Gave scan_fhog_pyramid an optional template argument that lets you

define a custom version of HOG feature extraction.

dlib/image_processing/scan_fhog_pyramid.h

@@ -13,10 +13,60 @@
 namespace dlib
 {
 
+// ----------------------------------------------------------------------------------------
+
+    class default_fhog_feature_extractor
+    {
+    public:
+        inline rectangle image_to_feats (
+            const rectangle& rect,
+            int cell_size,
+            int filter_rows_padding,
+            int filter_cols_padding
+        ) const
+        {
+            return image_to_fhog(rect, cell_size, filter_rows_padding, filter_cols_padding);
+        }
+
+        inline rectangle feats_to_image (
+            const rectangle& rect,
+            int cell_size,
+            int filter_rows_padding,
+            int filter_cols_padding
+        ) const
+        {
+            return fhog_to_image(rect, cell_size, filter_rows_padding, filter_cols_padding);
+        }
+
+        template <
+            typename image_type
+            >
+        void operator()(
+            const image_type& img, 
+            dlib::array<array2d<float> >& hog, 
+            int cell_size,
+            int filter_rows_padding,
+            int filter_cols_padding
+        ) const
+        {
+            extract_fhog_features(img,hog,cell_size,filter_rows_padding,filter_cols_padding);
+        }
+
+        inline long get_num_planes (
+        ) const
+        {
+            return 31;
+        }
+    };
+
+    inline void serialize   (const default_fhog_feature_extractor&, std::ostream&) {}
+    inline void deserialize (default_fhog_feature_extractor&, std::istream&) {}
+
 // ----------------------------------------------------------------------------------------
 
     template <
-        typename Pyramid_type
+        typename Pyramid_type,
+        typename Feature_extractor_type = default_fhog_feature_extractor
         >
     class scan_fhog_pyramid : noncopyable
     {
@@ -26,10 +76,15 @@ namespace dlib
         typedef matrix<double,0,1> feature_vector_type;
 
         typedef Pyramid_type pyramid_type;
+        typedef Feature_extractor_type feature_extractor_type;
 
         scan_fhog_pyramid (
         );  
 
+        explicit scan_fhog_pyramid (
+            const feature_extractor_type& fe_
+        );  
+
         template <
             typename image_type
             >
@@ -106,6 +161,9 @@ namespace dlib
         unsigned long get_max_pyramid_levels (
         ) const;
 
+        const feature_extractor_type& get_feature_extractor(
+        ) const { return fe; }
+
         void set_max_pyramid_levels (
             unsigned long max_levels
         );
@@ -187,9 +245,9 @@ namespace dlib
                 );
 
             fhog_filterbank temp;
-            temp.filters.resize(31);
-            temp.row_filters.resize(31);
-            temp.col_filters.resize(31);
+            temp.filters.resize(fe.get_num_planes());
+            temp.row_filters.resize(fe.get_num_planes());
+            temp.col_filters.resize(fe.get_num_planes());
 
             // load filters from w
             unsigned long width, height;
@@ -279,15 +337,15 @@ namespace dlib
             return height;
         }
 
-        template <typename T>
+        template <typename T, typename U>
         friend void serialize (
-            const scan_fhog_pyramid<T>& item,
+            const scan_fhog_pyramid<T,U>& item,
             std::ostream& out
         );
 
-        template <typename T>
+        template <typename T, typename U>
         friend void deserialize (
-            scan_fhog_pyramid<T>& item,
+            scan_fhog_pyramid<T,U>& item,
             std::istream& in 
         );
 
@@ -297,7 +355,8 @@ namespace dlib
             unsigned long& height
         ) const
         {
-            const rectangle temp = grow_rect(image_to_fhog(centered_rect(point(0,0),window_width,window_height), cell_size), padding);
+            const rectangle rect = centered_rect(point(0,0),window_width,window_height);
+            const rectangle temp = grow_rect(fe.image_to_feats(rect, cell_size, 1, 1), padding);
             width = temp.width();
             height = temp.height();
         }
@@ -336,6 +395,7 @@ namespace dlib
             array2d<float>& saliency_image
         );
 
+        feature_extractor_type fe;
         array<fhog_image> feats;
         int cell_size;
         unsigned long padding; 
@@ -346,12 +406,24 @@ namespace dlib
         unsigned long min_pyramid_layer_height;
         double nuclear_norm_regularization_strength;
 
