XY is the image-plane. The  Z-axis represents the direction in depth. All 3D shapes are recovered from the image on the image-plane. The aspect-ratios of these 3D shapes are different and they vary with the change of the slant of the symmetry-plane. In this animation, the slant of the symmetry-plane is varied from 24° to 55°. When the slant is close to 90° or 0°, the aspect-ratio of the 3D shapes recovered ranges from zero to infinity.