Interactive Hair Rendering Under Environment Lighting

Environment lighting is an effective way to model the complex natural illumination usually found in the real world. Over the past few years, much work has been devoted to interactive rendering under environment lighting, Despite the tremendous progress, one problem that remains unsolved is that of hair rendering under environment lighting. Existing interactive hair rendering techniques are all developed for relatively simple light sources, such as point and directional lights .We present a fast algorithm for interactive hair rendering with both single and multiple scattering effects under complex environment lighting. The outgoing radiance due to single scattering is determined by the integral of the product of the environment lighting,the scattering function, and the transmittance. We approximate the environment light by a set of spherical radial basis functions (SRBFs) and thus convert the outgoing radiance integral into the sum of radiance contributions of all SRBF lights. For each SRBF light, we factor out the effective transmittance to represent the radiance integral as the product of two terms: the transmittance and the convolution of the SRBF light and the scattering function. Observing that the convolution term is independent of the hair geometry, we precompute it for commonly-used scattering models, and reduce the run-time computation to table lookups. We further propose a technique, called the convolution optical depth map, to efficiently approximate the effective transmittance by filtering the optical depth maps generated at the center of the SRBF. As for the multiple scattering computation, we adopt an existing method, that of the dual scattering approximation proposed by Zinke et al.. Experiments show that our algorithm can achieve interactive frame rates, while the rendering results are comparable to those produced by off-line algorithms.