PERCEPTION OF LOCAL ORIENTATION FROM SHADED IMAGES

The perception of local orientation from shaded images was examined. I n Experiment 1, subjects viewed a boundaryless Gaussian hill and judge d local orientation using both a gauge figure and a pointing method. O ne subject reported an internally consistent surface which was incompa tible with the judged light-source direction and model used to generat e the image. The remaining subjects reported a surface similar to the generating one, and analysis of their results indicated a contour of z ero difference between response and generating slants. This contour of zero slant difference was explored in three subsequent experiments us ing the pointing technique. These experiments investigated possible in fluences of luminance artifact (Experiment 2), perception of global or ientation (Experiment 3), and self-occluding contours (Experiment 4). All three of these experiments yielded results similar to those of Exp eriment 1, with distinct contours of zero slant difference. This conto ur was explored for relationships with the simulated slant of the gene rating surface and the differential structure of image intensity. This analysis indicated that the contour of zero slant difference was appr oximately a line of constant slant which shared large regions of adjac ency to the zero crossings of the second directional derivative of ima ge intensity.