An Interface for Sketching 314 Curves

closely matches the one she is used to namely pencil and paper. The ability to specify nonplanar 3D curves is of fundamental importance in 3D modeling and animation systems. Effective techniques for specifying such curves using 2D input devices are desirable, but existing methods typically require the user to edit the curve from several viewpoints. We present a novel method for specifying 3D curves with 2D input from a single viewpoint. The user first draws the curve as it appears from the current viewpoint, and then draws its shadow on the floor plane. The system correlates the curve with its shadow to compute the curve’s 3D shape. This method is more “natural” than existing methods in that it leverages skills that many artists and designers have developed from work with pencil and paper. CR Categories: 1.3.6 [Computer Graphics]: Methodology and Techniques-Interaction Techniques The technique we present is an extension of the idea used in [ll, 201 that a point in 3D can be determined from its image-space projection together with that of its “shadow.” (The “shadow” is just the vertical projection of the point onto some horizontal surface.) We apply this idea to a connected set of 3D points to define a curve. With this approach, the user sketches a curve directly into a scene in two strokes: Iirst drawing the curve as it appears from the current viewpoint, and then sketching its approximate “shadow.” The effect is to redefine the 3D shape of the curve while leaving its appearance unchanged. The user can then refine portions of the curve by over-sketching either its projected image or that of its shadow. Although this technique is less precise than existing ones, it lets the user quickly sketch a reasonably correct shape that may be further refined with more conventional methods.