+        void init()
+        {
+            cell_size = 8;
+            padding = 1;
+            window_width = 64;
+            window_height = 64;
+            max_pyramid_levels = 1000;
+            min_pyramid_layer_width = 64;
+            min_pyramid_layer_height = 64;
+            nuclear_norm_regularization_strength = 0;
+        }
+
     };
 
 // ----------------------------------------------------------------------------------------
 
-    template <typename T>
-    rectangle scan_fhog_pyramid<T>::
+    template <typename T, typename U>
+    rectangle scan_fhog_pyramid<T,U>::
     apply_filters_to_fhog (
         const fhog_filterbank& w,
         const fhog_image& feats,
@@ -361,7 +433,7 @@ namespace dlib
         const unsigned long num_separable_filters = w.num_separable_filters();
         rectangle area;
         // use the separable filters if they would be faster than running the regular filters.
-        if (num_separable_filters > 31*std::min(w.filters[0].nr(),w.filters[0].nc())/3.0)
+        if (num_separable_filters > w.filters.size()*std::min(w.filters[0].nr(),w.filters[0].nc())/3.0)
         {
             area = spatially_filter_image(feats[0], saliency_image, w.filters[0]);
             for (unsigned long i = 1; i < w.filters.size(); ++i)
@@ -402,14 +474,15 @@ namespace dlib
 
 // ----------------------------------------------------------------------------------------
 
-    template <typename T>
+    template <typename T, typename U>
     void serialize (
-        const scan_fhog_pyramid<T>& item,
+        const scan_fhog_pyramid<T,U>& item,
         std::ostream& out
     )
     {
         int version = 1;
         serialize(version, out);
+        serialize(item.fe, out);
         serialize(item.feats, out);
         serialize(item.cell_size, out);
         serialize(item.padding, out);
@@ -424,9 +497,9 @@ namespace dlib
 
 // ----------------------------------------------------------------------------------------
 
-    template <typename T>
+    template <typename T, typename U>
     void deserialize (
-        scan_fhog_pyramid<T>& item,
+        scan_fhog_pyramid<T,U>& item,
         std::istream& in 
     )
     {
@@ -435,6 +508,7 @@ namespace dlib
         if (version != 1)
             throw serialization_error("Unsupported version found when deserializing a scan_fhog_pyramid object.");
 
+        deserialize(item.fe, in);
         deserialize(item.feats, in);
         deserialize(item.cell_size, in);
         deserialize(item.padding, in);
@@ -462,31 +536,41 @@ namespace dlib
 // ----------------------------------------------------------------------------------------
 
     template <
-        typename Pyramid_type
+        typename Pyramid_type,
+        typename feature_extractor_type
         >
-    scan_fhog_pyramid<Pyramid_type>::
+    scan_fhog_pyramid<Pyramid_type,feature_extractor_type>::
     scan_fhog_pyramid (
-    ) : 
-        cell_size(8),
-        padding(1),
-        window_width(64),
-        window_height(64),
-        max_pyramid_levels(1000),
-        min_pyramid_layer_width(64),
-        min_pyramid_layer_height(64),
-        nuclear_norm_regularization_strength(0)
+    ) 
     {
+        init();
     }
 
 // ----------------------------------------------------------------------------------------
 
     template <
-        typename Pyramid_type
+        typename Pyramid_type,
+        typename feature_extractor_type
+        >
+    scan_fhog_pyramid<Pyramid_type,feature_extractor_type>::
+    scan_fhog_pyramid (
+        const feature_extractor_type& fe_
+    ) 
+    {
+        init();
+        fe = fe_;
+    }
+
+// ----------------------------------------------------------------------------------------
+
+    template <
+        typename Pyramid_type,
+        typename feature_extractor_type
         >
     template <
         typename image_type
         >
-    void scan_fhog_pyramid<Pyramid_type>::
+    void scan_fhog_pyramid<Pyramid_type,feature_extractor_type>::
     load (
         const image_type& img
     )
@@ -514,18 +598,22 @@ namespace dlib
         typedef typename image_type::mem_manager_type mem_manager_type;
 
