There are several ways to apply NPR shading models using hardware [1,2]. In our ``Interactive Technical Illustration'' paper [4], we chose to use environment maps because they provide the most flexibility in the shading model. This effectively allows us to evaluate a lighting model at every normal/reflection direction in the visible hemisphere in eye-space.
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Shading is rendered using one of three modes. The first two are the diffuse and metallic shading models [2] presented in Section 8. In its simplest form the diffuse cool to warm shading model interpolates from a cool (blue-green) to a warm (yellow-orange) color based on the surface normal. This cool-to-warm diffuse shading is shown in Figure 6(a). The third method is an adaptation of this cool to warm shading, simulating the more dramatic shading effects sometimes used by artists. Figure 6(b) illustrates the effect achieved when the reflected light from the left of the object produces a back-splash of light opposite the direct lighting source. This is accomplished by modifying the model of [2] with a simple multiplier:
where and p are free parameters which, for this image, are set to 0.76 and 0.78, respectively.We evaluated the whole shading equation in a Phong environment map. In using an environment map as shown in Figure 5, all normals in eye-space are mapped to a 2D texture. This shading only is valid in eye-space, but it is possible to extend these to view-independent environment maps [6]. The cool-to-warm and light ``splashback'' terms are a function of the light direction and could be implemented with this representation. However, the Phong term would have to be computed for each view even though a single light source could be implemented as a single 1D texture map instead of a full 2D texture map.