From 03a34a69d1ce11ab58144337be0aa24096d5920d Mon Sep 17 00:00:00 2001
From: Davis King <davis@dlib.net>
Date: Sun, 8 Dec 2013 10:46:51 -0500
Subject: [PATCH] Added threshold_filter_singular_values()

---
 dlib/image_processing/scan_fhog_pyramid.h     | 64 +++++++++++++++++++
 .../scan_fhog_pyramid_abstract.h              | 27 ++++++++
 2 files changed, 91 insertions(+)

diff --git a/dlib/image_processing/scan_fhog_pyramid.h b/dlib/image_processing/scan_fhog_pyramid.h
index cf9fa468b..c81b8cf77 100644
--- a/dlib/image_processing/scan_fhog_pyramid.h
+++ b/dlib/image_processing/scan_fhog_pyramid.h
@@ -916,6 +916,70 @@ namespace dlib
         return fb.num_separable_filters();
     }
 
+// ----------------------------------------------------------------------------------------
+
+    template <
+        typename Pyramid_type
+        >
+    object_detector<scan_fhog_pyramid<Pyramid_type> > threshold_filter_singular_values (
+        const object_detector<scan_fhog_pyramid<Pyramid_type> >& detector,
+        double thresh,
+        const unsigned long weight_index = 0
+    )
+    {
+        // make sure requires clause is not broken
+        DLIB_ASSERT(thresh > 0 ,
+            "\t object_detector threshold_filter_singular_values()"
+            << "\n\t Invalid inputs were given to this function."
+            << "\n\t thresh: " << thresh 
+        );
+
+        DLIB_ASSERT(weight_index < detector.num_detectors(),
+            "\t object_detector threshold_filter_singular_values()"
+            << "\n\t Invalid arguments were given to this function. "
+            << "\n\t weight_index:             " << weight_index
+            << "\n\t detector.num_detectors(): " << detector.num_detectors()
+            );
+        DLIB_ASSERT(detector.get_w(weight_index).size() >= detector.get_scanner().get_num_dimensions() ,
+            "\t object_detector threshold_filter_singular_values()"
+            << "\n\t Invalid arguments were given to this function. "
+            << "\n\t detector.get_w(weight_index).size():         " << detector.get_w(weight_index).size()
+            << "\n\t detector.get_scanner().get_num_dimensions(): " << detector.get_scanner().get_num_dimensions()
+            );
+
+
+        const unsigned long width = detector.get_scanner().get_fhog_window_width();
+        const unsigned long height = detector.get_scanner().get_fhog_window_height();
+        const long size = width*height;
+
+        std::vector<matrix<double,0,1> > detector_weights;
+        for (unsigned long j = 0; j < detector.num_detectors(); ++j)
+        {
+            matrix<double,0,1> weights = detector.get_w(j);
+
+            if (j == weight_index)
+            {
+                matrix<double> u,v,w,f;
+                for (int i = 0; i < 31; ++i)
+                {
+                    f = reshape(rowm(weights, range(i*size, (i+1)*size-1)), height, width);
+
+                    svd3(f, u,w,v);
+                    const double scaled_thresh = std::max(1e-3, max(w)*thresh);
+                    w = round_zeros(w, scaled_thresh);
+                    f = u*diagm(w)*trans(v);
+
+                    set_rowm(weights,range(i*size, (i+1)*size-1)) = reshape_to_column_vector(f);
+                }
+            }
+            detector_weights.push_back(weights);
+        }
+        
+        return object_detector<scan_fhog_pyramid<Pyramid_type> >(detector.get_scanner(), 
+                                                                 detector.get_overlap_tester(),
+                                                                 detector_weights);
+    }
+
 // ----------------------------------------------------------------------------------------
 
     template <
diff --git a/dlib/image_processing/scan_fhog_pyramid_abstract.h b/dlib/image_processing/scan_fhog_pyramid_abstract.h
index 86354d224..3b70adfc9 100644
--- a/dlib/image_processing/scan_fhog_pyramid_abstract.h
+++ b/dlib/image_processing/scan_fhog_pyramid_abstract.h
@@ -54,6 +54,33 @@ namespace dlib
               defined by detector.get_w(weight_index).
     !*/
 
+// ----------------------------------------------------------------------------------------
+
+    template <
+        typename Pyramid_type
+        >
+    object_detector<scan_fhog_pyramid<Pyramid_type> > threshold_filter_singular_values (
+        const object_detector<scan_fhog_pyramid<Pyramid_type> >& detector,
+        double thresh,
+        const unsigned long weight_index = 0
+    );
+    /*!
+        requires
+            - thresh > 0
+            - weight_index < detector.num_detectors()
+            - detector.get_w(weight_index).size() >= detector.get_scanner().get_num_dimensions()
+              (i.e. the detector must have been populated with a HOG filter)
+        ensures
+            - Removes all components of the filters in the given detector that have
+              singular values that are smaller than the given threshold.  Therefore, this
+              function allows you to control how many separable filters are in a detector.
+              In particular, as thresh gets larger the quantity
+              num_separable_filters(threshold_filter_singular_values(detector,thresh,weight_index),weight_index)
+              will generally get smaller and therefore give a faster running detector.
+              However, note that at some point a large enough thresh will drop too much
+              information from the filters and their accuracy will suffer.  
+    !*/
+
 // ----------------------------------------------------------------------------------------
 
     template <