Game Development Reference

In-Depth Information

8D

BSSRDF

Homogenous material

No surface scattering

Bidirectional Texture

Function (BTF)

Bidirectional Surface-Scattering

Distribution Function (BSSDF)

6D

Fixed light

Fixed shot

Fixed position

No surface

scattering

Bidirectional Reflectance Distribution

Function (BRDF)

Luminance field

4D

Reflectance field

Flat and diffuse

surface

2D

2D Texture

Isotropic BRDF

Figure 15.2
Hierarchy of reflectance functions

takes us close to the concept of
phase function
which defines the behaviour of a

participating environment
(non-surface component like gas or vapour) vis-à-vis light

scattering (Glassner, 1995). The BRDF characterises the surfaces, whereas the phase

function characterises the volumes. A modelling approach valid for the BRDF can

naturally apply to the BTDF, BSDF or the phase function, if it is not specific from a

physical point of view.

To conclude, it is necessary to mention that in first approximation, the BRDF sup-

poses that the light energy incident on a point
x
of a surface is reflected from the same

point. In fact, a fraction of the light generally penetrates
inside
the material, is reflected

within the microscopic structure of the material and is reflected outside the surface only

at a point more or less away from
x
. Nonetheless, if we consider these microscopic

phenomena as completely unpredictable, approximation of the unique point is macro-

scopically exact since the energy penetrating the surface and then coming out of it

is distributed uniformly on the surface. However, this type of simplification is inade-

quate to get a perfect modelling of certain types of materials (for example, translucent).

We thus introduce the Bidirectional surface scattering reflectance distribution function

(BSSRDF) (Nicodemus et al., 1977) which includes this discrepancy between the inci-

dent point and reflection point. This function was used only recently in image synthesis

due to its complexity. We can quote Jensen et al. (2001), Jensen and Buhler (2002),

Dorsey et al. (1999), Hanrahan and Kruegar (1993), and Pharr and Hanrahan (2000),

all of which are works related to the problem of light scattering under the surface of a

material.

Classification of BRDF models

The research of a
typology
of different BRDF models, i.e. the study of their charac-

teristics and classification attracted more geophysicists than image synthesis experts.

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