Lace Curtain: Interactive Animation of Transparent Woven Fabric
Based on Microfacet BSDF Model
Tatsuya Matsumoto*Kensuke Tobitani
Sosuke Kaji
Noriko Nagata
Research Center for Kansei Value Creation,
Graduate School of Science and Technology, Kwansei Gakuin University

1. Introduction
Realistic and interactively generated computer graphics areneeded in various fields, such as for CGI-animated movie and ineducation. To express the realistic appearance of various materials,
it is important to represent the specific optical properties for eachmaterial.
In this study, we focus on woven fabric, which is frequently
expressed in computer graphics. Woven fabrics such as lacecurtains have specific transparency, and change their appearance
drastically by their deformation. To express the specific factors ofwoven fabrics under any environment, we proposed themicrofacet BSDF model [Nomura et al. 2011]. We also generate
animation with interactive frame rate using a ray tracing engine toexpress the realistic appearance caused by the deformation.

(a) Rendering image in a state of rest

Figure 2. Results of rendering based on the microfacet BSDF
model.
BTDF model has newly added specific parameters for woven
fabric such as density of threads.

2. Physically based model for woven cloth
2.1 Measurements of woven cloth
To compare the BTDF model with measurement BTDF, wemeasured optic properties of the four fabrics samples using theBRDF instrument OGM-3 (Optical Gyro Measuring Machine).
Measurement was performed to utilize metal halide as light source
which is similar to natural light. 4800 points were measured bychanging the angle of camera and light.

3. Interactive animation of woven fabric
We used ray-tracing as the rendering algorithm to express therealistic reflected and transmitted light, and used ambient
occlusion for global illumination. To achieve the interactive frame
rate with ray-tracing, rendering algorithm was implemented byusing Nvidia’s OptiX ray-tracing engine [Steven et al. 2010]. The
microfacet BSDF model was directly implemented in the closest
hit program as shading function in order to represent the specificappearance of woven fabric. We also implemented particle based
cloth simulation for the fabric behavior.
Figure 2 showed the results of rendering. The rendering environments are a 3.40 GHz Intel Core i7 processor, 8GB of RAM andNvidia Quadro K4000 2 GB graphics processor. We have achieved a frame rate of 10.5 fps under a resolution of 800 × 600. We
can observe that the transmitted light of woven fabric under thedeformation, and the transmission factor differs according to theregion of curtains.

2.2 Woven cloth microfacet BSDF model

4. Conclusions

Figure 1. Comparison between the measured BTDF and BTDF
model (elevation angle of camera isand light is , azimuth
angle of camera isand light is)

We proposed microfacet BSDF model to express the specifictransparency of woven fabric, and generate animation with cloth
simulation in order to represent the changing appearance under
the fabric behavior. In this study, we used ambient occlusion todetermine global illumination. However, ambient occlusion
cannot simulate the caustics of lace curtain. As future works, weintend to use photon mapping for the global illumination that wasimproved to sample photon based on a woven cloth BTDF model.

To estimate the best-fit model for measurement BTDF of fabrics,
we compared the approximate curves calculated from six typicalmodels (Phong, Ward, Ashikhmin-Shirly, Cook – Trance, HGF,and GGX) with the measured BTDF curves. Figure 1 showed theresult of comparison between the measured BTDF and themodeled BTDF. We can observe that the GGX model is the bestfit model for measurement BTDF data. Therefore, we proposed a
new microfacet BSDF model based on the GGX model in order torepresent the transparency of woven fabrics. The micorofacet

References
NOMURA, S., et al. 2011. Lace curtain: Modeling and RenderingWoven Cloth using Microfacet BSDF-production of a catalog
of curtain animations-. In ACM SIGGRAPH 2011 posters.

*e-mail:tatsuya0809@kwansei.ac.jp
Permission to make digital or hard copies of part or all of this work for personal orclassroom use is granted without fee provided that copies are not made or distributedfor commercial advantage and that copies bear this notice and the full citation on thefirst page. Copyrights for components of this work owned by others than ACM must behonored. Abstracting with credit is permitted. To copy otherwise, to republish, to post on
servers, or to redistribute to lists, requires prior specific permission and/or a fee.
Request permissions from permissions@acm.org.
I3D 2015, February 27 – March 01, 2015, San Francisco, CA.
2015 Copyright held by the Owner/Author. Publication rights licensed to ACM.
ACM 978-1-4503-3392-4/15/02 $15.00
http://dx.doi.org/10.1145/2699276.2721399(b) Rendering image in a state of motion

STEVEN, G. P., et al. 2010. OptiX: A General Purpose Ray Tracing
Engine. ACM Trans on Graphics 29, 4 (July), 66:1–66:13.

133