A framework for rendering complex scattering effects on hair

The appearance of hair plays a critical role in synthesizing realistic looking human characters. However, due to the high complexity in hair geometry and the scattering nature of hair fibers, rendering hair with photorealistic quality and at interactive speeds remains as an open problem in computer graphics. Previous approaches attempt to simplify the scattering model to only tackle a specific aspect of the scattering effects. In this paper, we present a new approach to simultaneously render complex scattering effects including volumetric shadows, transparency, and antialiasing under a unified framework. Our solution uses a shadow-ray path to produce volumetric self-shadows and an additional view-ray path to produce transparency. To compute and accumulate the contribution of individual hair fibers along each (shadow or view) path, we develop a new GPU-based k-buffer technique that can efficiently locate the K nearest scattering locations and combine them in the correct order. Compared with existing multi-layer based approaches[Kim and Neumann 2001; Yuksel and Keyser 2008; Sintorn and Assarsson 2009], we show that our k-buffer solution can more accurately reproduce the shadowing and transparency effects. Further, we present an anti-aliasing scheme that directly builds upon the k-buffer. We implement all three effects (volumetric shadows, transparency, and anti-aliasing) under a unified rendering pipeline. Experiments on complex hair models demonstrate that our new solution produces near photorealistic hair rendering at very interactive speed.