Game Development Reference
In-Depth Information
The following are some common BRDF models:
•
The Lambertian model
: This perfectly represents diffuse (matte) surfaces
with a constant BRDF.
•
The Phong reflectance model
: This is a phenomenological model akin to
plastic-like specularity.
•
The Blinn-Phong model
: This resembles Phong but allows certain quantities
to be interpolated and reduces computational overhead.
•
The Torrance-Sparrow model
: This is a general model that represents
surfaces as distributions of perfect specular microfacets.
•
The Cook-Torrance model
: This is a specular-microfacet model
(Torrance-Sparrow) that accounts for wavelength and thus color shifting.
•
The Ward's anisotropic model
: This is a specular-microfacet model with an
elliptical-Gaussian distribution function dependent on the surface tangent
orientation (in addition to the surface normal).
We will cover the Lambertian (for diffuse light/materials) and Blinn-Phong
models for specular effect. The reason they are most favored is because of lesser
computational overhead.
Lambertian reflectance/diffuse reflection
Lambertian reflectance
is the property that defines an ideal diffusely reflecting
surface. The apparent brightness of such a surface to an observer is the same
regardless of the observer's angle of view. The luminous intensity obeys Lambert's
cosine law.
This technique causes all closed polygons (such as a triangle within a 3D mesh) to
reflect light equally in all directions when rendered. In effect, a point rotated around
its normal vector will not change the way it reflects light. However, the point will
change the way it reflects light if it is tilted away from its initial normal vector. The
reflection is calculated by taking the dot product of the surface's normal vector and a
normalized light-direction vector pointing from the surface to the light source. This
number is then multiplied by the color of the surface and the intensity of the light
hitting the surface, as shown in the following formula:
I =-L.NCI
D
L
