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rotated to match the rotation induced by the deformation; as described in the
previous paragraph, this rotation can be approximated by translating the pixels.
To calculate the change in shadowing, rays are cast from the surface point in the
postdeformation model in the direction of pixels in the reflectance function. If
the cast ray is occluded, the pixel is excluded from the summation. In reality, the
deformation also changes the local interreflection, but this is not considered.
Mark Sagar was of the opinion that reflectance field rendering had much room
for improvement. Nonetheless, the results were more realistic than the production
staff had expected. This reaffirmed the benefits of using image-based techniques,
and expanded the potential use of the light stage in movie production. The next
task would be to provide digital artists the flexibility to directly control the diffuse
and specular components calculated from the reflectance functions. This would
allowlightstagedatatobeusedinmorecreative ways. The light stage has already
been used in other ways than capturing the reflectance field. One application
in movie projects has been to fit parameters to shading models. Improvement in
the speed of image capture has made this process easier for artists. Image-based
geometry reconstruction (Chapter 5) is another potential application of a light
stage.
8.3 Separation of Direct and Global Illumination
The term “global illumination” normally refers to all the lighting in an environ-
ment, including interreflection and scattering effects. As noted in Chapter 2,
global illumination can be regarded as having two components: the direct com-
ponent, which is caused by light from light sources reflecting directly from scene
surfaces to the eye or camera, and the indirect component, which comes frommul-
tiple reflections. The indirect component is often called the “global component,”
especially in literature concerned with the separation of the two components. This
convention is followed in the present chapter: “global” and “indirect” are often
used interchangeably. 14
The BRDF models introduced in this chapter often separate the reflection
into diffuse and specular components. The face reflectance recovery methods
described in the previous section do something similar; in fact, in the separation
of the reflectance field into diffuse and specular components is the key to syn-
thesizing reflectance functions for an arbitrary viewpoint. A related separation,
14 To complicate matters further, the term “radiosity” is sometimes used to describe indirect lighting,
especially from interreflection. This usage arose in the 1980s when the predominant (if not the only)
method of computing global illumination was the radiosity method. In recent years this somewhat
inaccurate use of “radiosity” has largely been replaced by the term “global illumination.”
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