SURFACE GRADIENTS, CONTOURS AND THE PERCEPTION OF SURFACE ATTITUDE INIMAGES OF COMPLEX SCENES

Sophisticated computer graphics were used to generate images of three- dimensional blocks-world scenes to investigate the perception of surfa ce attitude. Three types of surface primitive (planar blocks, cylinder s, and ellipsoids) were combined to form structured settings. The expe riments were designed to investigate whether surface-based information such as gradients in shading and texture provide any significant adva ntage in attitude judgments over information derived from object conto urs. Images of shaded, textured, and line-drawn surfaces formed the st imulus set. The subjects' task consisted of setting an attitude probe on different parts of the scene so that the probe appeared to be local ly coplanar with the perceived surfaces. Analysis of settings accordin g to attitude components, slant and till, shows remarkable agreement i n slant settings for the shaded and line-drawn scenes but poor correla tion between shaded and textured scenes. Similarly, tilt was also easi ly judged in shaded and line-drawn scenes and the experiments indicate that explicit surface boundaries are important for stable tilt percep tion. In general, the results suggest that, for the simple surfaces em ployed here, surface cues provide little extra information beyond that which is derived from contours.