Minimal information to determine affine shape equivalence

Participants judged the affine equivalence of 2 simultaneously presented 4- point patterns. Performance level (d') varied between 1.5 and 2.7, dependin g on the information available for solving the correspondence problem (insu fficient in Experiment la, superfluous in Experiment 1b, and minimal in Exp eriments 1c, 2a, 2b) and on the exposure time (unlimited in Experiments 1 a nd 2a and 500 ms in Experiment 2b), but it did not vary much with the compl exity of the affine transformation (rotation and slant in Experiment 1 and same plus tilt in Experiment 2). Performance in Experiment 3 was lower with 3-point patterns than with 4-point patterns, whereas blocking the trials a ccording to the affine transformation parameters had little effect. Determi ning affine shape equivalence with minimal-information displays is based on a fast assessment of qualitatively or quasi-invariant properties such as c onvexity/concavity, parallelism, and collinearity.