Real-time Cloth Simulation with Sparse Particles

Contact
Masaki Oshita

Akifumi Makinouchi

Kyushu University, Japanmoshita@db.is.kyushu-u.ac.jp

Physically-based dynamic simulation is a common technique forcloth animation. The clothes are modeled as a combination ofmass-distributed particles and elastic forces that work between theparticles. Typically between a few thousand and a hundred thousand particles are required to simulate cloth movement. Althoughmany techniques have been developed for fast and robust simulation,1 dynamic simulation requires too much computational time toanimate clothes in real-time.
In this sketch, a novel technique for real-time cloth simulation ispresented. The method combines dynamic simulation and geometric techniques. Only a small number of particles (a few hundred)
are controlled using dynamic simulation to simulate global cloth
behaviors such as waving and bending. The cloth surface is thensmoothed based on elastic forces applied to each particle and thedistance between each pair of adjacent particles. Using this geometric smoothing, local cloth behaviors such as twists and wrinkles areefficiently simulated (Figure 1).

3 (a), and 4,096 triangles used for the smoothed surface – Figure
3 (b). The computational time was approximately 30 milliseconds for each frame of the 30 Hz simulation. As a result,the skirt was animated in real-time. The proposed method
has potential for use in computer games, virtual studios, andvirtual fashion shows.
References
1. Baraff, D. & Witkin, A. (1998). Large steps in cloth simulation. In Proceedingsof SIGGRAPH 98, 43-54.
2. Vlachos, A., Peters, J., Boyd, C., & Mitchell, J. (2001). Curved PN triangles.
Proceedings of the 2001 ACM Symposium on Interactive 3D Graphics.

Proposed Method

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In the proposed method, PN triangles2 is used to smooth a clothmesh. This is a smoothing technique that substitutes a three-sided
cubic Bézier patch for each triangular face of a mesh. It facilitates
control of the smoothing mesh using the normal of the vertices,while other smoothing techniques such as subdivision (e.g.
Catmull-Clark) and parametric (e.g. NURBS) surfaces generate asmoothed mesh from the vertices of a simple mesh. In addition, PNtriangles ensures that the generated surface matches the original
mesh on each vertex.
The cloth surface is smoothed by controlling particle normals andedge length using the PN triangles. Figure 2 illustrates the smoothing method. Figure 2 (a) is the original mesh and (b) is thesmoothed surface using PN triangles without normal control. The
normal of each particle is computed from the elastic forces that areapplied to the particle from the adjacent faces. The elastic forces
work so as to make the tangent of the cloth surface parallel withthe elastic forces. Therefore we compute the normal direction bybalancing the vectors that are orthogonal to the elastic forces –Figure 2 (c). In addition, when the length of an edge is shorter thanits original length, the midpoint of the curve is moved so as tomaintain the original edge length – Figure 2 (d). As a result, thesmoothed surface reflects the elastic forces and maintains surface
dimensions.

Figure 1. An example of the proposed approach:(a) a small number of particles (25 particles in this case)are controlled using dynamic simulation; (b) the cloth
surface is then smoothed using geometrical techniques(2,048 triangles were rendered).

Figure 2. Smoothing method: (a) a section of an original
mesh -- the middle edge is shortened and the elastic forces
are working so as to part the two vertices; (b) the smoothedsurface by PN triangles; (c) with normal control; and(d) edge length control.

Implementation and Results
A particle-based dynamic simulation using the proposed method
has been implemented. The proposed method is very simple and iseasy to implement and integrate with an existing particle-based system. In terms of dynamic simulation, existing techniques1 could beused. However, the proposed method efficiently smoothes the cloth
surface in response to the distance between adjacent particles by
setting low stiffness of the cloth, while standard systems maintainthe shape of the cloth by setting high stiffness.
Figure 3 shows a snapshot of the resulting animation. Forty particles of the skirt were controlled using dynamic simulation – Figure

Conference Abstracts and Applications

Sketches & Applications

Kyushu University

Figure 3. A snapshot from an animation of a skirt:(a) the controlled mesh (40 particles); and (b) the smoothedsurface (4,096 triangles).