In the previous section we showed that the ability for illumination cues to ``glue'' an object to another surface does not depend strongly on the details of the illumination cues, provided they are present. For applications where accurate spatial perception is more important than subjective realism, even coarse approximations to indirect illumination, conventional shadows, and diffuse shadows are able to indicate contact in a rendered image. To show the value of this in applications where interactive performance is of critical concern, we have developed an algorithm which is capable of generating perceptually effective contact cues using very crude approximations to shadows and interreflection.3This algorithm is simpler and faster than methods for accurately rendering shadows interactively (e.g., [30]) and is orders of magnitude more efficient than current methods for generating physically based indirect lighting effects. Because it uses projected textures, it also works for irregular ground planes. Unlike obscurance maps [24], this technique is effective in dynamic environments, though with a loss of visual realism.
Figure 9a shows an image generated on a low-end workstation at interactive rates using this technique. Figure 9b shows the same scene rendered with a highly accurate renderer, running on a high-end processor and requiring close to an hour of CPU time. Although the second image is subjectively more realistic, it does not visually imply contact much more effectively than the approximation.