         // build our feature pyramid
-        extract_fhog_features(img, feats[0], cell_size,height,width);
+        fe(img, feats[0], cell_size,height,width);
+        DLIB_ASSERT(feats[0].size() == fe.get_num_planes(), 
+            "Invalid feature extractor used with dlib::scan_fhog_pyramid.  The output does not have the \n"
+            "indicated number of planes.");
+
         if (feats.size() > 1)
         {
             array2d<pixel_type,mem_manager_type> temp1, temp2;
             pyr(img, temp1);
-            extract_fhog_features(temp1, feats[1], cell_size,height,width);
+            fe(temp1, feats[1], cell_size,height,width);
             swap(temp1,temp2);
 
             for (unsigned long i = 2; i < feats.size(); ++i)
             {
                 pyr(temp2, temp1);
-                extract_fhog_features(temp1, feats[i], cell_size,height,width);
+                fe(temp1, feats[i], cell_size,height,width);
                 swap(temp1,temp2);
             }
         }
@@ -534,9 +622,10 @@ namespace dlib
 // ----------------------------------------------------------------------------------------
 
     template <
-        typename Pyramid_type
+        typename Pyramid_type,
+        typename feature_extractor_type
         >
-    bool scan_fhog_pyramid<Pyramid_type>::
+    bool scan_fhog_pyramid<Pyramid_type,feature_extractor_type>::
     is_loaded_with_image (
     ) const
     {
@@ -546,9 +635,10 @@ namespace dlib
 // ----------------------------------------------------------------------------------------
 
     template <
-        typename Pyramid_type
+        typename Pyramid_type,
+        typename feature_extractor_type
         >
-    void scan_fhog_pyramid<Pyramid_type>::
+    void scan_fhog_pyramid<Pyramid_type,feature_extractor_type>::
     copy_configuration (
         const scan_fhog_pyramid& item
     )
@@ -561,14 +651,16 @@ namespace dlib
         min_pyramid_layer_width = item.min_pyramid_layer_width;
         min_pyramid_layer_height = item.min_pyramid_layer_height;
         nuclear_norm_regularization_strength = item.nuclear_norm_regularization_strength;
+        fe = item.fe;
     }
 
 // ----------------------------------------------------------------------------------------
 
     template <
-        typename Pyramid_type
+        typename Pyramid_type,
+        typename feature_extractor_type
         >
-    unsigned long scan_fhog_pyramid<Pyramid_type>::
+    unsigned long scan_fhog_pyramid<Pyramid_type,feature_extractor_type>::
     get_num_detection_templates (
     ) const
     {
@@ -578,9 +670,10 @@ namespace dlib
 // ----------------------------------------------------------------------------------------
 
     template <
-        typename Pyramid_type
+        typename Pyramid_type,
+        typename feature_extractor_type
         >
-    unsigned long scan_fhog_pyramid<Pyramid_type>::
+    unsigned long scan_fhog_pyramid<Pyramid_type,feature_extractor_type>::
     get_num_movable_components_per_detection_template (
     ) const
     {
@@ -590,23 +683,25 @@ namespace dlib
 // ----------------------------------------------------------------------------------------
 
     template <
-        typename Pyramid_type
+        typename Pyramid_type,
+        typename feature_extractor_type
         >
-    long scan_fhog_pyramid<Pyramid_type>::
+    long scan_fhog_pyramid<Pyramid_type,feature_extractor_type>::
     get_num_dimensions (
     ) const
     {
         unsigned long width, height;
         compute_fhog_window_size(width,height);
-        return width*height*31;
+        return width*height*fe.get_num_planes();
     }
 
 // ----------------------------------------------------------------------------------------
 
     template <
-        typename Pyramid_type
+        typename Pyramid_type,
+        typename feature_extractor_type
         >
-    unsigned long scan_fhog_pyramid<Pyramid_type>::
+    unsigned long scan_fhog_pyramid<Pyramid_type,feature_extractor_type>::
     get_max_pyramid_levels (
     ) const
     {
@@ -616,9 +711,10 @@ namespace dlib
 // ----------------------------------------------------------------------------------------
 
     template <
-        typename Pyramid_type
+        typename Pyramid_type,
+        typename feature_extractor_type
         >
-    void scan_fhog_pyramid<Pyramid_type>::
+    void scan_fhog_pyramid<Pyramid_type,feature_extractor_type>::
     set_max_pyramid_levels (
         unsigned long max_levels
     )
@@ -637,9 +733,10 @@ namespace dlib
 // ----------------------------------------------------------------------------------------
 
