Made the hough_transform's functions for getting line coordinates use dpoint

instead of point.

dlib/image_transforms/hough_transform.h

@@ -68,12 +68,12 @@ namespace dlib
         long nc(
         ) const { return _size; }
 
-        std::pair<point, point> get_line (
-            const point& p
+        std::pair<dpoint, dpoint> get_line (
+            const dpoint& p
         ) const
         {
             DLIB_ASSERT(rectangle(0,0,size()-1,size()-1).contains(p) == true,
-                "\t pair<point,point> hough_transform::get_line(point)"
+                "\t pair<dpoint,dpoint> hough_transform::get_line(dpoint)"
                 << "\n\t Invalid arguments given to this function."
                 << "\n\t p:      " << p 
                 << "\n\t size(): " << size()
@@ -90,8 +90,8 @@ namespace dlib
             const dpoint cent = center(box);
             dpoint v1 = cent + dpoint(size()+1000,0) + dpoint(0,radius);
             dpoint v2 = cent - dpoint(size()+1000,0) + dpoint(0,radius);
-            point p1 = rotate_point(cent, v1, theta);
-            point p2 = rotate_point(cent, v2, theta);
+            dpoint p1 = rotate_point(cent, v1, theta);
+            dpoint p2 = rotate_point(cent, v2, theta);
 
             clip_line_to_rectangle(box, p1, p2);
 
@@ -99,7 +99,7 @@ namespace dlib
         }
 
         double get_line_angle_in_degrees (
-            const point& p 
+            const dpoint& p 
         ) const
         {
             double angle, radius;
@@ -108,7 +108,7 @@ namespace dlib
         }
 
         void get_line_properties (
-            const point& p,
+            const dpoint& p,
             double& angle_in_degrees,
             double& radius
         ) const

dlib/image_transforms/hough_transform_abstract.h

@@ -65,8 +65,8 @@ namespace dlib
                 - returns size()
         !*/
 
-        std::pair<point, point> get_line (
-            const point& p
+        std::pair<dpoint, dpoint> get_line (
+            const dpoint& p
         ) const;
         /*!
             requires
@@ -79,7 +79,7 @@ namespace dlib
         !*/
 
         double get_line_angle_in_degrees (
-            const point& p 
+            const dpoint& p 
         ) const;
         /*!
             requires
@@ -91,7 +91,7 @@ namespace dlib
         !*/
 
         void get_line_properties (
-            const point& p,
+            const dpoint& p,
             double& angle_in_degrees,
             double& radius
         ) const;
@@ -107,6 +107,10 @@ namespace dlib
                 - #radius == the distance from the center of the input image, measured in
                   pixels, and the line corresponding to the Hough transform point p.
                   Moreover: -sqrt(size()*size()/2) <= #radius <= sqrt(size()*size()/2)
+                - Note that the line properties are calculated to sub-pixel accuracy.  That
+                  is, p doesn't have to contain integer values, it can reference locations
+                  between pixels and the appropriate calculation will be done to find the
+                  corresponding line.
         !*/
 
         template <