Colormaps for Shaded Surfaces: Stepped vs Smooth

It is common in scientific visualization for one 2D scalar field variable to be colormapped and draped onto a shaded surface representing a second variable displayed as a height field. The experiments reported here investigate the properties that make a colormap suitable for this application; specifically, how much colormap accuracy is lost because of draping and shading as well as the degree to which the colormap impacts the perception of surface shape. A new task is used to evaluate surface shape perception; it involves study participants clicking on the peaks of hills and the bottoms of valleys in the display. The results provided surprisingly little support for the hypothesis that colormaps varying greatly in luminance impacted surface shape perception more than those that are close to isoluminant. Stepped colormaps proved to be more accurately read. However, stepped colormaps also interfered much more with the perception of surface shape. The implication of these results are that when selecting a colormap to be used for draping, careful consideration should be given to which of the two variables is more important, the one represented by the shaded surface, or the one which is colormapped and draped.