Graphics Reference
In-Depth Information
(
θ
i
,
φ
i
)
(
u
i
,
v
i
)
(
θ
i
,
φ
i
)
(
θ
r
,
φ
r
)
(
u
r
,
v
r
)
(
u
r
,
v
r
)
(
θ
r
,
φ
r
)
A
A
(b)
Figure 8.24
(a) The reflectance field is defined on a closed surface
A
surrounding the object. It gives
reflected radiance at a position
(
u
r
,
v
r
)
on the surface
A
in direction
(
θ
r
,
φ
r
)
from incident
light at position
(
u
i
,
v
i
)
and incident direction
(
θ
i
,
phi
i
)
. (b) If the light source and view-
point are sufficiently distant, the variation in incident position can be dropped, and the
surface
A
can be regarded as surface of the object.
(a)
(From [Debevec et al. 00] c
2000
ACM, Inc. Included here by permission.)
The purpose of the reflectance field is to recover the appearance of an object
(a human face) from an arbitrary viewing direction under illumination from an-
other arbitrary direction. It is constructed from photographs of each part of the
object taken from different viewpoints under different lighting conditions. The
reflectance field is an function of eight variables defined on a closed surface
A
,
such as a sphere or a cuboid, that encompasses the object (
Figure 8.24(a)
)
:
R
(
u
i
,
v
i
,
θ
i
,
φ
i
;
u
r
,
v
r
,
θ
r
,
φ
r
)
.
The parameters are the position
u
i
,
v
i
and direction
θ
i
,
φ
i
of light incident on
φ
r
of reflected radiance. The
reflectance field
R
can be regarded as a function of two 4D light fields: one for
incident light and one for the reflected light. If the surface
A
coincides with the
physical surface of the object, the reflectance field is equivalent to the BSSRDF
(equivalently, subsurface scattering can be expressed as a reflectance field.
In acquiring the reflectance field, the surface
A
can be thought of as being co-
incident with the object surface. The positions
the surface, and the position
u
r
,
v
r
and direction
θ
r
,
can be regarded
as parameters on the object itself (
Figure 8.24(b)
)
. That is, the light source and
viewpoint are sufficiently far away that the light direction is essentially the same
from a point on the reflectance field surface and a corresponding point on the ob-
ject surface. In fact, for a distant light source, the light direction can be taken as
constant across the entire object, in which case the reflectance field reduces to a
(
u
i
,
v
i
)
and
(
u
r
,
v
r
)
