3.1.1 Ambient Lighting

  The ambient term l_a in Equation 1 is a crude approximation used in conventional ray tracers to avoid computing an indirect lighting term. It is not meant to be physically accurate, but instead to illuminate those areas that are not directly lit by the luminaires. Given this, its main failing is that the uniform intensity causes diffuse objects to appear flat when the surface faces away from the light source. One way to avoid this is to put a fill-light at the eye point, but we feel this is distracting for a moving viewpoint. An alternative is to allow the ambient coefficient to vary with position and orientation: . This can be a simple heuristic, or based on radiosity solutions [13]. Our motivation for using a more sophisticated ambient term is to allow shading variation on surfaces that are not directly lit without the computational cost of adding additional lights. This can be accomplished by assuming the ambient term arises due to illumination from a background divided evenly between two intensities A and B (Figure 8). The angle will vary from zero to radians. For the surface will only ``see'' A so the ambient term will be A. As increases the ambient term will gradually change to (A+B)/2 at , and finally change to B as the surface fully faces the bottom hemisphere. Nusselt's analog [4] allows us to derive the full relationship:

Either the user can set A and B algorithmically or by hand. We set ours by hand, but some heuristics can aid in selection. If we envision the hemisphere-pair as approximating indirect lighting of an object in a room, then the ``walls'' opposite the light are well illuminated and bright. So the hemisphere can be roughly aligned to the light source, with the hemisphere in the direction of the light source darker than the one pointing away from the light source. As advocated by Gooch et al. [12], we can accent the shape using a cool-to-warm color shift by making sure the light source is yellow (warm) and the hemisphere facing away from the light is blue (cool). Our ambient approximation is shown in Figure 9 and is not measurably slower than a constant ambient component for non-trivial models.

  

Figure 8: A surface is illuminated by a hemisphere with colors A and B.

  

 

Figure 9: Left: simple ambient approximation. Right: directionally varying ambient approximation.