     template <
-        typename Pyramid_type
+        typename Pyramid_type,
+        typename feature_extractor_type
         >
-    void scan_fhog_pyramid<Pyramid_type>::
+    void scan_fhog_pyramid<Pyramid_type,feature_extractor_type>::
     detect (
         const fhog_filterbank& w,
         std::vector<std::pair<double, rectangle> >& dets,
@@ -678,7 +775,7 @@ namespace dlib
                     // if we found a detection
                     if (saliency_image[r][c] >= thresh)
                     {
-                        rectangle rect = fhog_to_image(centered_rect(point(c,r),width-2*padding,height-2*padding), cell_size, height,width);
+                        rectangle rect = fe.feats_to_image(centered_rect(point(c,r),width-2*padding,height-2*padding), cell_size, height,width);
                         rect = pyr.rect_up(rect, l);
                         dets.push_back(std::make_pair(saliency_image[r][c], rect));
                     }
@@ -692,9 +789,10 @@ namespace dlib
 // ----------------------------------------------------------------------------------------
 
     template <
-        typename Pyramid_type
+        typename Pyramid_type,
+        typename feature_extractor_type
         >
-    const rectangle scan_fhog_pyramid<Pyramid_type>::
+    const rectangle scan_fhog_pyramid<Pyramid_type,feature_extractor_type>::
     get_best_matching_rect (
         const rectangle& rect
     ) const
@@ -708,9 +806,10 @@ namespace dlib
 // ----------------------------------------------------------------------------------------
 
     template <
-        typename Pyramid_type
+        typename Pyramid_type,
+        typename feature_extractor_type
         >
-    void scan_fhog_pyramid<Pyramid_type>::
+    void scan_fhog_pyramid<Pyramid_type,feature_extractor_type>::
     get_mapped_rect_and_metadata (
         const unsigned long number_pyramid_levels,
         const rectangle& rect,
@@ -731,9 +830,9 @@ namespace dlib
         // window. 
         for (unsigned long l = 0; l < number_pyramid_levels; ++l)
         {
-            const rectangle rect_fhog_space = image_to_fhog(pyr.rect_down(rect,l), cell_size, height,width);
+            const rectangle rect_fhog_space = fe.image_to_feats(pyr.rect_down(rect,l), cell_size, height,width);
 
-            const rectangle win_image_space = pyr.rect_up(fhog_to_image(centered_rect(center(rect_fhog_space),width-2*padding,height-2*padding), cell_size, height,width), l);
+            const rectangle win_image_space = pyr.rect_up(fe.feats_to_image(centered_rect(center(rect_fhog_space),width-2*padding,height-2*padding), cell_size, height,width), l);
 
             const double match_score = get_match_score(win_image_space, rect); 
             if (match_score > best_match_score)
@@ -746,15 +845,16 @@ namespace dlib
             if (rect_fhog_space.area() <= 1) 
                 break;
         }
-        mapped_rect = pyr.rect_up(fhog_to_image(shrink_rect(fhog_rect,padding), cell_size,height,width),best_level);
+        mapped_rect = pyr.rect_up(fe.feats_to_image(shrink_rect(fhog_rect,padding), cell_size,height,width),best_level);
     }
 
 // ----------------------------------------------------------------------------------------
 
     template <
-        typename Pyramid_type
+        typename Pyramid_type,
+        typename feature_extractor_type
         >
-    full_object_detection scan_fhog_pyramid<Pyramid_type>::
+    full_object_detection scan_fhog_pyramid<Pyramid_type,feature_extractor_type>::
     get_full_object_detection (
         const rectangle& rect,
         const feature_vector_type& 
@@ -766,9 +866,10 @@ namespace dlib
 // ----------------------------------------------------------------------------------------
 
     template <
-        typename Pyramid_type
+        typename Pyramid_type,
+        typename feature_extractor_type
         >
-    void scan_fhog_pyramid<Pyramid_type>::
+    void scan_fhog_pyramid<Pyramid_type,feature_extractor_type>::
     get_feature_vector (
         const full_object_detection& obj,
         feature_vector_type& psi
@@ -814,9 +915,10 @@ namespace dlib
 // ----------------------------------------------------------------------------------------
 
     template <
-        typename Pyramid_type
+        typename Pyramid_type,
+        typename feature_extractor_type
         >
-    void scan_fhog_pyramid<Pyramid_type>::
+    void scan_fhog_pyramid<Pyramid_type,feature_extractor_type>::
     set_min_pyramid_layer_size (
         unsigned long width,
         unsigned long height 
@@ -838,9 +940,10 @@ namespace dlib
 // ----------------------------------------------------------------------------------------
 
