A regular hexagon, when partially occluded in a coincidental way, appears distorted. Da Pos and Zambianchi (1996) labelled the effect “the Gerbino illusion” and indicated its antecedent in a variant of the Poggendorff illusion studied by Burmester (1896, fig. 15). As noticed by Metzger (1953, fig. 187; 1975, fig. 194) the same distortion can be observed in a real three-dimensional setting, when a far Gothic arch is coincidentally occluded by a near pillar.
