Illuminant and Viewpoint Biases from Embossed Surfaces

Abstract

The human visual system uses a-priori constraints for the estimation of surface shape from images. We propose here a robust paradigm to study individual observers' assumptions about the illuminant and viewpoint positions.

In the study of illumination, the stimuli consisted of parallel, sinusoidally shaped, striped regions, alternating between wide and narrow. Narrow stripes alternating between white and black separated the uniform grey stripes, representing slanted edges in light and in shadow. The stimulus had the shape-from-shading ambiguity: either the wide or the narrow stripes could be seen as ‘in-front’, consistent with different assumed tilts of the illuminant. In a brief flash of a randomly oriented stimulus, observers stated whether the narrow or wide stripes appeared in the foreground. The results showed a strong bias for a light-from-above-left assumption (as in Howard et al, 1990 Perception 19 523 – 530; Sun and Perona, 1996 Investigative Ophthalmology and Visual Science 37 935). This bias was 20 to 30 deg to the left of vertical. Slower reaction times were obtained for more ambiguous figures.

The same shape judgment task was used with an unshaded stimulus where the only depth cue was image contour. The same curvy, striped figure was portrayed with image contours at the edges of the stripes, as well as surface markings orthogonal to the depth variation, resulting in a shape-from-contour cue. We have previously reported indirect evidence for a bias of viewpoint above the object, that is observers interpret surface normals as pointing upward (Mamassian, 1995 Perception 24 Supplement, 35). Our observers' shape judgments were consistent with this bias.