The effects of lighting direction and elevation on judgements of shape-from-shading.

Knowledge of the lighting direction is crucial in recovering shape-from-shading. Many studies have shown that we adopt a lighting-from-above assumption when the true lighting direction is unknown or ambiguous. Others have shown that perceived lighting direction depends on the orientation structure in the shading pattern itself. Many studies have tested the effects of lighting azimuth variations on the perception of shape-from-shading. We test the effects of varying the declination (elevation) of the light source. We generated images of three different surfaces (differing in their degree of anisotropy) illuminated with spotlights at 6 angles of declination (0°, frontal to 75° degrees, oblique) with constant azimuth. These surfaces were generated from Gabor noise textures with surface heights determined by the ‘grey’ values of each texture. We tested images at two orientations: upright (with the surface lit from the right), or rotated (lit from above). Varying declination affected the contrast of the resulting shading patterns so we also tested images after normalising their RMS contrasts. Observers set the tilt and slant of a probe disk to match the perceived 3D surface orientation. We obtained observations at nine locations per image. We found that tilt setting for the most anisotropic surface depended on image orientation whereas for the isotropic surfaces tilt was determined by the lighting-from-above prior. Slant settings tended to increase in proportion to local luminance/contrast, and normalising RMS contrast greatly reduced the range of slant settings. In conclusion we found that shape judgments in isotropic shading patterns are more affected by the lighting-from-above-prior than those for anisotropic surfaces and that slant judgments depend on local luminance/contrast and are affected by changes in the declination of the light source; when this affects the contrast of the shading pattern.

Meeting abstract presented at VSS 2012