Added another overload of extract_fhog_features(). This one returns the

features as a column vector but by reference instead of by value.

dlib/image_transforms/fhog.h

@@ -658,24 +658,42 @@ namespace dlib
 // ----------------------------------------------------------------------------------------
 
     template <
-        typename image_type
+        typename image_type,
+        typename T
         >
-    matrix<double,0,1> extract_fhog_features(
+    void extract_fhog_features(
         const image_type& img, 
+        matrix<T,0,1>& feats,
         int cell_size = 8,
         int filter_rows_padding = 1,
         int filter_cols_padding = 1
     )
     {
-        dlib::array<array2d<double> > hog;
+        dlib::array<array2d<T> > hog;
         extract_fhog_features(img, hog, cell_size, filter_rows_padding, filter_cols_padding);
-        matrix<double,0,1> temp(hog.size()*hog[0].size());
+        feats.set_size(hog.size()*hog[0].size());
         for (unsigned long i = 0; i < hog.size(); ++i)
         {
             const long size = hog[i].size();
-            set_rowm(temp, range(i*size, (i+1)*size-1)) = reshape_to_column_vector(mat(hog[i]));
+            set_rowm(feats, range(i*size, (i+1)*size-1)) = reshape_to_column_vector(mat(hog[i]));
         }
-        return temp;
+    }
+
+// ----------------------------------------------------------------------------------------
+
+    template <
+        typename image_type
+        >
+    matrix<double,0,1> extract_fhog_features(
+        const image_type& img, 
+        int cell_size = 8,
+        int filter_rows_padding = 1,
+        int filter_cols_padding = 1
+    )
+    {
+        matrix<double,0,1> feats;
+        extract_fhog_features(img, feats, cell_size, filter_rows_padding, filter_cols_padding);
+        return feats;
     }
 
 // ----------------------------------------------------------------------------------------

dlib/image_transforms/fhog_abstract.h

@@ -137,6 +137,34 @@ namespace dlib
               H[0], then H[1], then H[2], and so on in ascending index order.
     !*/
 
+// ----------------------------------------------------------------------------------------
+
+    template <
+        typename image_type,
+        typename T
+        >
+    void extract_fhog_features(
+        const image_type& img, 
+        matrix<T,0,1>& feats,
+        int cell_size = 8,
+        int filter_rows_padding = 1,
+        int filter_cols_padding = 1
+    );
+    /*!
+        requires
+            - cell_size > 0
+            - filter_rows_padding > 0
+            - filter_cols_padding > 0
+            - image_type  == is an implementation of array2d/array2d_kernel_abstract.h
+            - img contains some kind of pixel type. 
+              (i.e. pixel_traits<typename image_type::type> is defined)
+            - T is float, double, or long double
+        ensures
+            - This function is identical to the above version of extract_fhog_features()
+              that returns a matrix<double,0,1> except that it returns the matrix here
+              through a reference argument instead of returning it by value.
+    !*/
+
 // ----------------------------------------------------------------------------------------
 
     inline point image_to_fhog (