Graphics Reference
In-Depth Information
The Phong BSDF has three parameters. The Lambertian constant
k
L
controls
the color and intensity of matte reflection. The analogous
k
g
controls the color
and intensity of glossy reflection, which includes highlights produced by glossy
reflection of bright light sources. A perfectly smooth reflective surface has a mir-
rorlike appearance. Rougher surfaces diffuse the mirror image, which produces
the glossy appearance. The term including
k
g
produces a teardrop-shaped lobe
near the mirror-reflection direction when
f
s
is graphed, so
k
g
is often referred to as
the magnitude of the glossy (or specular) lobe.
The smoothness parameter
s
describes how smooth the surface is, on an arbi-
trary scale. Low numbers, like
s
=
60, produce fairly broad highlights. This is a
good model for surfaces like leather, finished wood, and dull plastics. High num-
bers, like
s
=
2000, produce sharper reflections. This is a better model for car
paint, glazed ceramics, and metals.
The scale of
s
is not perceptually linear. For example,
s
=
120 does not pro-
duce highlights that have half the extent of
s
=
60 ones. It is therefore a good idea
to expose a perceptual “shininess” parameter
σ ∈
[
0, 1
]
to artists and map it to
s
with a function such as
s
=
8192
(
1
−σ
)
.
Most insulators exhibit colorless highlights, so
k
g
is typically chosen to either
be constant across color channels or have a hue opposite
k
L
in order to sum to a
gray or white appearance. Metals tend to have nearly zero Lambertian reflectance
and a
k
g
that matches the perceived color of the metal; examples include gold,
copper, silver, and brass.
The normalization factor
(
8
+
s
)
)
increases the intensity of highlights as
they grow sharper. This makes
s
and
k
G
somewhat perceptually orthogonal and
makes the energy conservation constraint simply
k
L
+
k
g
≤
/
(
8
π
1. The “8”s appear
from rounding the constants in the true solution for the integral of the glossy term
over the hemisphere to the nearest integer.
We say that an object or medium is translucent when we can “see through it,” such
as with glass, fog, or a window screen. For that to happen, some light from beyond
the object must be able to pass through it to reach our eyes.
The phenomenon of
translucency
occurs when multiple scene locations
directly contribute to the energy at a point in screen space. Under the ray optics
modeled in this chapter, light rays do not interact with one another. For exam-
ple, two flashlight beams pass through each other. Because they don't interact, we
can consider the energy contribution from each light ray independently. We then
sum the contribution of all rays to a point. The property of light that describes
this behavior (at least macroscopically) is called
superposition.
This property is
what allows us to consider different wavelengths (colors) independently as well
as describe light scattering for individual rays yet render all the light in a scene.
As with any other scene point, the incoming energy at a point on the image
plane may arrive from multiple locations. The camera aperture blocks a majority
of incoming directions, and in the limiting case of a pinhole camera, it blocks
all but a single direction. In that case, a single ray exiting the virtual camera
describes the path (albeit backward) along which light must have arrived. Yet in
the presence of translucent surfaces, there may be multiple scene points along
that eye ray that contribute because those points need not fully obscure the light
coming from beyond them.
