In this section we show that shadows and interreflection provide powerful perceptual cues for physical contact between objects and surfaces. As a result, effective rendering of shadowing and indirect lighting can provide benefits even in applications such as virtual reality, where realism in and of itself is not the primary goal.
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Illumination generates powerful perceptual cues indicating that two objects are in physical contact. Figure 2a shows a block sitting on a flat surface, rendered with a mix of direct and diffuse illumination but without any consideration of shadows or interreflections. Figure 2b shows the same configuration, rendered with the addition of a soft shadow and interreflections. There is now a strong sense of contact not apparent in the previous figure. Figure 2c shows a rendering with a shadow but no interreflections. Note that the sense of the front of the object being in contact is diminished compared to Figure 2b. Figure 2d shows the same configuration, this time rendered with interreflections but not a shadow. Note that the interreflections are not at all prominent, but they have a large influence on the perception of contact when compared to Figure 2a. Also note that in Figure 2d that the interreflections ``glues'' the front of the object just as well as the shadow glues the side of the object in Figure 2c. Figures 3a-3d show the same effects for an object positioned just above an extended surface. Figures 2b, 2d and 3b, 3d are effective in portraying whether the object is in contact. This contradicts the idea that indirect lighting is an esoteric effect largely of interest only when photorealistic realism is required. In fact, indirect lighting clearly complements shadowing in establishing a sense of contact. When used on its own as a contact cue, indirect lighting is of comparable effectiveness to shadows, despite being less visually prominent.
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Strong shadow cues disappear under diffuse illumination. Figure 4a shows a block illuminated by lighting that is approximately uniform, without any account being taken of the interactions between the lighting, object, and surface. Figure 4b shows the same arrangement of light source and scene geometry, this time rendered with accurate light transport. While the effect is subtle, it clearly signals contact between object and surface. The information provided about spatial proximity is even more apparent in Figure 4c, in which the block is lifted slightly off of the surface. Comparing Figures 4a and 4b emphasize that a lighting effect whose presence may not be obvious in isolation can still be an important visual cue for spatial organization.
To confirm the subjective sense that both shadows and interrfections act as strong contact cues, a controlled psychological experiment was run [25]. Subjects viewed rendered images of the block and ground plane with no shadows or interrfections, just a shadow, just an interreflection, a shadow and interrfections, two shadows (Figure 5a), two interreflections (Figure 5b), and two shadows and two interreflections. Two sets of images were produced, one with the block in contact with the ground plane and one with the block just above the ground plane. The subjects were quite accurate in their determination of contact when either shadows or interreflections were present. Maximum accuracy came when both shadows and interreflections were present in the image, however the accuracy was only slightly better than either shadows or interreflections. When neither shadows nor interreflections were present, responses were at chance rates, indicating that no other information was available in the images signaling contact.
Applications involving real-time interaction with complex virtual environments need to present users with a clear sense of spatial organization and contact. Based on what we have just shown, shadows and indirect illumination are likely to play an important role in achieving this objective. This is a disquieting observation, since accurate rendering of interreflection is extremely costly. Shadows, while computationally less complex, still require substantial resources to render. This is particularly true when soft shadowing is done. As a result, it is important to probe more deeply into the role shadows and interreflection play in signaling contact between one object and another.