Figure 2. Screenshot of teddy and sample 3D models created using the system.

Figure 3. modeling session in teddy. the user can create a 3D model using simple sketching operations.

the silhouette of the base primitive, and the system generates the corresponding 3D geometry. The user then draws a stroke across the model and the system cuts the model at the line. The user can also add parts to the base model by drawing two strokes. Figure 2 right shows several 3D models created using the system.

Several sketching systems for free-form shapes were developed after Teddy. The original Teddy system used polygonal meshes, but some later systems experimented with other representations such as voxels20 and implicit surfaces. 2 Some systems extended the interface to support subsequent editing by direct manipulation. ShapeShop21 represents a model as a collection of blob primitives and allows the user to move or scale each primitive. Fibermesh19 keeps the original stroke as a control curve on the model surface and allows the user to adjust the shape by deforming the curves. However, all these systems are designed for purely virtual 3D models without consideration of the physical properties of materials. Plushie is innovative in that it shows the feasibility of using a sketching interface for free-form shapes in the design of physical objects.

3. DESiGninG Ph YSiCaL moDELS With a ComPutER Another key aspect of our work is the tight integration of physical simulation into the 3D modeling process. In traditional applications, modeling and simulation are completely separate. A virtual model is created in 3D modeling software without considering any physical constraints, and it is then passed to a simulation environment. If the simulation result reveals a problem, the user returns to the model to fix the problem. We made this process more efficient by running the simulation concurrently with the

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modeling to create only models that are physically realizable. In this way, the user can more efficiently explore the design dimensions within realistic constraints. From the user’s point of view, the model generated by the system may not correspond exactly to the shape that was input, but it will be a physically realizable shape reflecting the input shape.

Some recent systems have tried to incorporate fast physical simulation into an interactive design process. Igarashi and Hughes developed a mark-based interface for putting clothing on a virtual character, 10 and Decaudin et al. proposed a system for designing an original garment via sketching. 6 Both used simple geometric simulations to represent the physical properties of cloth material. Masry and Lipson described a system in which the user can quickly build a CAD model by sketching and immediately apply a finite element analysis to the model. 15 However, the model construction is computed before the simulation in these systems, and no dynamic feedback loop exists between the simulation result and the original user input.

Several efforts have also been made to support the design of physical objects by end users in the computer graphics research community. Mitani and Suzuki16 and Shatz et al. 22 presented automatic segmentation of a 3D model into surface patches that can be perfectly flattened onto a plane without distortion for constructing paper craft models. Similarly, Julius et al. proposed similar method for plush toys14 allowing small distortion. Pillow system17 facilitates the manual segmentation of a model by providing automatic flattening and by showing the result of physical simulation. These systems make plush-toy design more accessible, but the fundamental challenge of creating an original plush toy is still unresolved.

4. thE PLuShiE SYStEm

The system consists of two windows: one shows the 3D plush-toy model being constructed and the other shows the corresponding 2D pattern (Figure 4). The user works on the 3D view, interactively building the 3D model by using a sketching interface. The 2D view is mainly for reference but the user can also edit the 2D pattern directly when desired. The 3D model is produced from a physical simulation of the assembled 2D pattern. After each input from the user,

Figure 4. a screen snapshot of the Plushie system.