Added get_next_double_click() to the image_ex example.

examples/image_ex.cpp

@@ -69,9 +69,19 @@ int main(int argc, char** argv)
         // also make a window to display the original image
         image_window my_window2(img, "Original Image");
 
+        // Sometimes you want to get input from the user about which pixels are important
+        // for some task.  You can do this easily by trapping user clicks as shown below.
+        // This loop executes every time the user double clicks on some image pixel and it
+        // will terminate once the user closes the window.
+        point p;
+        while (my_window.get_next_double_click(p))
+        {
+            cout << "User double clicked on pixel:         " << p << endl;
+            cout << "edge pixel value at this location is: " << (int)edge_image[p.y()][p.x()] << endl;
+        }
+
         // wait until the user closes the windows before we let the program 
         // terminate.
-        my_window.wait_until_closed();
         win_hot.wait_until_closed();
         my_window2.wait_until_closed();
     }