Perceived 3D metric (or Euclidean) shape is merely ambiguous, not systematically distorted.

Many studies have reported that perceived shape is systematically distorted, but Lind et al. (Inf Vis 2:51-57, 2003) and Todd and Norman (Percept Psychophys 65:31-47, 2003) both found that distortions varied with tasks and observers. We now investigated the hypothesis that perception of 3D metric (or Euclidean) shape is ambiguous rather than systematically distorted by testing whether variations in context would systematically alter apparent distortions. The task was to adjust the aspect ratio of an ellipse on a computer screen to match the cross-section of a target elliptical cylinder object viewed in either frontoparallel elliptical cross-section (2D) or elliptical cross-section in depth (3D). Three different groups were tested using two tasks and two different ranges of aspect ratio: Group 1) 2D(Small) → 3D(Large), Group 2) 2D(Large) → 3D(Small), Group 3a) 2D(Small) → 3D(Small), and Group 3b) 2D(Large) → 3D(Large). Observers performed the 2D task accurately. This provided the context. The results showed the expected order of slopes when judged aspect ratios were regressed on actual aspect ratios: Group 1 (SL) < Group 3 (SS and LL) < Group 2 (LS). The ambiguity of perceived 3D aspect ratios allowed the range of aspect ratios experienced in the 2D task to affect the 3D judgments systematically. Nevertheless, when the 2D and 3D ranges of aspect ratios were the same (LL and SS) and the 2D were judged accurately, this did not yield accurate 3D judgments. The results supported the hypothesis that perceived 3D metric shape is merely ambiguous rather than systematically distorted.