
166 Image Processing for Cinema
light world without reference sheets of white paper” [243]. The sensation of
white will be generated by the area of maximum radiance in all three bands
(this is the von Kries’ model or “white-patch” assumption, although Land
doesn’t cite it); this area could be used as reference, but Land didn’t know
how our visual system could “ascertain the reflectance of an area without in
effect placing a comparison standard next to the area” [243]. The solution he
proposed consisted of comparing far-away points through paths: the ratio of
the values at the two end-points of the path can be computed as the sequential
product of the