The pint glass illusion: Exploring the misjudgment of curvature and length for two and three-dimensional objects: AVA Christmas Meeting Abstracts 2023

Humans are exquisitely sensitive in curvature detection and discrimination, but misjudge the extent of curvature. A largely unexplored phenomenon is the underestimation of the circumference length of curved objects. Anecdotal, and some experimental evidence (Rivera-Aparicio & Firestone, 2018 J Vision 18(10) 1318–1318), suggests that when comparing the circumference of a Pint glass to its height, subjects significantly underestimate the circumference (Pint Glass Illusion, PGI). I present results from two experiments exploring the dependence of the PGI on segment length and type of curve for 2D and 3D shapes. (i) 10 subjects were presented with circular and elliptical arcs (2D) of varying segment length (central angle, CA = 45°–360°) and aspect ratios (AR = 1, 0.5, 0.25). Subjects adjusted a straight line to the arc length. Contrary to the PGI, results show that for all segment lengths, subjects on average overestimated the length with the largest overestimation for CA = 90° and 180° (∼40%), with the most accurate estimation for the full circle and ellipses (∼20%). (ii) To investigate whether results were due to the planar presentation, subjects also judged the arc length of 3D cylindroids (height 14 cm). For elliptical cylindroids with AR = 0.25 (CA = 45°, 90°), subjects underestimate the arc length (∼ 8%). For AR = 0.5 (CA = 45°and 90°), observers overestimate the arc length after which the performance is very accurate for semi and full ellipses and circles. The observed overestimation and most accurate performance for full ellipses and circles implies that the PGI might depend on different strategies, for example, the comparison of height to circumference.