How well does motion convey an object's shape? It depends on your viewpoint

Classical structure from motion schemes tend not to differentiate explicitly among viewpoints. However, intuitively it seems easier to answer specific questions about objects’ geometry from some viewpoints that others. Correspondingly, people seem to recognise “bad” viewpoints, and may seek out “good” ones. An experiment tested this intuition. Subjects answered questions about geometric relations between two edges in a computer gent:rated structure, using motion cues. The structure could be just the two target edges (giving three re-identifiable points), or an approximate prism defined by the target edges and two others almost parallel to them (giving six re-identifiable points). Projection involved moderate perspective. Vantage point was defined by imagining the edges lay in the equatorial plane of a sphere, meeting at the centre. Each trial presented views of the structure from points on a circle centred at one location on the sphere. Fifteen such centres were chosen, roughly evenly distributed across o...