extract_fhog_features() that happened when very small

images were given.

dlib/image_transforms/fhog.h

@@ -193,6 +193,12 @@ namespace dlib
             const int cells_nr = (int)((double)img.nr()/(double)cell_size + 0.5);
             const int cells_nc = (int)((double)img.nc()/(double)cell_size + 0.5);
 
+            if (cells_nr == 0 || cells_nc == 0)
+            {
+                hog.clear();
+                return;
+            }
+
             array2d<matrix<float,18,1> > hist(cells_nr, cells_nc);
             for (long r = 0; r < hist.nr(); ++r)
             {
@@ -208,6 +214,11 @@ namespace dlib
             // memory for HOG features
             const int hog_nr = std::max(cells_nr-2, 0);
             const int hog_nc = std::max(cells_nc-2, 0);
+            if (hog_nr == 0 || hog_nc == 0)
+            {
+                hog.clear();
+                return;
+            }
             init_hog(hog, hog_nr, hog_nc);
 
             const int visible_nr = cells_nr*cell_size;

dlib/test/fhog.cpp

@@ -76,9 +76,38 @@ namespace
             }
         }
 
+        void test_on_small()
+        {
+            print_spinner();
+            array2d<unsigned char> img;
+            dlib::array<array2d<float> > hog;
+
+            // do this just to make sure it doesn't crash on small images
+            for (int i = 0; i < 10; ++i)
+            {
+                img.set_size(i,i);
+                assign_all_pixels(img, i);
+                extract_fhog_features(img, hog);
+            }
+            for (int i = 1; i < 10; ++i)
+            {
+                img.set_size(i,i+1);
+                assign_all_pixels(img, i);
+                extract_fhog_features(img, hog);
+            }
+            for (int i = 1; i < 10; ++i)
+            {
+                img.set_size(i+1,i);
+                assign_all_pixels(img, i);
+                extract_fhog_features(img, hog);
+            }
+        }
+
         void perform_test (
         )
         {
+            test_on_small();
+
             print_spinner();
             // load the testing data
             array2d<rgb_pixel> img;