"vtkPicker may return more than one vtkProp3D, since more than one bounding box may be intersected.
vtkPicker returns an unsorted list of props that were hit, and a list of the corresponding world points of the hits.
For the vtkProp3D that is closest to the camera, vtkPicker returns the pick coordinates in world and untransformed mapper space,
the prop itself, the data set, and the mapper.
However, it is unclear how to obtain the closest vtkProp3D. picker.getActors() works but there is no picker.getClosetActor() function.