 The illusion is limited, however; phantograms lack some cues for depth perception such as convincing parallax, so the viewer must be stationary at the illusion's "sweet spot", a specific point at which the phantogram is designed to be most convincing.The anamorphic distortion of the source image crucial to the illusion can be understood by likening the images to projections of a 3D object onto a plane (e.g.a sheet of paper) originating from the location of the viewer's eyes.