Shape perception from shading under natural lighting conditions.

摘要

Much of the recent research on the visual perception of shape from shading has employed computer generated images, which often do not include several basic components of shading that are common in natural scenes, such as cast shadows, indirect illuminations and specular highlights. In addition, the ambiguity in shaded images has been a puzzle for the studies on "shape perception from shading" since the 18th century.; The present research was designed to investigate the effects of the three components (i.e. cast shadows, indirect illuminations and specular highlights) on the shape perception and also the process of resolving the ambiguity in shaded images.; In the first three experiments, convex and concave half spheroids were used as stimuli. The presence of each of the three components could be turned on or off independently, the illumination direction could be from top or bottom, and the images were presented to observers in all possible combinations. Both the perception of the sign of curvature and the depth magnitude were tested. The results revealed clear biases in observers' judgments of the sign of curvature (i.e. convexity bias and overhead illumination bias). Different observers had different biases. Observers' estimations of depth magnitude were affected by the sign of curvature, the presence or absence of cast shadows and the shininess of surfaces.; The fourth and fifth experiments used images of randomly shaped objects as stimuli. Shape perception was very accurate comparing to the real shapes and it was also reliable across subjects and different illumination directions. The perception of complex shapes seemed to be more stable than convex and concave bumps. Even when the occluding contour of the complex shapes was masked off, shape perception was still very accurate and reliable across observers and illumination directions.; The estimations of illumination directions were also tested. The results showed close relationship between the estimated illumination direction and the judgment of the sign of curvature for convex and concave bumps. However, the illumination information may not be necessary for the perception of complex shapes.