diff --git a/common/rfb/UpdateTracker.cxx b/common/rfb/UpdateTracker.cxx
index 14ac49d..79f43f6 100644
--- a/common/rfb/UpdateTracker.cxx
+++ b/common/rfb/UpdateTracker.cxx
@@ -60,6 +60,10 @@
     ut->add_changed(tmp);
 }
 
+void ClippingUpdateTracker::set_video_area(const Rect &rect) {
+  ut->set_video_area(rect.intersect(clipRect));
+}
+
 // SimpleUpdateTracker
 
 SimpleUpdateTracker::SimpleUpdateTracker(bool use_copyrect) {
@@ -135,6 +139,18 @@
   return;
 }
 
+void SimpleUpdateTracker::set_video_area(const Rect &rect)
+{
+  video_area = rect;
+}
+
+//
+// subtract() is called to mark some region as unchanged. We just remove that
+// region from both `copied' and `changed' regions. Note that `video_area' is
+// not affected intentionally; we assume that video is continuously changing,
+// so it should always be treated as "changed".
+//
+
 void SimpleUpdateTracker::subtract(const Region& region) {
   copied.assign_subtract(region);
   changed.assign_subtract(region);
@@ -142,10 +158,15 @@
 
 void SimpleUpdateTracker::getUpdateInfo(UpdateInfo* info, const Region& clip)
 {
-  copied.assign_subtract(changed);
+  changed.assign_subtract(video_area);
+  copied.assign_subtract(changed.union_(video_area));
   info->changed = changed.intersect(clip);
   info->copied = copied.intersect(clip);
   info->copy_delta = copy_delta;
+  // FIXME: Using get_bounding_rect() is not what should actually be done!
+  //        We should use the biggest inner rectangle of the `clip' instead.
+  //        From the other side, `clip' is usually just a sole rectangle.
+  info->video_area = video_area.intersect(clip.get_bounding_rect());
 }
 
 void SimpleUpdateTracker::copyTo(UpdateTracker* to) const {
@@ -153,4 +174,5 @@
     to->add_copied(copied, copy_delta);
   if (!changed.is_empty())
     to->add_changed(changed);
+  to->set_video_area(video_area);
 }