Takustraße 7 D-14195 Berlin-Dahlem Germany Konrad-Zuse-Zentrum f¨ ur Informationstechnik Berlin ALEXANDER WIEBEL ,F RANS M. VOS UND HANS -C HRISTIAN HEGE Perception-Oriented Picking of Structures in Direct Volumetric Renderings ZIB-Report 11-45 (Dezember 2011)