A Cue to Shading: Elongations near Intensity Maxima

The human visual system is often able to recognize shading patterns and to discriminate them from surface reflectance patterns. To understand how this ability is possible, we investigate what makes shading patterns special. We study a statistical property of shading patterns, namely that they tend to be more elongated near intensity maxima. Second-order derivatives of shading and of surface height are compared, and it is shown that intensities typically have an elongated structure relative to surface heights, and this elongation is more extreme near intensity maxima. This elongation property is formalized in terms of a skewness statistic on the aspect ratios of iso-intensity vs. iso-height curves.