added definition of gaussian adaptive threshold

closes boostorg#316

include/boost/gil/image_processing/threshold.hpp

@@ -16,10 +16,12 @@
 #include <vector>
 #include <cmath>
 
+#include <boost/assert.hpp>
+
 #include <boost/gil/image.hpp>
 #include <boost/gil/extension/numeric/kernel.hpp>
 #include <boost/gil/extension/numeric/convolve.hpp>
-#include <boost/assert.hpp>
+#include <boost/gil/image_processing/numeric.hpp>
 
 namespace boost { namespace gil {
 
@@ -403,15 +405,15 @@ void threshold_adaptive
     typedef typename channel_type<SrcView>::type source_channel_t;
     typedef typename channel_type<DstView>::type result_channel_t;
 
+    image<typename SrcView::value_type> temp_img(src_view.width(), src_view.height());
+    typename image<typename SrcView::value_type>::view_t temp_view = view(temp_img);
+    SrcView temp_conv(temp_view);
+
     if (method == threshold_adaptive_method::mean)
     {
         std::vector<float> mean_kernel_values(kernel_size, 1.0f/kernel_size);
         kernel_1d<float> kernel(mean_kernel_values.begin(), kernel_size, kernel_size/2);
 
-        image<typename SrcView::value_type> temp_img(src_view.width(), src_view.height());
-        typename image<typename SrcView::value_type>::view_t temp_view = view(temp_img);
-        SrcView temp_conv(temp_view);
-
         convolve_1d<pixel<float, typename SrcView::value_type::layout_t>>(
             src_view, kernel, temp_view
         );
@@ -429,6 +431,34 @@ void threshold_adaptive
                     { return px > (threshold - constant) ? 0 : max_value; });
         }
     }
+    else if (method == threshold_adaptive_method::gaussian)
+    {
+        gray32f_image_t gaussian_kernel_values(kernel_size, kernel_size);
+        generate_gaussian_kernel(view(gaussian_kernel_values), 1.0);
+
+
+        kernel_2d<float> kernel(
+            view(gaussian_kernel_values).begin(),
+            kernel_size * kernel_size,
+            kernel_size / 2,
+            kernel_size / 2
+        );
+
+        convolve_2d(src_view, kernel, temp_view);
+
+        if (direction == threshold_direction::regular)
+        {
+            detail::adaptive_impl<source_channel_t, result_channel_t>(src_view, temp_conv, dst_view,
+                [max_value, constant](source_channel_t px, source_channel_t threshold) -> result_channel_t
+            { return px > (threshold - constant) ? max_value : 0; });
+        }
+        else
+        {
+            detail::adaptive_impl<source_channel_t, result_channel_t>(src_view, temp_conv, dst_view,
+                [max_value, constant](source_channel_t px, source_channel_t threshold) -> result_channel_t
+            { return px > (threshold - constant) ? 0 : max_value; });
+        }
+    }
 }
 
 template <typename SrcView, typename DstView>