Abstract
While structure-from-motion (SFM) studies have largely focused on the perception of rigidly moving 3D objects, more recent studies have shown that observers are good at perceiving certain kinds of non-rigid transformations (Jain & Zaidi, 2011). Our aim is to understand what types of non-rigid transformations are perceivable in SFM. In previous work (Choi et al., VSS, 2019) we compared the perception of two non-rigid transformations: part-orientation change (part-wise rigid motion, e.g. articulation of limbs) and part-length change. We observed a misperception of length change as orientation change. This misperception existed for each observer over a range of length-change values. Here we further investigated the misperception of non-rigid length change. Stimuli consisted of an ellipsoid with a protruding part (perpendicular to the main body, fully visible in the silhouette) that non-rigidly changed in length as the whole object rotated back and forth. We observed a misperception of length change as a rigidly-attached part with an “illusory” non-orthogonal horizontal angle relative to the ellipsoid. In an adjustment task, observers matched the perceived angle between the part and the ellipsoid for seven levels of length change. We compared the perceived horizontal angle to model predictions based on a reinterpretation of length change as a fixed non-orthogonal angle between the part and the body. We equated the ratios of the projected lengths of the part from the starting length:ending length for both ground truth and reinterpretation. We could then make predictions of perceived horizontal angle for each magnitude of length change. Even with no free parameters, the model closely tracked observers’ data. Together, the results show that the visual system is biased towards part-wise rigid interpretations of SFM (such as biological articulation of limbs).