
156 Image Processing for Cinema
– Color perception works based on perceived color differences.
– Nearby elements have greater influence than distant ones.
– Any element of any scene can be replaced by a stimulus perceived as
achromatic (gray or white).
– The difference between the scene element and the achromatic stimulus
associated with it is what determines the color perceived at that element.
For uniformly illuminated scenes, Judd found in 1940 that the achromatic
stimulus can be estimated easily, having the same chromaticity as the scene
average and about half the luminance: this is precisely the “Gray world” ap-
proach to white balance and color constancy ...