Perspective and the Rotating Trapezoid*

Abstract

Judgments of direction of motion were obtained for computer-generated motion-picture sequences of rectangles, trapezoids, circles, and ellipses rotating about a vertical axis and displayed at five perspectives, simulating viewing distances from infinity to close proximity to the figure. Accuracy ranged from 25% or less at the farthest perspective to close to 100% at the nearest perspective for rectangles and trapezoids. The Ames effect (perceived rotation of the rectangle and oscillation of the trapezoid) was found at intermediate perspectives. Rectangles and trapezoids were also generated with perspective variations in the horizontal or vertical dimensions only. Results for the vertical-perspective sequences were similar, but less accuracy was found for the other displays. With sufficiently close viewing distances, vertical-perspective changes resulting from rotary motion give rise to correct direction judgments but subjects resort to secondary information sources, such as relative sizes of vertical sides, when greater viewing distances render information from motion perspective ambiguous.