Approximation to new model

  Our model cannot be implemented directly in high-level graphics packages that use Phong shading. However, we can use the Phong lighting model as a basis for approximating our model. This is in the spirit of the non-linear approximation to global illumination used by Walter et al. [24]. In most graphics systems (e.g. OpenGL) we can use negative colors for the lights. We can approximate Equation 2 by two lights in directions $\mbox{${\bf \hat{l}}$}$ and - $-\mbox{${\bf \hat{l}}$}$ with intensities (k_warm - k_cool)/2 and (k_cool - k_warm)/2 respectively, and an ambient term of (k_cool + k_warm)/2. This assumes the object color is set to white. We turn off the Phong highlight because the negative blue light causes jarring artifacts. Highlights could be added on systems with accumulation buffers [11].

This approximation is shown compared to traditional Phong shading and the exact model in Figure 11. Like Walter et al., we need different light colors for each object. We could avoid these artifacts by using accumulation techniques which are available in many graphics libraries.

Edge lines for highly complex objects can be generated interactively using Markosian et al.'s technique [14]. This only works for polygonal objects, so higher-order geometric models must be tessellated to apply that technique. On high-end systems, image-processing techniques [19] could be made interactive. For metals on a conventional API, we cannot just use a light source. However, either environment maps or texture maps can be used to produce alternating light and dark stripes.