     template <
-        typename Pyramid_type
+        typename Pyramid_type,
+        typename feature_extractor_type
         >
-    unsigned long scan_fhog_pyramid<Pyramid_type>::
+    unsigned long scan_fhog_pyramid<Pyramid_type,feature_extractor_type>::
     get_min_pyramid_layer_width (
     ) const
     {
@@ -850,9 +953,10 @@ namespace dlib
 // ----------------------------------------------------------------------------------------
 
     template <
-        typename Pyramid_type
+        typename Pyramid_type,
+        typename feature_extractor_type
         >
-    unsigned long scan_fhog_pyramid<Pyramid_type>::
+    unsigned long scan_fhog_pyramid<Pyramid_type,feature_extractor_type>::
     get_min_pyramid_layer_height (
     ) const
     {
@@ -863,10 +967,11 @@ namespace dlib
 // ----------------------------------------------------------------------------------------
 
     template <
-        typename Pyramid_type
+        typename Pyramid_type,
+        typename feature_extractor_type
         >
     matrix<unsigned char> draw_fhog (
-        const object_detector<scan_fhog_pyramid<Pyramid_type> >& detector,
+        const object_detector<scan_fhog_pyramid<Pyramid_type,feature_extractor_type> >& detector,
         const unsigned long weight_index = 0,
         const long cell_draw_size = 15
     )
@@ -887,17 +992,18 @@ namespace dlib
             << "\n\t detector.get_scanner().get_num_dimensions(): " << detector.get_scanner().get_num_dimensions()
             );
 
-        typename scan_fhog_pyramid<Pyramid_type>::fhog_filterbank fb = detector.get_scanner().build_fhog_filterbank(detector.get_w(weight_index));
+        typename scan_fhog_pyramid<Pyramid_type,feature_extractor_type>::fhog_filterbank fb = detector.get_scanner().build_fhog_filterbank(detector.get_w(weight_index));
         return draw_fhog(fb.get_filters(),cell_draw_size);
     }
 
 // ----------------------------------------------------------------------------------------
 
     template <
-        typename Pyramid_type
+        typename Pyramid_type,
+        typename feature_extractor_type
         >
     unsigned long num_separable_filters (
-        const object_detector<scan_fhog_pyramid<Pyramid_type> >& detector,
+        const object_detector<scan_fhog_pyramid<Pyramid_type,feature_extractor_type> >& detector,
         const unsigned long weight_index = 0
     )
     {
@@ -915,17 +1021,18 @@ namespace dlib
             << "\n\t detector.get_scanner().get_num_dimensions(): " << detector.get_scanner().get_num_dimensions()
             );
 
-        typename scan_fhog_pyramid<Pyramid_type>::fhog_filterbank fb = detector.get_scanner().build_fhog_filterbank(detector.get_w(weight_index));
+        typename scan_fhog_pyramid<Pyramid_type,feature_extractor_type>::fhog_filterbank fb = detector.get_scanner().build_fhog_filterbank(detector.get_w(weight_index));
         return fb.num_separable_filters();
     }
 
 // ----------------------------------------------------------------------------------------
 
     template <
-        typename Pyramid_type
+        typename Pyramid_type,
+        typename feature_extractor_type
         >
-    object_detector<scan_fhog_pyramid<Pyramid_type> > threshold_filter_singular_values (
-        const object_detector<scan_fhog_pyramid<Pyramid_type> >& detector,
+    object_detector<scan_fhog_pyramid<Pyramid_type,feature_extractor_type> > threshold_filter_singular_values (
+        const object_detector<scan_fhog_pyramid<Pyramid_type,feature_extractor_type> >& detector,
         double thresh,
         const unsigned long weight_index = 0
     )
@@ -953,6 +1060,7 @@ namespace dlib
 
         const unsigned long width = detector.get_scanner().get_fhog_window_width();
         const unsigned long height = detector.get_scanner().get_fhog_window_height();
+        const long num_planes = detector.get_scanner().get_feature_extractor().get_num_planes();
         const long size = width*height;
 
         std::vector<matrix<double,0,1> > detector_weights;
@@ -963,7 +1071,7 @@ namespace dlib
             if (j == weight_index)
             {
                 matrix<double> u,v,w,f;
-                for (int i = 0; i < 31; ++i)
+                for (long i = 0; i < num_planes; ++i)
                 {
                     f = reshape(rowm(weights, range(i*size, (i+1)*size-1)), height, width);
 
