Multi-Scale Modeling and Rendering of Granular Materials

We address the problem of modeling and rendering granular materials—such as large structures made of sand, snow, or sugar— where an aggregate object is composed of many randomly oriented, but discernible grains.

These materials pose a particular challenge as the complex scattering properties of individual grains, and their packing arrangement, can have a dramatic effect on the large-scale appearanceoftheaggregateobject. Weproposeamulti-scalemodeling and rendering framework that adapts to the structure of scattered lightatdifferentscales. Werelyonpathtracingtheindividualgrains only at the finest scale, and—by decoupling individual grains from their arrangement—we develop a modular approach for simulating longer-scale light transport. We model light interactions within and across grains as separate processes and leverage this decomposition to derive parameters for classical radiative transport, including standardvolumetricpathtracingandadiffusionmethodthatcanquickly summarize the large scale transport due to many grain interactions. We require only a one-time precomputation per exemplar grain, which we can then reuse for arbitrary aggregate shapes and a continuum of different packing rates and scales of grains. We demonstrate our method on scenes containing mixtures of tens of millions of individual, complex, specular grains that would be otherwise infeasible to render with standard techniques.