Graphics Reference
In-Depth Information
Deformation
p
p
Figure 8.33
A more general deformation changes the local geometry, which affects occlusion.
Of course, deformation of the model involves an actual change in the geome-
try. This affects local interreflection and shadowing: a change in the local surface
geometry can change the visibility of the light sources (
Figure 8.33
)
. Resampling
the predeformation reflectance functions cannot capture this. However, because
there is a model for the geometry, shadow rays can be cast from the point to be
rendered over the hemisphere of directions using the deformed model. This tech-
nique was applied when the animatable reflectance field technique was used in
the movie
Spider Man 2
, as described next.
8.2.6 Use of the Light Stage in Movie Production
The method described in the “Animatable Facial Reflectance Fields” paper was
used in the movie
Spider Man 2
[Raimi 04] to calculate the reflectance of a face
that deformed due to animation. The texture of the faces of the Spider Man char-
acter and his enemy “Doctor Octopus” were reconstructed with the use of a light
stage. The person charged with this task was Mark Sagar, a coauthor of the pa-
per “Acquiring the Reflectance Field of a Human Face” described above and an
expert in rendering faces.
The process of acquiring the reflectance of the faces was based on the method
in the above paper, but in
Spider Man 2
, there was a need to acquire the reflectance
for the entire surface of the head, including the sides and back. Images therefore
had to be captured from more viewing directions. Also, in the rendering system
used in movie production, rendering is performed by passing the shaders diffuse
and specular values. Consequently, the reflectance functions are separated into
diffuse and specular components (as they are in the methods described above) but
these components are not recombined; they are stored in separate texture maps.
The process from image capture to rendering consists of the following steps (see
also
Figure 8.34
)
.
1. A reflectance function for each point on the surface of the face is con-
structed from a set of images captured using the light stage.
2. The reflectance functions are split into diffuse and specular components;
the surface normal direction of the point is recovered at the same time.
