The 3D evaluation of design shapes is an essential step in product styling. Thus, physical models of final-stage design alternatives have been made in tradition, and recently the effort to substitute them with CAD models has been tried. Whereas, designers in the early phase of the design-development stage where almost design concepts are determined, still use raster-type 2D graphics S/Ws. It causes not only the difficulty of evaluating 3D shapes but also the serious severance of a digital dis-connectivity with downstream processes. This paper presents a method of directly constructing 3D plane-symmetric freeform curves with a sketch interface, as the first step of developing a sketch-based 3D-freeform shape creation S/W for designers. A curve drawn by the designer within the see-through box in an arbitrary perspective view, is simultaneously converted to a real space curve without the 3D ambiguity problem except only special cases to be specified.
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