@@ -978,7 +1086,7 @@ namespace dlib
             detector_weights.push_back(weights);
         }
         
-        return object_detector<scan_fhog_pyramid<Pyramid_type> >(detector.get_scanner(), 
+        return object_detector<scan_fhog_pyramid<Pyramid_type,feature_extractor_type> >(detector.get_scanner(), 
                                                                  detector.get_overlap_tester(),
                                                                  detector_weights);
     }
@@ -987,19 +1095,21 @@ namespace dlib
 
     template <
         typename Pyramid_type,
+        typename feature_extractor_type,
         typename svm_struct_prob_type
         >
     void configure_nuclear_norm_regularizer (
-        const scan_fhog_pyramid<Pyramid_type>& scanner,
+        const scan_fhog_pyramid<Pyramid_type,feature_extractor_type>& scanner,
         svm_struct_prob_type& prob
     )
     { 
         const double strength = scanner.get_nuclear_norm_regularization_strength();
+        const long num_planes = scanner.get_feature_extractor().get_num_planes();
         if (strength != 0)
         {
             const unsigned long width = scanner.get_fhog_window_width();
             const unsigned long height = scanner.get_fhog_window_height();
-            for (int i = 0; i < 31; ++i)
+            for (long i = 0; i < num_planes; ++i)
             {
                 prob.add_nuclear_norm_regularizer(i*width*height, height, width, strength);
             }
@@ -1010,17 +1120,18 @@ namespace dlib
 // ----------------------------------------------------------------------------------------
 
     template <
-        typename Pyramid_type
+        typename Pyramid_type,
+        typename feature_extractor_type
         >
-    struct processed_weight_vector<scan_fhog_pyramid<Pyramid_type> >
+    struct processed_weight_vector<scan_fhog_pyramid<Pyramid_type,feature_extractor_type> >
     {
         processed_weight_vector(){}
 
         typedef matrix<double,0,1> feature_vector_type;
-        typedef typename scan_fhog_pyramid<Pyramid_type>::fhog_filterbank fhog_filterbank;
+        typedef typename scan_fhog_pyramid<Pyramid_type,feature_extractor_type>::fhog_filterbank fhog_filterbank;
 
         void init (
-            const scan_fhog_pyramid<Pyramid_type>& scanner
+            const scan_fhog_pyramid<Pyramid_type,feature_extractor_type>& scanner
         ) 
         {
             fb = scanner.build_fhog_filterbank(w);

dlib/image_processing/scan_fhog_pyramid_abstract.h

@@ -13,10 +13,11 @@ namespace dlib
 // ----------------------------------------------------------------------------------------
 
     template <
-        typename Pyramid_type
+        typename Pyramid_type,
+        typename feature_extractor_type
         >
     matrix<unsigned char> draw_fhog (
-        const object_detector<scan_fhog_pyramid<Pyramid_type> >& detector,
+        const object_detector<scan_fhog_pyramid<Pyramid_type,feature_extractor_type> >& detector,
         const unsigned long weight_index = 0,
         const long cell_draw_size = 15
     );
@@ -37,10 +38,11 @@ namespace dlib
 // ----------------------------------------------------------------------------------------
 
     template <
-        typename Pyramid_type
+        typename Pyramid_type,
+        typename feature_extractor_type
         >
     unsigned long num_separable_filters (
-        const object_detector<scan_fhog_pyramid<Pyramid_type> >& detector,
+        const object_detector<scan_fhog_pyramid<Pyramid_type,feature_extractor_type> >& detector,
         const unsigned long weight_index = 0
     );
     /*!
@@ -57,10 +59,11 @@ namespace dlib
 // ----------------------------------------------------------------------------------------
 
     template <
-        typename Pyramid_type
+        typename Pyramid_type,
+        typename feature_extractor_type
         >
-    object_detector<scan_fhog_pyramid<Pyramid_type> > threshold_filter_singular_values (
-        const object_detector<scan_fhog_pyramid<Pyramid_type> >& detector,
+    object_detector<scan_fhog_pyramid<Pyramid_type,feature_extractor_type> > threshold_filter_singular_values (
+        const object_detector<scan_fhog_pyramid<Pyramid_type,feature_extractor_type> >& detector,
         double thresh,
         const unsigned long weight_index = 0
     );
@@ -84,8 +87,115 @@ namespace dlib
 
