small bugfix

semseg/SemSegContextTree.cpp

@@ -1620,7 +1620,7 @@ void SemSegContextTree::addFeatureMaps (
     }
   }
 
-  int classes = labelmapback.size();
+  int classes = classNames->numClasses();
 
   if ( useFeat3 )
     imgData.addChannel ( classes );