Simulating and Rendering Wet Hair

彬彬

1 Introduction

Simulating hair dynamics is one of the most challenging tasks in

computer animation due to its complexity and required realism.

The huge amount of hair make the computation and rendering very

cumbersome, especially when influenced by visual effect elements

such as water and fire. In this paper we propose a physically-based

model to capture the characteristics of wet hair. Hair and fur in our

system can interact with water particles and change the parameters

such as weight and stiffness for physics simulation. A fast cohesion

process is then applied to wet hair at each time step to achieve realistic

clumping effects. We also introduces a shading model that

produces convincing appearance for wet hair

2 Dynamics of Wet Hair

Figure 2: The cohesion and diffusion processes. (a) The clumping

force brings the affected joints closer when the virtual spheres make

contacts. (b) As the wetness increases, the virtual spheres become

larger and collide with other spheres. (c) The simulation result after

cohesion and diffusion processes.

We model a hair strand with a series of rigid bodies connected by

joints, and represent the water by a group of particles. When a hair

segment collides with water particles, the wetness and the weight

of the segment increase, and its physical behaviors change accordingly.

We observe that the volume of the wet hair decreases drastically

[Ward and Lin 2004]. This is caused by the cohesion of water that

makes wet hair clump into bundles and groups. To improve efficiency,

only a subset of hair is selected to perform the cohesion

process that simulates the clumping effects. Figure 2 illustrates the

cohesion and diffusion processes. A virtual sphere with variable

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radius controlled by the wetness is placed at each joint to represent

the affecting space of the clumping force. When the hair segment

is hit by a water particle, the sphere attached to the connected joint

becomes larger as the wetness increases. When it is large enough to

make contacts with neighboring spheres, the clumping force brings

the affected joints closer. This way we can simulate the detailed

clumping effects for individual hair segments according to their

wetness.

We then apply a diffusion process on each joint to simulate the water

absorption and diffusion in the hair fiber. If a joint is changed

in the previous cohesion process, its neighboring joints on the same

hair strand within a diffusion range are also affected. The clumping

effect is smoothed along the hair strands by the diffusion process,

resulting in a much more natural look.

3 Shading

Our hair shading model is inspired by the physically-based approach

described in [Gupta and Magnenat-Thalmann 2007]. The

wetness of each hair segment is used to modify primary highlight,

secondary highlight, and transmission of the hair shader. The final

rendered images exhibit darker colors and higher specular reflections.

Additional parameters are provided to artists for further

controlling the degree of wetness.

4 Results and Conclusion

We have proposed an efficient method for simulating and rendering

wet hair. For the results presented in this paper, 10,000 out of

100,000 hair strands are chosen to perform the simulation. The cohesion

and diffusion processes take about two seconds per frame to

finish on a regular personal computer.

References

GUPTA, R., AND MAGNENAT-THALMANN, N. 2007. Interactive

rendering of optical effects in wet hair. In Proceedings of the

2007 ACM symposium on Virtual reality software and technology,

ACM, New York, NY, USA, VRST ’07, 133–140.

WARD, K., AND LIN, M. C. 2004. Modeling hair influenced by

water and styling products. In In International Conference on

Computer Animation and Social Agents (CASA), 207–214.

彬彬

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