 // ----------------------------------------------------------------------------------------
 
+    class default_fhog_feature_extractor
+    {
+        /*!
+            WHAT THIS OBJECT REPRESENTS
+                The scan_fhog_pyramid object defined below is primarily meant to be used
+                with the feature extraction technique implemented by extract_fhog_features().  
+                This technique can generally be understood as taking an input image and
+                outputting a multi-planed output image of floating point numbers that
+                somehow describe the image contents.  Since there are many ways to define
+                how this feature mapping is performed, the scan_fhog_pyramid allows you to
+                replace the extract_fhog_features() method with a customized method of your
+                choosing.  To do this you implement a class with the same interface as
+                default_fhog_feature_extractor.  
+
+                Therefore, the point of default_fhog_feature_extractor is two fold.  First,
+                it provides the default FHOG feature extraction method used by scan_fhog_pyramid.
+                Second, it serves to document the interface you need to implement to define 
+                your own custom HOG style feature extraction. 
+        !*/
+
+    public:
+
+        rectangle image_to_feats (
+            const rectangle& rect,
+            int cell_size,
+            int filter_rows_padding,
+            int filter_cols_padding
+        ) const { return image_to_fhog(rect, cell_size, filter_rows_padding, filter_cols_padding); }
+        /*!
+            requires
+                - cell_size > 0
+                - filter_rows_padding > 0
+                - filter_cols_padding > 0
+            ensures
+                - Maps a rectangle from the coordinates in an input image to the corresponding
+                  area in the output feature image.
+        !*/
+
+        rectangle feats_to_image (
+            const rectangle& rect,
+            int cell_size,
+            int filter_rows_padding,
+            int filter_cols_padding
+        ) const { return fhog_to_image(rect, cell_size, filter_rows_padding, filter_cols_padding); }
+        /*!
+            requires
+                - cell_size > 0
+                - filter_rows_padding > 0
+                - filter_cols_padding > 0
+            ensures
+                - Maps a rectangle from the coordinates of the hog feature image back to
+                  the input image.
+                - Mapping from feature space to image space is an invertible
+                  transformation.  That is, for any rectangle R we have:
+                    R == image_to_feats(feats_to_image(R,cell_size,filter_rows_padding,filter_cols_padding),
+                                                         cell_size,filter_rows_padding,filter_cols_padding).
+        !*/
+
         template <
-        typename Pyramid_type
+            typename image_type
+            >
+        void operator()(
+            const image_type& img, 
+            dlib::array<array2d<float> >& hog, 
+            int cell_size,
+            int filter_rows_padding,
+            int filter_cols_padding
+        ) const { extract_fhog_features(img,hog,cell_size,filter_rows_padding,filter_cols_padding); }
+        /*!
+            requires
+                - 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)
+            ensures
+                - Extracts FHOG features by calling extract_fhog_features().  The results are
+                  stored into #hog.  Note that if you are implementing your own feature extractor you can
+                  pretty much do whatever you want in terms of feature extraction so long as the following
+                  conditions are met:
+                    - #hog.size() == get_num_planes()
+                    - Each image plane in of #hog has the same dimensions.
+                    - for all valid i, r, and c:
+                        - #hog[i][r][c] == a feature value describing the image content centered at the 
+                          following pixel location in img: 
+                            feats_to_image(point(c,r),cell_size,filter_rows_padding,filter_cols_padding)
+        !*/
+
+        inline long get_num_planes (
+        ) const { return 31; }
+        /*!
+            ensures
+                - returns the number of planes in the hog image output by the operator()
+                  method.
+        !*/
+    };
+
+    inline void serialize   (const default_fhog_feature_extractor&, std::ostream&) {}
+    inline void deserialize (default_fhog_feature_extractor&, std::istream&) {}
+    /*!
+        Provides serialization support.  Note that there is no state in the default hog
+        feature extractor so these functions do nothing.  But if you define a custom
+        feature extractor then make sure you remember to serialize any state in your
+        feature extractor.
+    !*/
+
+// ----------------------------------------------------------------------------------------
+
+    template <
+        typename Pyramid_type,
+        typename Feature_extractor_type = default_fhog_feature_extractor
         >
     class scan_fhog_pyramid : noncopyable
     {
@@ -95,6 +205,10 @@ namespace dlib
                   dlib/image_transforms/image_pyramid_abstract.h or an object with a
                   compatible interface
 
