FLEXIBLE APPEARANCE MODEL - 3D Face Processing: Modeling, Analysis and Synthesis - page 63

2. FlexibleAppearance Model
In this section‚ we present a flexible appearance model with reduced de-
pendency on illuminations and individuals. We develop an efficient method to
model illumination effects from a single face image [Wen et al.‚ 2003]. The
illumination model can be used to reduce illumination dependency of the ap-
pearance model in both analysis and synthesis. The flexible appearance model
also utilize ratio-image technique to reduce person dependency in a principled
way [Wen et al.‚ 2003‚ Wen and Huang‚ 2003]. We discuss the two components
in the Section 2.1 and 2.2. Analysis and synthesis using this flexible model will
be presented in Chapter 7 and 8‚ respectively.

2.1 Reduce illumination dependency based on illumination

modeling

2.1.1 Radiance environment map (REM)

Environment map [Greene‚ 1986‚ Miller and Hoffman‚ 1984] is a technique
in computer graphics for capturing environment illuminations from all direc-
tions using a sphere. The captured lighting environment can be used together
with surface reflectance properties (i.e. “Bidirectional Reflectance Distribution
Function” (BRDF)) to generate photo-realistic rendering of objects. For ap-
plications dealing with images‚ such as image/video analysis and image-based
rendering‚ it is difficult to recover accurate surface reflectance properties from
images. Another issue is that it could be time consuming to integrate illumina-
tion and complex surface reflectance properties at rendering time.
In these scenarios‚ environment maps that pre-integrate with BRDF can be
useful. Radiance environment map is proposed by Greene [Greene‚ 1986] and
Cabral et al. [Cabral et al.‚ 1999] for real-time rendering of objects rotating
in lighting environment. Given any sphere with constant “Bidirectional Re-
flectance Distribution Function” (BRDF)‚ its radiance environment map records
its reflected radiance for each point on the sphere. Then‚ for any surface with
the same reflectance material as that of the sphere‚ its reflected radiance at any
point can be found from the radiance environment map by simply looking up
the normal. This method is very fast and produces photo-realistic results.
One limitation of this method is that one needs to generate a radiance en-
vironment map for each different material. For some objects where every
point's material may be different‚ it would be difficult to apply this method. To
overcome this limitation‚ we observe that‚ for diffuse objects‚ the ratio image
technique [Riklin-Raviv and Shashua‚ 1999‚ Liu et al.‚ 2001a] can be used to
remove the material dependency of the radiance environment map‚ thus mak-
ing it possible to relight surfaces which have variable rerlectance properties.
Details of the ratio-image technique will be described in Section 2.2.