Abstract
This course is about recent advances in the challenging field of physically-based appearance modeling of cloth. Apart from geometrical complexity, optical complexity presents complications as highly anisotropic single and multiple scattering effects often dominate the appearance. Many types of fibers are highly translucent and multiple scattering significantly influences the observed color. Since a cloth model may potentially consist of billions of fibers, finding a viable level of geometrical abstraction is difficult. After explaining the general structure of several types of textiles, we give an overview of different approaches that have been proposed to render cloth. As the micro-geometry of cloth can be represented using an explicit representation of a fiber assembly, we continue by explaining optical properties of fibers; these can be derived from first principles of physics such as absorption or index of refraction. Understanding light scattering from fibers is essential, when a physically-based cloth renderer is designed. However, as storing these fibers explicitly is often too costly, more efficient statistical descriptions of cloth have also been proposed that can be used together with volumetric rendering techniques to allow for physically-based image synthesis, while retaining most of the flexibility of explicit methods. A major part of this course will focus on these approaches. We discuss the theory and practice of physically-based rendering of anisotropic media. The discussion begins with a review of linear transport theory, upon which current methods for rendering volumetric cloth are based. Relevant implementation details are discussed at each stage, and the final result will be the pseudocode of a Monte Carlo path tracer for volumetric cloth representations. Although rendering of cloth is a very specialized task, many of the concepts, developed in this field, can be used for rendering other materials with complex micro-geometry as well.
Highlights
Textiles are an essential component of most virtual scenes: The appearance of human avatars relies on realistic virtual clothing; other textiles such as carpets and curtains are common indoor elements
As first shown by Marschner et al [2003] in his seminal work on hair rendering, light scattering can be well approximated by a direct surface reflection component (R), light that gets transmitted through the fibers (TT) and back-scattered light that got internally reflected (TRT)
For example, the first condition holds in case of hair, fur and for many types of synthetic fibers. In this case internal light transport may not be neglected, but since the curvature is small compared to the radius of the fiber and since the light gets attenuated due to volumetric absorption, the error introduced by this approximation is still acceptable
Summary
Textiles are an essential component of most virtual scenes: The appearance of human avatars relies on realistic virtual clothing; other textiles such as carpets and curtains are common indoor elements. Recent techniques that represent micro-geometry as fiber assemblies or in volumetric ways model more aspects of the material explicitly. This makes them attractive for predictive rendering in the context of computer aided design (CAD) of materials. [Zhao et al 2011] model cloth based on micro CT data and photographs and apply this framework for rendering Another volumetric approach has been presented by [Schroder et al 2011b] who render woven materials using virtual scattering events: Instead of representing all fibers explicitly, only their statistical distribution is stored in a volume.
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