Chromatic shadows for improved perception

Soft shadows are effective depth and shape cues. However, traditional shadowing algorithms decrease the luminance in shadow areas. The features in shadow become dark and thus shadowing causes information hiding. For this reason, in shadowed areas, medical illustrators decrease the luminance less and compensate the lower luminance range by adding color, i.e., by introducing a chromatic component. This paper presents a novel technique which enables an interactive setup of an illustrative shadow representation for preventing overdarkening of important structures. We introduce a scalar attribute for every voxel denoted as shadowiness and propose a shadow transfer function that maps the shadowiness to a color and a blend factor. Typically, the blend factor increases linearly with the shadowiness. We then let the original object color blend with the shadow color according to the blend factor. We suggest a specific shadow transfer function, designed together with a medical illustrator which shifts the shadow color towards blue. This shadow transfer function is quantitatively evaluated with respect to relative depth and surface perception.