Graphics Reference
In-Depth Information
(a) Self-shadowing
(b) Self-masking
Figure 8.5
Local occlusion by nearby microgeometry elements has two forms: (a) self-shadowing is
the blocking of incident light; (b) self-masking is the blocking of reflected light.
as well as general increasing specularity near grazing reflection. As described in
Chapter 1, Fresnel reflection is derived by considering light as a wave instead of
using the wave path approach of geometric optics. The term “Fresnel reflection”
is often used more loosely to describe reflection effects dependent on the wave
nature of light. In some contexts it is synonymous with increasing specularity
near grazing.
Another factor regarding off-specular reflection is light occlusion due to rough-
ness of the surface itself. Real surfaces that appear smooth to the unaided eye can
be very rough at the microscopic level. For example, a typical billiard ball scaled
to the size of the Earth would have mountains much higher than Mount Everst.
The microscopic topography—the hills and valleys and crags and tors—are part
of the surface
microgeometry
. Occlusion due to microgeometry can be split into
two forms:
self-shadowing
stops light from hitting a particular microscopic sur-
face point;
self-masking
blocks light reflected from a surface point (
Figure 8.5
)
.
Local occlusion in general is most prominent near grazing reflection. Fresnel
reflectance increases the amount of reflected light near grazing, while local occlu-
sion reduces it. In this sense the effects partially cancel each other out; however
the effect of Fresnel reflection dominates very close to grazing directions.
8.1.4 Microfacet-Based BRDF Models
Surface reflection has been studied in the field of optics since long before the in-
vention of the digital computer. As early as 1924, physicists were modeling light
reflection by treating the surface as if it were made of microscopic flat mirror-like
surfaces, now known as
microfacets
. Much of this work, however, was limited
to the plane of incidence. In 1967, Kenneth E. Torrance and E. M. Sparrow
published a microfacet-based surface reflection model that better matched ob-
served off-specular peaks [Torrance and Sparrow 67]. The model, now known as
the Torrance-Sparrow reflectance model was actually developed in the context of
modeling heat transfer, but it is a genuine BRDF model and is therefore applicable
to rendering.
