VisibilitySensor

  VisibilitySensor { 
      exposedField SFVec3f center   0 0 0      # (-,)
      exposedField SFBool  enabled  TRUE
      exposedField SFVec3f size     0 0 0      # [0,)
      eventOut     SFTime  enterTime
      eventOut     SFTime  exitTime
      eventOut     SFBool  isActive
    }