Controlling the appearance of specular microstructures

The focus of this dissertation is the appearance modeling of specular microstructures. We limit our study to microspheres, microcylinders, and a mesh of interwoven microcylinders. These microstructures are involved in the appearance of rainbows, hair, and cloth. There have been many studies on the appearance modeling of these three subjects. However, previous models either fail to reproduce specific appearances or they do so at the cost of time-consuming parameter tweaking. The main goal of this dissertation is to present novel practical appearance models that do not suffer from these shortcomings. Each one of our novel appearance models has a robust physical basis and incorporates intuitive parameters that control the final appearance. Our appearance models facilitate the reproduction of a wider range of desired appearances for rendering rainbows, hair, and cloth. First, we present a novel appearance model for rendering rainbows. We introduce a ray tracing framework for simulating light interactions by particles with arbitrary shapes. We validate our approach against the Lorenz-Mie theory for spherical water drops. We also show that our model can predict the light scattering behavior of large water drops which have non-spherical shapes where the Lorenz-Mie theory is not applicable. By considering the physically- based shape of water drops, as well as the effect of the sun's inclination, we are first to present a simulation of twinned rainbows. Next, we present a novel hair appearance model that addresses the problem of art-directability for physically-based appearance models. We introduce a novel approach for creating an art-directable hair shading model from existing physically based models. Through a user study, we show that this system is easier to use compared to both physically based and ad hoc shading models. Our appearance model has been integrated into the production pipeline at the Walt Disney Animation Studios and has been used in the production of the animated feature film Tangled. Finally, we present a practical appearance model for rendering cloth fabrics. Our model is based on extensive measurements of Bidirectional Reflectance Distribution Function (BRDF) of several cloth samples and threads. Based on these measurements we present a novel BRDF model for threads. Using this model and statistical tangent distributions of cloth threads inside a weaving pattern, we can reproduce the appearance of a wide range of cloth fabrics. We also introduce a novel shadowing and masking term for cloth fabrics, which is important in grazing angle viewing and lightin