The NIFTI format uses packed RGB, but the older Analyze format, in its
earliest incarnations, used planar RGB.  For the sake of compatibility
with these old Analyze files, or with NIFTI files that were created from
them without a planar-to-packed conversion, a PlanarRGB option has been
added to the vtkNIFTIImageReader and the vtkNIFTIImageWriter.