3D schematic of the central “ring” region of NGC 6781 showing its orientation with respect to us, in which the cylindrical barrel, barrel cavity, and inner wall of the cavity, as well as the locations of the multipoint observations, are identified as a visual guide for readers. Note that the gray-scale image represents an isodensity surface of the barrel structure and that the density distribution is NOT necessarily confined within the surface shown here. This image is reproduced from the work by Schwarz & Monteiro (2006) with permission of the American Astronomical Society.