Added comments

examples/video_tracking_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 Bulk Synchronous Parallel (BSP)
-    processing tools from the dlib C++ Library.  These tools allow you to easily setup a
-    number of processes running on different computers which cooperate to compute some
-    result.  
+    This example shows how to use the correlation_tracker from the dlib C++ library.  This
+    object lets you track the position of an object as it moves from frame to frame in a
+    video sequence.  To use it, you give the correlation_tracker the bounding box of the
+    object you want to track in the current video frame.  Then it will identify the
+    location of the object in subsequent frames.
 
+    In this particular example, we are going to run on the video sequence that comes with
+    dlib, which can be found in the examples/video_frames folder.  This video shows a juice
+    box sitting on a table and someone is waving the camera around.  The task is to track the
+    position of the juice box as the camera moves around.
 */
 
 #include <dlib/image_processing.h>
@@ -26,6 +31,7 @@ int main(int argc, char** argv) try
         return 1;
     }
 
+    // Get the list of video frames.  
     std::vector<file> files = get_files_in_directory_tree(argv[1], match_ending(".jpg"));
     std::sort(files.begin(), files.end());
     if (files.size() == 0)
@@ -34,12 +40,17 @@ int main(int argc, char** argv) try
         return 1;
     }
 
+    // Load the first frame.  
     array2d<unsigned char> img;
     load_image(img, files[0]);
-
+    // Now create a tracker and start a track on the juice box.  If you look at the first
+    // frame you will see that the juice box is centered at pixel point(92,110) and 38
+    // pixels wide and 86 pixels tall.
     correlation_tracker tracker;
     tracker.start_track(img, centered_rect(point(93,110), 38, 86));
 
+    // Now run the tracker.  All we have to do is call tracker.update() and it will keep
+    // track of the juice box!
     image_window win;
     for (unsigned long i = 1; i < files.size(); ++i)
     {