Realistic fabric rendering is still a significant challenge due to their complex structures and varying fiber properties. We present a new fabric shading technique, which models both reflection and transmission using a hybrid of ray and wave optics methods, grounded in simulation data. We target fabrics woven from yarns, each formed by twisting together one or more plies, which further contain twisted fibers. Our model is based on simulations that predict the scattering of a narrow Gaussian beam by a single ply. Comparing results from full-wave simulations and path tracing, we found that ray optics can accurately simulate the average far field scattering from an ensemble of plies, but not the variation among individual ply instances, and ray tracing overlooks important diffraction effects. Following these observations, our model is built from ray simulations performed for many ply instances, with simulation data fitted by Gaussian mixtures to be used during rendering. Wave simulations are used to calibrate noise functions that account for instance-to-instance variation, and an aperture diffraction model is used to handle light passing between plies and yarns. The result is a hybrid model capable of producing realistic appearance and highlight structure in fabrics, while capturing spatial break-ups and irregularities and simulating the subtle color shifts and blurriness that occur in transmission. We validate our results by comparing rendered images with photographs, demonstrating the effectiveness of our approach in achieving realistic cloth rendering.

Many animals exhibit structural colors, which are often iridescent, meaning that the perceived colors change with illumination conditions and viewing perspectives. Biological iridescence is usually caused by multilayers or other periodic structures in animal tissues, which selectively reflect light of certain wavelengths and often result in a shiny appearance---which almost always comes with spatially varying highlights, thanks to randomness and irregularities in the structures. Previous models for biological iridescence tend to each target one specific structure, and most models only compute large-area averages, overlooking spatial variation in iridescent appearance. In this work, we build appearance models for biological iridescence using bird feathers as our case study, investigating different types of feathers with a variety of structural coloration mechanisms. We propose an approximate wave simulation method that takes advantage of quasi-regular structures while efficiently modeling the effects of natural structural irregularities. We further propose a method to distill our simulation results into distributions of BRDFs, generated using noise functions, that preserve relevant statistical properties of the simulated BRDFs. This allows us to model the spatially varying, glittery appearance commonly seen on feathers. Our BRDFs are practical and efficient, and we present renderings of multiple types of iridescent feathers with comparisons to photographic images.

Computing light reflection from rough surfaces is an important topic in computer graphics. Reflection models developed based on geometric optics fail to capture wave effects such as diffraction and interference, while existing models based on physical optics approximations give erroneous predictions under many circumstances (e.g. when multiple scattering from the surface cannot be ignored). We present a scalable 3D full-wave simulator for computing reference solutions to surface scattering problems, which can be used to evaluate and guide the development of approximate models for rendering. We investigate the range of validity for some existing wave optics based reflection models; our results confirm these models for low-roughness surfaces but also show that prior rendering methods do not accurately predict the scattering behavior of some types of surfaces. Our simulator is based on the boundary element method (BEM) and accelerated using the adaptive integral method (AIM), and is implemented to execute on modern GPUs. We demonstrate the simulator on domains up to 60 × 60 × 10 wavelengths, involving surface samples with significant height variations. Furthermore, we propose a new system for efficiently computing BRDF values for large numbers of incident and outgoing directions at once, by combining small simulations to characterize larger areas. Our simulator will be released as an open-source toolkit for computing surface scattering.