Added an example program for the new hough_transform object.

examples/CMakeLists.txt

@@ -39,6 +39,7 @@ add_example(fhog_object_detector_ex)
 add_example(file_to_code_ex)
 add_example(graph_labeling_ex)
 add_example(gui_api_ex)
+add_example(hough_transform_ex)
 add_example(image_ex)
 add_example(integrate_function_adapt_simp_ex)
 add_example(iosockstream_ex)

examples/hough_transform_ex.cpp

+// The contents of this file are in the public domain. See LICENSE_FOR_EXAMPLE_PROGRAMS.txt
+/*
+
+    This is an example illustrating the use of the Hough transform tool in the
+    dlib C++ Library.
+
+
+    In this example we are going to draw a line on an image and then use the
+    Hough transform to detect the location of the line.  Moreover, we do this in
+    a loop that changes the line's position slightly each iteration, which gives
+    a pretty animation of the Hough transform in action.
+*/
+
+#include <dlib/gui_widgets.h>
+#include <dlib/image_transforms.h>
+
+using namespace dlib;
+
+int main()
+{
+    // First let's make a 400x400 image.  This will form the input to the Hough transform.
+    array2d<unsigned char> img(400,400);
+    // Now we make a hough_transform object.  The 300 here means that the Hough transform
+    // will operate on a 300x300 subwindow of its input image.  
+    hough_transform ht(300);
+
+    image_window win, win2;
+    double angle1 = 0;
+    double angle2 = 0;
+    while(true)
+    {
+        // Generate a line segment that is rotating around inside the image.  The line is
+        // generated based on the values in angle1 and angle2. So each iteration creates a
+        // slightly different line.
+        angle1 += pi/130;
+        angle2 += pi/400;
+        const point cent = center(get_rect(img));  
+        // A point 90 pixels away from the center of the image but rotated by angle1.
+        const point arc = rotate_point(cent, cent + point(90,0), angle1); 
+        // Now make a line that goes though arc but rotate it by angle2.
+        const point l = rotate_point(arc, arc + point(500,0), angle2);
+        const point r = rotate_point(arc, arc - point(500,0), angle2);
+
+
+        // Next, blank out the input image and then draw our line on it.
+        assign_all_pixels(img, 0);
+        draw_line(img, l, r, 255);
+
+         
+        const point offset(50,50);
+        array2d<int> himg;
+        // pick the window inside img on which we will run the Hough transform.
+        const rectangle box = translate_rect(get_rect(ht),offset);
+        // Now let's compute the hough transform for a subwindow in the image.  In
+        // particular, we run it on the 300x300 subwindow with an upper left corner at the
+        // pixel point(50,50).  The output is stored in himg.
+        ht(img, box, himg);
+        // Now that we have the transformed image, the Hough image pixel with the largest
+        // value should indicate where the line is.  So we find the coordinates of the
+        // largest pixel:
+        point p = max_point(mat(himg));
+        // And then ask the ht object for the line segment in the original image that
+        // corresponds to this point in Hough transform space.
+        std::pair<point,point> line = ht.get_line(p);
+
+        // Finally, let's display all these things on the screen.  We copy the original
+        // input image into a color image and then draw the detected line on top in red.
+        array2d<rgb_pixel> temp;
+        assign_image(temp, img);
+        // Note that we must offset the output line to account for our offset subwindow.
+        // We do this by just adding in the offset to the line endpoints. 
+        draw_line(temp, line.first+offset, line.second+offset, rgb_pixel(255,0,0));
+        win.clear_overlay();
+        win.set_image(temp);
+        // Also show the subwindow we ran the Hough transform on as a green box.  You will
+        // see that the detected line is exactly contained within this box and also
+        // overlaps the original line.
+        win.add_overlay(box, rgb_pixel(0,255,0));
+
+        // We can also display the Hough transform itself using the jet color scheme.
+        win2.set_image(jet(himg));
+    }
+}
+