Fixed a bug in gaussian_blur() that caused messed up outputs when big sigma

values were used on some pixel types.

dlib/image_transforms/spatial_filtering.h

@@ -1233,15 +1233,25 @@ namespace dlib
             << "\n\t is_same_object(in_img,out_img): " << is_same_object(in_img,out_img) 
         );
 
-        typedef typename pixel_traits<typename image_traits<out_image_type>::pixel_type>::basic_pixel_type type;
-        typedef typename promote<type>::type ptype;
-
-        const matrix<ptype,0,1>& filt = create_gaussian_filter<ptype>(sigma, max_size);
-
-        ptype scale = sum(filt);
-        scale = scale*scale;
-
-        return spatially_filter_image_separable(in_img, out_img, filt, filt, scale);
+        if (sigma < 18)
+        {
+            typedef typename pixel_traits<typename image_traits<out_image_type>::pixel_type>::basic_pixel_type type;
+            typedef typename promote<type>::type ptype;
+            const matrix<ptype,0,1>& filt = create_gaussian_filter<ptype>(sigma, max_size);
+            ptype scale = sum(filt);
+            scale = scale*scale;
+            return spatially_filter_image_separable(in_img, out_img, filt, filt, scale);
+        }
+        else
+        {
+            // For large sigma we need to use a type with a lot of precision to avoid
+            // numerical problems.  So we use double here.
+            typedef double ptype;
+            const matrix<ptype,0,1>& filt = create_gaussian_filter<ptype>(sigma, max_size);
+            ptype scale = sum(filt);
+            scale = scale*scale;
+            return spatially_filter_image_separable(in_img, out_img, filt, filt, scale);
+        }
 
     }