To achieve its results, the shading model approximates each hair fiber as a transparent elliptical cylinder, with an absorbing interior and a surface covered with tilted scales (cuticle angle). Light scattering on a model of a hair fiber. The main result of the paper is that hair scattering exhibits three highlights: a primary highlight, which is a direct specular reflection from the surface of a hair strand a secondary highlight (shifted away from the primary), the result of light penetrating into the hair (transmission), reflecting internally, then exiting at a different location and a transmitted component, dealing with light transmitting directly through the hair (as in illumination from back lighting). ![]() The Marschner paper bases its model of human hair on empirical measurements of light scattering from a single hair fiber. The challenge was to match the RenderMan shader to the fur on the Kong maquette, made with dirty Yak’s hair. So a series of refinements and optimizations were added to get the necessary look for Kong. Out of the box, the Marschner model gave a clean ‘shampoo commercial’ look. The details of the paper were implemented as a RenderMan shader and modified to give it the flexibility needed to render Kong’s rough ill-kept black hair, as well as Ann’s nicely-groomed blond hair. Weta based the illumination model for the hair in Kong on the 2002 Siggraph paper “Light Scattering from Human Hair Fibers” by Stephen Marschner et al. In the end, Weta’s massive renderfarm generated the fur on a per-frame basis, foregoing the cost of moving huge cache files around the network. This decision was arrived at by weighing the cost of network bandwidth (considerable when moving multi-gigabyte fur caches) against the cost of recomputing the fur every time. Interestingly, a decision was made not to cache the fur generated by the DSO, but to regenerate it every frame. In the end, the DSO generated approximately 3 to 3.5 million hairs for Kong, although this varied considerably depending on the shot. This was important as the look of Kong was heavily art-directed many iterations could be quickly tested to refine the desired look. All without changing the clumping behaviors applied afterwards. The hair, for example, could be bent in a certain direction. ![]() These attributes include length, baldness, density and other control parameters for deformers.īy layering these deformers, the artists (or fur groomers) were able to quickly create, and change, the look of Kong’s fur in a procedural and non-destructive way. To control the fur deformers, artists must specify a variety of attributes across the surface, using texture maps or per-vertex primitive variables (painted in Maya’s Artisan). Kong, for example, had different “groom programs” for his arms, head, chest, etc. Within a single character, there can be a number of “groom programs” that specify the fur characteristics for different parts of the body. It affects surrounding hairs, within a region of tolerance. The wire deformer is a curve, with an arbitrary number of CVs, that grows from the skin and can be modeled. The clump deformer, as the name suggests, allows artists to bend surrounding hairs towards a central hair. ![]() ![]() They range from simple deformers (such as noise and wave), to more complicated directional deformers (such as clump and wire deformers). Image © 2005 Universal Pictures.ĭeformers are the tools that the groom TDs use to create the desired look. The hair grows straight out of the surface, then “groomed” by deformers which add life to the fur. The hair is created from follicles, which are spread across the subdivision surface the hair is being generated from. The hair system is implemented as a DSO: a procedural plug-in for RenderMan called wmFur, written by Martin Preston. Note: the VFX studio now relies on its in-house spectral renderer called Manuka for rendering. In this latest retro RenderMan story, where befores & afters is featuring previously published Pixar materials, we look at what Weta Digital carried out back then with the aid of RenderMan for Kong.
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