Interactive Hair Rendering and Appearance Editing under Environment Lighting

晃晃

Interactive Hair Rendering and Appearance Editing under Environment Lighting

Kun Xu1 Li-Qian Ma1 Bo Ren1 Rui Wang2 Shi-Min Hu1

1 TNList, Department of Computer Science and Technology, Tsinghua University, Beijing2 Department of Computer Science, University of Massachusetts, Amherst







Abstract

We present an interactive algorithm for hair rendering and ap-

pearance editing under complex environment lighting represented

as spherical radial basis functions (SRBFs). Our main contribu-

tion is to derive a compact 1D circular Gaussian representation

that can accurately model the hair scattering function introduced

by [Marschner et al. 2003]. The primary benefit of this represen-

tation is that it enables us to evaluate, at***n-time, closed-form in-

tegrals of the scattering function with each SRBF light, resulting

in efficient computation of both single and multiple scatterings. In

contrast to previous work, our algorithm computes the rendering

integrals entirely on the fly and does not depend on expensive pre-

computation. Thus we allow the user to dynamically change the

hair scattering parameters, which can vary spatially. Analyses show

that our 1D circular Gaussian representation is both accurate and

concise. In addition, our algorithm incorporates the eccentricity of

the hair. We implement our algorithm on the GPU, achieving in-

teractive hair rendering and simultaneous appearance editing under

complex environment maps for the first time.

Keywords: Hair rendering, environment lighting, appearance edit-

ing, SRBFs, circular Gaussian, single and multiple scattering, GPU.

In hair rendering, it is often desirable to support the dynamic chang-

ing of hair’s scattering properties. This enables artists and designers

to edit hair appearance at will, and receive realistic rendering feed-

backs at interactive speed. Existing methods already support such

capability under simple lighting, such as a few point or directional

lights [Zinke et al. 2008; Yuksel and Keyser 2008; Shinya et al.

2010; Sadeghi et al. 2010]. However, it remains a challenge to ren-

der and simultaneously edit hair appearance under complex light-

ing such as environment maps [Debevec and Malik 1997]. Such

lighting is important to convey the rich look of the hair in natural

illumination conditions.

Themain difficulty with environment lighting is the large number of

directional lights that must be considered. An effective solution is

to approximate the environment map as a set of spherical radial ba-

sis functions (SRBFs), yielding a low-dimensional representation.

This approach has been studied in the context of precomputed light

transport and BRDF aprpoximation [Tsai and Shih 2006; Green

et al. 2007; Wang et al. 2009]. For hair rendering, Ren et al. [2010]

proposed to integrate the hair scattering function with each SRBF

light to produce realistic rendering effects. Their method incorpo-

rates both single and multiple scatterings. Unfortunately, as they

precompute the integrals of the scattering function with sampled

SRBF lights into 4D tables, their method requires fixing hair scat-

tering parameters at precomputation time, disabling online editing.

In this paper, our goal is to enable realistic rendering and simulta-

neous editing of hair appearance under complex environment light-

ing. Similar to previous work, we represent an environment map

using SRBF lights. Our main contribution is to derive a compact 1D

circular Gaussian representation that can accurately model the hair

scattering function introduced by [Marschner et al. 2003]. Analyses

show that our representation is both concise and accurate. Exploit-

ing the properties of Gaussian functions, the primary benefit of this

representation is that it enables the***n-time evaluation of closed-

form integrals of the scattering function with SRBF lights. This

results in efficient computation of both single and multiple scatter-

ings, without the need of expensive precomputation. By using this

approach, our algorithm evaluates the rendering integrals entirely

on the fly, allowing the user to dynamically change hair scattering

parameters at will. Our approach can be seen as an accurate model

of the scattering function that is particularly suitable for integration

with SRBF lights. In addition, our algorithm successfully incorpo-

rates the eccentricity of the hair, which is important for capturing

the hair’s rich look.

We provide a GPU implementation that achieves interactive rates

for rendering and editing of spatially-varying hair appearance un-

der environment maps. We believe the ability to dynamically adjust

the hair appearance under natural lighting provides artists a conve-

nient way for designing hairs. Our method is the first to achieve

such capability, including the editing of spatially-varying hair pa-

rameters and hair eccentricity effects. Figure 1 shows an example

of our results captured at***n-time.









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