Purpose. To see how perceived shape varies with changes in illumination. Methods. In shape from shading the structure of surface geometry is derived from smooth variations in image luminance by associating the image luminance field with distal surface attitude with respect to the illuminant. However, if shape is derived solely according to this relationship then even for simple surfaces derived shape will vary with the movement of the light source (first order illumination). The effects of illuminant direction on shape from shading was investigated by using different computer generated stimuli of the same shape illuminated from different positions. Two factors were identified as possible contributors to shape stability under such conditions: surface albedo and surface markings. The first factor is important because for dark surfaces (low albedo) image luminance is almost entirely a function of the first order illumination and so shape variance should be more marked for dark surfaces than for white ones. Surface markings are also relevant because the 3D information they provide remains invariant under illumination changes. We tested the influence of these two factors in two experiments using a local attitude probe. The probe consisted of a circle with a central needle orthographically projected at various locations across the stimulus. Subjects adjusted the projection of the probe so that it appeared to lie flat on the depicted surface. The locations of the probe formed the lattice of a triangulation which could be used to form a 3D representation (reconstruction) of the perceived surface according to the subjects attitude settings. Results. In all cases reconstructions from subjects settings varied systematically according to the direction of the primary illuminant; however, this variation was reduced, but not eliminated, for surfaces with high albedo and those with surface markings. Conclusion. Shape from shading appears to be derived according to a simple brighter-is-closer principle and much of the observed stability in shape under variations in illumination must be a result of contraints imposed by surface markings and, possibly, outline contour.
|Journal||Investigative Ophthalmology and Visual Science|
|Publication status||Published - 15 Feb 1996|