User draw lines directly on screen using available tools (ellipse, lasso or polygon). Lines are then extruded to create shapes that are combined using boolean operations (union, intersection or subtraction) so that complex shapes can be easily defined. Finally, the resulting shape is used to clip the 3DHEAD model thanks to a VolumeClippingGroup.
SoVolumeRender, SoVolumeClippingGroup, SoCSGShape, SbExtrusionGenerator, SoLassoScreenDrawer, SoEllipseScreenDrawer, SoPolygonScreenDrawer