To evaluate the appearances of objects and materials, it is necessary to assess both color and geometric attributes of appearance. Color stimuli can be synthesized by mixing proper amounts of three primary colored lights to match visually any unknown. Color is thus tridimensional. By contrast, the geometric attributes (gloss, texture, and distinctness-of-image) cannot be synthesized. Instead, different perceived attributes are selected on the basis of best correlation of measurement results with visual ratings. The best measurement methods are identified by ASTM and other organizations. In practice, however, only the most obvious sources of geometric appearance differences can be measured. Complete analyses of geometric appearance, based on all directions of light distribution, would be too cumbersome to be usable.