Graphics Reference
In-Depth Information
lighting condition of a different person's face image assuming the two faces
have similar skin albedos.
1.1 Relighting with radiance environment maps
Using the technique described in Section 2.1.3 of Chapter 6, the radiance
environment map (REM) based illumination model can be approximated from
a single face image. Based on the illumination model, we can then synthesize
face image in novel lighting environments in various scenarios, including: (1)
face rotating in the same lighting environment; (2) illumination effects transfer
from one face to another; and (3) interactively lighting effects editing.
1.1.1 Relighting when rotating in the same lighting condition
When an object is rotated in the same lighting condition, the intensity of
the object will change due to incident lighting changes. let
denote the
irradiance defined by equation 6.1. For any given point
on the object, suppose
its normal is rotated from
to
Assuming the object is diffuse, and let
denote the reflectance coefficient at
then the intensities at
before and after
rotation are respectively:
From equation 8.1 and 8.2 we have
From the definition of the radiance environment map (equation 6.2), we have
Comparing equation 8.3 and 8.4, we have
What this equation says is that for any point on the object, the ratio between
its intensity after rotation and its intensity before rotation is equal to the in-
tensity ratio of two points on the radiance environment map. Therefore given
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