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Marina Bloj, Alexa I. Ruppertsberg; The role of mutual illumination in gradient formation. Journal of Vision 2006;6(6):246. doi: 10.1167/6.6.246.
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Psychophysical (Bloj et al., Nature, 402; Gilchrist & Jacobsen, Perception, 13) and computational (Funt & Drew, IEEE PAMI, 15; Nayar et al., Int. J Comp Vis, 6) studies have postulated that mutual illumination (indirect illumination due to light reflected by surfaces in a scene) provides cues for colour and shape perception. We have previously shown that RADIANCE can be used to accurately simulate the chromaticity and luminance of scenes (Ruppertsberg & Bloj, JOSA-A, in press). Here, we use this rendering package to study and analyse how mutual illumination varies as a function of light and surface position, in an effort to understand what potential cues are available to the visual system for the recovery of scene layout and colour. Study scenes included corners made from two cards with various hinge angles and different illumination angles as well as a cylinders and other objects. For the corners the scatter plot of the chromaticity profile for a given illumination angle forms a straight line in the CIE chromaticity plane, which shifts systematically toward the “whiter” region of the plane as the illumination angle increases. The slopes of the luminance profiles become steeper and the overall luminance level increases. For the cylinder scene the resulting chromaticity profile is close to a straight line in the chromaticity plane and the luminance profile is non-linear. In summary we find that the colours of the involved objects and lights determine chromaticity profiles of colour gradients, and the luminance profiles convey information about geometry, including illumination angle and/or object shape.
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