--- a/Framework/Widgets/IWorldSceneMouseTracker.h	Mon Aug 27 16:22:08 2018 +0200
+++ b/Framework/Widgets/IWorldSceneMouseTracker.h	Tue Aug 28 10:01:31 2018 +0200
@@ -13,7 +13,7 @@
  * WITHOUT ANY WARRANTY; without even the implied warranty of
  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  * Affero General Public License for more details.
- * 
+ *
  * You should have received a copy of the GNU Affero General Public License
  * along with this program. If not, see <http://www.gnu.org/licenses/>.
  **/
@@ -25,16 +25,20 @@
 
 namespace OrthancStone
 {
+
+  // this is tracking a mouse in scene coordinates/mm unlike
+  // the IMouseTracker that is tracking a mouse
+  // in screen coordinates/pixels.
   class IWorldSceneMouseTracker : public boost::noncopyable
   {
   public:
     virtual ~IWorldSceneMouseTracker()
     {
     }
-    
+
     virtual void Render(CairoContext& context,
                         double zoom) = 0;
-    
+
     virtual void MouseUp() = 0;
 
     virtual void MouseMove(double x,