Photo-Realistic Rendering of Blond Hair A. Zinke, G. Sobottka, A. Weber Institut f ¨ ur Informatik II, Universit¨ at Bonn, Germany Email: {zinke, sobottka, weber}@cs.uni-bonn.de Abstract We present a method for photo-realistic hair ren- dering, which is based on light scattering from (hu- man) hair fibers. In contrast to existing approaches our method gives realistic results also for light hair types and for close-ups. These results are obtained by using a “near-field” model taking into account different scattering components and indirect illu- mination. Our near-field model is a generalization of previously published methods that can be seen as far-field models. Nevertheless our more general shading model can be used computationally as effi- ciently as the previous ones. 1 Introduction Modeling human hair is one of the great challenges of computer graphics. Although a lot of progress has been made in the simulation of hair dynamics [3], [1], modeling of hair geometries [6], and in the development of interactive hair styling tools [2], [12] it is still not possible to manage the complexity of human scalp hair in real-time. In the realm of rendering the partially contra- dicting overall goals of realism versus rendering time have led to the introduction of a variety of hair rendering techniques considering both back- lighting and self shadowing effects and making in- tense use of pixel-blending, shadow buffers, deep- shadow maps, opacity maps or textures: [4], [9], [13], [7], [8], [10]. A recent milestone with respect to the goal of photo-realism has been [11]. In this article Marschner et al. propose a model for light scatter- ing from human hair fibers. It is in particular suit- able as a shading model for direct illumination by distant light sources and a distant viewer, as it uses a “far-field” scattering model. However, for lights close to the hair, close-ups or indirect illumination it is not a priori guaranteed that all of the approximations are still valid. When hair- hair scattering or local scattering phenomena have to be taken into account the scattering model has to be generalized. This generalization is especially necessary for the photo-realistic rendering of blond hair types, for which effects of indirect illumination are much more prominent than for darker hair types. Our Contribution: In this paper we present an adequate generalized model for local scattering ef- fects, to which we will refer as near-field scattering model. This model is the theoretical foundation of our photo-realistic hair rendering framework, which achieves high quality for close-ups and indirect il- lumination, suitable for blond hair types. Further- more, we show that for light hair types the effects of indirect illumination from neighboring hair fibers can result in visible effects up to a recursion depth of about 5. We can use our approach to identify situations in which common techniques for simulating trans- parency effects—pixel blending or (deep) shadow maps—will not differ substantially from the results of a more accurate model. For these cases we present an approximation for indirect illumination. Rendering results of some exemplary hair scenes can be seen in the color plates at the end of the pa- per. 2 Preliminaries For a hair fiber with elliptical cross-section the lo- cal normalized tangent is denoted u and points into the direction of the hair tip; incoming and outgoing directions  ω i and  ω r may then be locally expressed in polar coordinates. Thus, it is possible to write  ωi =(θi ,ϕi ) and  ωr =(θr ,ϕr ) where θi and θr are the angles of inclination to the hair tangent (0 ◦ is perpendicular, -90 ◦ parallel to the fiber’s axis). VMV 2004 Stanford, USA, November 16–18, 2004