Transcript [John Kloetzli]

Rendering Fur with Three
Dimensional Textures
James T. Kajiya and Timothy L. Kay
Motivation

We want to be able to
render furry things!
http://www.trappersden.ca/Moxy.JPG
http://www.lesleymattuchio.com/Ladies.html
Previous Work

Geometrically accurate
– Brute-force rendering of individual hairs
– Very slow
– Severe aliasing due to micro-geometry

Particle-based systems for Volume
rendering
– Not optimal for ray tracing
Volume Densities

Blinn (1982)
– Method for rendering volume densities
– Volume of microscopic spheres
– Assumed Homogeneous media

Kajiya and Von Hersen (1984)
– Generalized to non-homogeneous media
– Volume rendering for ray tracing
Volume Densities

For each ray through the volume…
– Find the transparency of the surface
– Find the brightness of the surface
Volume densities
Three parts of the brightness integral
Visibility of the current
position in the volume
Sum of the light contribution
from each light source
The density of the surface
at the point
Texels

Replace volume densities with texels:
3D array containing microsurface data
– Density: relative projected area of a
surface inside the volume cell
– Frames: Local direction of the surface
– Lighting Model: The lighting model for
the local surface
Texels

Can we render texels
with the original
volume density
rendering equations?
– All line integrals go to
0
– Transparency and
brightness calculations
both go to 0!
Texels

Solution: replace all integrals with sums
– Transparency equation becomes
– Brightness equation becomes
Fur Rendering

Four steps to rendering fur:
– Create fur texel
– Map texels to world space
– Shoot rays into texel
– Calculate lighting
Fur Rendering

Create Fur Texel
– Each hair is a cylinder
– Place hairs randomly on a
surface
– Model overcoat and
undercoat
– Jiggle until it looks
“correct”
Fur Rendering

Map texels to world
space
– Create a continuous
mapping between
texel and model
– Interpolate between
texels to form a
bilinear patch
– No geometry is
needed
Fur Rendering

Shoot rays into texel
– Intersect rays with bilinear texel patch in world
space
– Convert intersection into texel space

Calculate Lighting
– Fur lighting model is diffuse and specular
– Diffuse based on cylindrical shape
– Specular is modified Phong lighting
Results
Results
Conclusion

Fur rendering using 3D textures
– New 3D texture model called texel
– New rendering equations for
microsurfaces
– Fur texel modeling technique
– Texel mapping/interpolation to form
objects without geometry
– Fur lighting BRDF
Pros/Cons

Pros
– Very realistic results!
– Complete description of method
– Great reference images for comparison with
faster methods
– Easily extendable

Cons
– Fairly slow (2 hrs on a 16 processor mainframe)
Questions