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Usability of Digital Cameras for Verifying Physically Based ...

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Figure 66: Surface reflectance values <strong>for</strong>m a comparatively small subset (gray<br />

volume) <strong>of</strong> all possible XYZ colors (produced by Andrea Weidlich).<br />

ues which describe a light – which can be arbitrarily saturated – and surface re-<br />

flectance values, which <strong>for</strong>m a comparatively small subset <strong>of</strong> all possible XYZ<br />

colors [Wys62]; see figure 66 <strong>for</strong> a qualitative sketch. While the shape <strong>of</strong> the sub-<br />

set is not correctly represented in this plot it still shows its key feature as far as<br />

the topic <strong>of</strong> this paper is concerned: even its most saturated colors are located at<br />

comparatively large distances from the gamut boundary, which means that com-<br />

binations <strong>of</strong> surface colors and light values are very unlikely to produce invalid<br />

results.<br />

Since the interactions which potentially produce invalid results are those where<br />

both operands are highly saturated colors (i.e. those which are already near the<br />

boundary <strong>of</strong> the solid <strong>of</strong> valid XYZ colors) this means that realistic light and sur-<br />

face interactions are not likely to produce the kind <strong>of</strong> problem we are trying to<br />

illustrate here.<br />

However, the theoretical weakness <strong>of</strong> XYZ space as a color space <strong>for</strong> render-<br />

ing calculations remains, especially since some kinds <strong>of</strong> light and matter inter-<br />

actions (such as calculations involving transparency, diffraction effects or disper-<br />

sion) can lead to situations where invalid colors are easily produced.<br />

93

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