Made the detection threshold an argument to get_surf_points() rather than having it hard

coded.  I also increased the default threshold to a more reasonable value.  The previous
value hardly excluded any points and gave a slightly worse average precision on a standard
test dataset.

dlib/image_keypoint/surf.h

@@ -237,13 +237,15 @@ namespace dlib
     template <typename image_type>
     const std::vector<surf_point> get_surf_points (
         const image_type& img,
-        long max_points
+        long max_points,
+        double detection_threshold = 30.0
     )
     {
-        DLIB_ASSERT(max_points > 0,
+        DLIB_ASSERT(max_points > 0 && detection_threshold >= 0,
             "\t std::vector<surf_point> get_surf_points()"
-            << "\n\t invalid arguments to this function"
+            << "\n\t Invalid arguments were given to this function."
             << "\n\t max_points:          " << max_points 
+            << "\n\t detection_threshold: " << detection_threshold 
         );
 
         // Figure out the proper scalar type we should use to work with these pixels.  
@@ -260,7 +262,7 @@ namespace dlib
 
         // now get all the interest points from the hessian pyramid
         std::vector<interest_point> points; 
-        get_interest_points(pyr, 0.10, points);
+        get_interest_points(pyr, detection_threshold, points);
         std::vector<surf_point> spoints;
 
         // sort all the points by how strong their detect is

dlib/image_keypoint/surf_abstract.h

@@ -126,11 +126,13 @@ namespace dlib
     template <typename image_type>
     const std::vector<surf_point> get_surf_points (
         const image_type& img,
-        long max_points
+        long max_points,
+        double detection_threshold = 30.0
     );
     /*!
         requires
             - max_points > 0
+            - detection_threshold >= 0
             - image_type == a type that implements the array2d/array2d_kernel_abstract.h interface
             - pixel_traits<image_type::type> must be defined
         ensures
@@ -146,6 +148,7 @@ namespace dlib
                       compute_surf_descriptor())
                     - V[i].angle == the angle of the SURF box at this point (calculated using 
                       compute_dominant_angle())
+                    - V[i].p.score >= detection_threshold
     !*/
 
 // ----------------------------------------------------------------------------------------