Rendering Fur with Three Dimensional Textures James T. Kajiya and Timothy L. Kay

Motivation We want to be able to render furry things! We want to be able to render furry things!

Previous Work Geometrically accurate Geometrically accurate –Brute-force rendering of individual hairs –Very slow –Severe aliasing due to micro-geometry Particle-based systems for Volume rendering Particle-based systems for Volume rendering –Not optimal for ray tracing

Volume Densities Blinn (1982) Blinn (1982) –Method for rendering volume densities –Volume of microscopic spheres –Assumed Homogeneous media Kajiya and Von Hersen (1984) Kajiya and Von Hersen (1984) –Generalized to non-homogeneous media –Volume rendering for ray tracing

Volume Densities For each ray through the volume… 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 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? 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 Solution: replace all integrals with sums –Transparency equation becomes –Brightness equation becomes

Fur Rendering Four steps to rendering fur: Four steps to rendering fur: –Create fur texel –Map texels to world space –Shoot rays into texel –Calculate lighting

Fur Rendering Create Fur Texel 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 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 Shoot rays into texel –Intersect rays with bilinear texel patch in world space –Convert intersection into texel space Calculate Lighting 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 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 Pros –Very realistic results! –Complete description of method –Great reference images for comparison with faster methods –Easily extendable Cons Cons –Fairly slow (2 hrs on a 16 processor mainframe)

Questions