Graphics Reference
In-Depth Information
• What mathematical or computational representation should we use for this
model?
Henceforth, we'll generally refer to the model of scattering as the BSDF,
except (a) when we're talking about reflection-only scattering, where terms like
“the Blinn-Phong BRDF” are common, and (b) briefly in our discussion of reci-
procity, and when we talk about subsurface scattering. In most equations, wher-
ever it makes sense we'll use
f
s
rather than
f
r
.
One of the most challenging problems in the numerical representation of the scat-
tering of light by surfaces is the difference between
specular
reflections—the
mirrorlike reflections that we see in extremely shiny surfaces—and
diffuse
scat-
tering, in which an incoming beam of light spreads out into light going in almost
every direction, as happens when it meets a surface made of flat latex paint. Most
of a light beam hitting a mirror scatters in one primary direction, but some small
amount scatters in other directions as well. By just about any measure of light
quantity, the scattering in the primary direction is a
huge
multiple of the scattering
in other directions. (A multiplier of 10
10
is well within reason.) And the falloff
in light quantity, as one moves away from the primary direction, is exceptionally
rapid as well. It therefore makes some sense to separate out the specular reflection
and treat it as a
pointwise
phenomenon, and regard the rest of the scattering as
a smooth function of outgoing direction. The same applies to the kind of inter-
material transmission described by Snell's law: Incoming light in one direction
essentially exits in some other direction. We'll call both of these
impulses
in the
scattering, and treat them separately from the diffuse effects.
We'll discuss the following types of scattering models.
•
Empirical/phenomenological models:
These are models crafted to sim-
ulate some observed scattering phenomenon. The Phong model of Sec-
tion 6.5.3 is an example. With little physical motivation, it's designed to
allow a user to vary between nearly Lambertian and highly glossy appear-
ances for a surface.
•
Measured/captured models:
These are models in which the BSDF is care-
fully measured and stored; when we need the BSDF for a particular pair of
directions
(
v
o
)
, we look for them (or directions near them) in a large
table of stored data, perhaps interpolating from nearby samples when nec-
essary.
•
Physically based models:
These are models based on some degree of
understanding of the physical interaction of light with materials. They
occupy the bulk of this chapter.
v
i
,
We cannot, for a passive material, have more light scattered from the surface
than arrives there. (An “active” object, like a photosensor that triggers a strobe
light whenever it detects light, can obviously emit more light than it receives.)
