title: Perception of illumination direction in images of 3-D convex objects: influence of surface materials and light fields.
abstract: We investigated the perception of illumination direction in images of 3-D convex objects under variations of light field and surface material properties. In a first experiment, we used an illumination-matching procedure in order to measure observers' ability to estimate the direction of illumination in images of 3-D polyhedra rendered under different light fields and illumination directions. Match deviations were larger in frontal direction than in rear directions, mainly counterclockwise in azimuth component, and diverged, in elevation component, from the image plane. In a second experiment, we examined whether the direction estimate was affected by the surface material type (BRDF), the light field, and the illumination direction. Angular deviations varied with material surface type and were largest in the test elevation direction 0 degrees. Elevation component deviations also differed with surface type and were larger in hemispherical diffuse lighting than in collimated lighting. These results suggest that the direction estimation is better with images of evenly distributed intensity gradients than with those of drastically varying gradients, and that the visual system may not take intensity variations due to the surface material or the light field into account in estimating the direction of illumination.
authors.affiliation: Department of Physics of Man, Helmholtz Institute, Utrecht University, Princetonplein 5, 3584 CC Utrecht, The Netherlands. bgkhang@sunyopt.edu
authors.forename: Byung-Geun
authors.initials: BG
authors.lastname: Khang
authors.forename: Jan J
authors.initials: JJ
authors.lastname: Koenderink
authors.forename: Astrid M L
authors.initials: AM
authors.lastname: Kappers
mesh_terms: D003463:Cues; D003867:Depth Perception; D006801:Humans; D008027:Light; D008029:Lighting; D008433:Mathematics; D009949:Orientation; D010364:Pattern Recognition, Visual; D010775:Photic Stimulation / Q000379:methods; D011601:Psychophysics; D012542:Scattering, Radiation; D013499:Surface Properties; D014796:Visual Perception