Traditional Shading of Matte Objects

  Traditional diffuse shading sets luminance proportional to the cosine of the angle between light direction and surface normal:

where I is the RGB color to be displayed for a given point on the surface, k_d is the RGB diffuse reflectance at the point, k_a is the RGB ambient illumination, is the unit vector in the direction of the light source, and is the unit surface normal vector at the point. This model is shown for k_d = 1 and k_a = 0 in Figure 11. This unsatisfactory image hides shape and material information in the dark regions. Both highlights and edge lines can provide additional information about the object. These are shown alone in Figure 12 with no shading. Edge lines and highlights could not be effectively added to Figure 11 because the highlights would be lost in the light regions and the edge lines would be lost in the dark regions.

  

Figure 11: Diffuse shaded image using Equation 1 with k_d = 1 and k_a = 0. Black shaded regions hide details, especially in the small claws; edge lines could not be seen if added. Highlights and fine details are lost in the white shaded regions.

  

Figure 12: Image with only highlights and edges. The edge lines provide divisions between object pieces and the highlights convey the direction of the light. Some shape information is lost, especially in the regions of high curvature of the object pieces. However, these highlights and edges could not be added to Figure 11 because the highlights would be invisible in the light regions and the silhouettes would be invisible in the dark regions.

To add edge lines to the shading in Equation 1, either of two standard heuristics could be used. First k_a could be raised until it is large enough that the dim shading is visually distinct from the black edge lines, but this would result in loss of fine details. Alternatively, a second light source could be added, which would add conflicting highlights and shading. To make the highlights visible on top of the shading, k_d could be lowered until it is visually distinct from white. An image with hand-tuned k_a and k_d is shown in Figure 13. This is the best achromatic image using one light source and traditional shading. This image is poor at communicating shape information, such as details in the claw nearest the bottom of the image, where it is colored the constant shade k_d k_a regardless of surface orientation.

  

Figure 13: Phong-shaded image with edge lines and k_d = 0.5 and k_a = 0.1. Like Figure 11, details are lost in the dark gray regions, especially in the small claws, where they are colored the constant shade of k_d k_a regardless of surface orientation. However, edge lines and highlights provide shape information that was gained in Figure 12, but could not be added to Figure 11.