Abstract
Specular reflections provide potentially useful information about an object's 3D structure for the moving (Roth & Black, 2006 ) and stationary case, and it has been shown that human observers use reflections when estimating the 3D shape of an object (Fleming, Torralba & Adelson 2004). What happens when a specular object rotates rigidly in depth? Depending on the reliability of the shape information, a rotating ellipsoid can appear non-rigid. If we provide an observer with rich information as to the cause of the optic flow, namely the specular reflections of a natural vs. a not-so-natural environment, will the rotating ellipsoid vary in apparent rigidness?
We used five superellipsoids with decreasing degrees of corner roundedness, ranging from a near-ellipsoidal to a near-cuboidal shape. We also manipulated the perceived shininess that a given environment map elicits by: a) making the pixel histogram of the map Gaussian b) scrambling the map's spatial phase spectrum. Observers viewed short movie clips of rotating superellipsoids under different environment maps and rated rigidness of the object on a scale from 1(least) to 5 (most rigid).
A natural illumination map provides an optic flow pattern consistent with reflections flowing on a surface, whereas a less natural map suggests pigment changes (that might be incorrectly interpreted as stuck to the surface). We hypothesized that ellipsoids which are perceived as shiny, may appear more rigid than those which are perceived as matte and textured. Results show an effect of shape (roundedess) on perceived rigidity, the most roundish object appearing least and the most squarish appearing most rigid, but no effect of the environment map. Human observers were not able to use the specular cues to shape and the specular flow of the rotating roundish ellipsoids - even when they appeared very shiny.