+            REQUIREMENTS ON Feature_extractor_type
+                - Must be a type with an interface compatible with the
+                  default_fhog_feature_extractor.
+
             INITIAL VALUE
                 - get_padding()   == 1
                 - get_cell_size() == 8
@@ -129,6 +243,7 @@ namespace dlib
     public:
         typedef matrix<double,0,1> feature_vector_type;
         typedef Pyramid_type pyramid_type;
+        typedef Feature_extractor_type feature_extractor_type;
 
         scan_fhog_pyramid (
         );  
@@ -137,6 +252,15 @@ namespace dlib
                 - this object is properly initialized
         !*/
 
+        scan_fhog_pyramid (
+            const feature_extractor_type& fe
+        );  
+        /*!
+            ensures
+                - this object is properly initialized
+                - #get_feature_extractor() == fe
+        !*/
+
         template <
             typename image_type
             >
@@ -154,6 +278,13 @@ namespace dlib
                   locations.  Call detect() to do this.
         !*/
 
+        const feature_extractor_type& get_feature_extractor(
+        ) const;
+        /*!
+            ensures
+                - returns a const reference to the feature extractor used by this object.
+        !*/
+
         bool is_loaded_with_image (
         ) const;
         /*!
@@ -197,7 +328,8 @@ namespace dlib
                 - Since we use a HOG feature representation, the detection procedure works
                   as follows:
                     Step 1. Make an image pyramid.
-                    Step 2. Convert each layer of the image pyramid into a 31 band HOG "image".
+                    Step 2. Convert each layer of the image pyramid into a multi-planed HOG "image".
+                    (the number of bands is given by get_feature_extractor().get_num_planes())
                     Step 3. Scan a linear classifier over each HOG image in the pyramid. 
                   Moreover, the HOG features quantize the input image into a grid of cells,
                   each cell being get_cell_size() by get_cell_size() pixels in size.  So
@@ -289,9 +421,9 @@ namespace dlib
         ) const;
         /*!
             ensures
-                - get_fhog_window_width()*get_fhog_window_height()*31
-                  (i.e. The number of features is equal to the size of the HOG window
-                  times 31 since there are 31 channels in the HOG feature representation.)
+                - returns get_fhog_window_width()*get_fhog_window_height()*get_feature_extractor().get_num_planes()
+                  (i.e. The number of features is equal to the size of the HOG window times
+                  the number of planes output by the feature extractor. )
         !*/
 
         inline unsigned long get_num_detection_templates (
@@ -375,7 +507,7 @@ namespace dlib
             ensures
                 - Creates and then returns a fhog_filterbank object FB such that:
                     - FB.get_num_dimensions() == get_num_dimensions()
-                    - FB.get_filters() == the values in weights unpacked into 31 filters.
+                    - FB.get_filters() == the values in weights unpacked into get_feature_extractor().get_num_planes() filters.
                     - FB.num_separable_filters() == the number of separable filters necessary to
                       represent all the filters in FB.get_filters().
         !*/
@@ -384,10 +516,10 @@ namespace dlib
         {
             /*!
                 WHAT THIS OBJECT REPRESENTS
-                    This object represents a HOG filter bank.  That is, the classifier that
-                    is slid over a HOG pyramid is a set of 31 linear filters, each
-                    get_fhog_window_width() rows by get_fhog_window_height() columns in
-                    size.  This object contains that set of 31 filters.  
+                    This object represents a HOG filter bank.  That is, the classifier that is 
+                    slid over a HOG pyramid is a set of get_feature_extractor().get_num_planes() 
+                    linear filters, each get_fhog_window_width() rows by get_fhog_window_height() 
+                    columns in size.  This object contains that set of filters.  
             !*/
 
         public:
@@ -402,7 +534,7 @@ namespace dlib
             ) const; 
             /*!
                 ensures
-                    - returns the set of 31 HOG filters in this object.
+                    - returns the set of HOG filters in this object.
             !*/
 
             unsigned long num_separable_filters(
@@ -510,7 +642,7 @@ namespace dlib
             ensures
                 - If the number of separable filters in a fhog_filterbank is small then the
                   filter bank can be scanned over an image much faster than a normal set of
-                  31 filters.  Therefore, this object provides the option to encourage
+                  filters.  Therefore, this object provides the option to encourage
                   machine learning methods that learn a HOG filter bank (i.e.
                   structural_object_detection_trainer) to select filter banks that have
                   this beneficial property.  In particular, the value returned by