Graphics Reference
In-Depth Information
9
Bidirectional Texture
Function
The appearance of an object depends on the response of the light reflected from
its surface to the eye of the observer. For the purposes of photorealistic rendering,
surface reflection was typically regarded at two levels: the
macrostructure
of an
object refers to the basic visual geometry; the
microstructure
is the microscopic
detail that affects light reflection, e.g., surface roughness and microgeometry, but
the detail itself is not actually visible. The representative example is the com-
bination of BRDFs and global illumination: BRDF models depend (implicitly, at
least) on the microscopic detail of the surfaces; global illumination simulates light
transport at the macroscopic scale.
The distinction between macrostructure and microstructure is made because
they involve different approximation techniques. A middle level of detail, called
the
mesostructure
, can also be separated. Mesostructures include fine structural
detail that is visible, but the individual features are not large enough to affect
global illumination directly. Examples of mesostructure include wood grain, de-
tail in cloth, plaster, brush strokes, etc. Traditionally, microstructure is handled
by the BRDF, and mesostructures are modeled by bump maps and displacement
maps.
A
bidirectional texture function
(BTF) is a way of representing mesostructure
using an image-based approach. It represents the changes in reflectance over a
small area using a pixel array or general set of textures. In practice, the textures
are photographs acquired under multiple viewing and lighting directions. A BTF
predicts all the effects caused by mesostructure without explicitly modeling the
actual geometry of the surface. Moreover, as the BTF is an image-based approach,
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