Graphics Reference
In-Depth Information
six-variable function
R
(
θ
i
,
φ
i
;
u
r
,
v
r
,
θ
r
,
φ
r
)
.
A BRDF models only the absolutely local reflection behavior at a single point.
The reflectance field includes everything that happens from the point of incidence
to the outgoing point. This includes near-field light interaction such as shadows,
interreflection, and subsurface scattering. Once the reflectance field is acquired,
the true appearance of the object from an arbitrary viewpoint and lighting direc-
tion can be accurately reproduced without having to perform any further render-
ing calculations. Because of the additivity of light,
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the appearance of the object
under several distant sources can be recovered by simply adding the images con-
structed for each separate source. In fact, the appearance under
any
incident light
field (i.e., environment map) can be recovered by sampling the incident direc-
tions and treating each sample as a separate light source. In this context “adding”
images refers to adding the radiance values directly; tone mapping or any other
nonlinear correction must be applied only to the final image.
Acquiring the reflectance field for a fixed viewing direction is relatively simple
because of the nature of the parameter set. For a fixed
φ
r
, the radiance at a
set of surface point samples
u
r
,
v
r
is recorded for a set of illumination directions
θ
i
and
θ
r
and
θ
r
. However, if the viewing direction is allowed to vary, the acquisition
of the reflectance field becomes more complicated. This is especially true for a
human face. The reason for this is that small changes in the viewing direction can
cause not-so-small changes in the appearance due to different parts of the object
slipping in and out of visibility and a general lack of understanding of physical
reflectance of a human face. In the “reflectance field” paper, the authors propose
a method of first acquiring the reflectance field with the viewing direction fixed in
the same direction as the camera, and then changing this reflectance field so that
it is applicable to an arbitrary viewing direction.
8.2.3 The Light Stage
Debevec and his colleagues devised a measurement apparatus known as a
light
stage
to acquire the reflectance field of an object by taking photographs from a
sequence of different lighting directions. A light stage has white spotlights of
equal brightness uniformly positioned on a spherical grid, all aimed at a subject
sitting on a fixed chair in the center of the grid
(
Figure 8.25
). The spotlights are
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Radiometric quantities are assumed to be additive, or linear, in all physical simulations; for ex-
ample, the radiance reflected from a surface illuminated by two light sources is equal to the sum of the
reflected radiance from the sources considered separately. However, human perception, camera and
film response, and tone mapping are not generally additive: the sum of two tone mapped images is not
the same as the tone mapped image of the sum of the original images.
