Skeleton verses Envelope

Many drawing/2D warping packages allow the user to specify the deformation of a picture by reshaping an envelope around the undeformed picture. This technique has been advocated for applications in 2D animation in as early as the 70's[15] and is still widely used in the latest systems[32]. A coordinate system transformation based on envelope deformation is quite different from a skeleton based one. The former is much more well defined as the coordinates within an envelope can usually be interpolated bi-parametrically from the bounding curves defining the envelope.

To control the whole coordinate system with one single skeleton, however, a proper deformation model must be defined to handle the singularities arising from extreme bending cases. Under this definition, the 'skeleton techniques' described in [15] should not be considered as a skeleton deformation.

Envelope based techniques may be suitable for manipulating a few pictures. When it comes to controlling individual strokes in a complicated drawing, however, it becomes as cumbersome as the use of lines to trace out brush trails. One could see why the place of general brush strokes are not so easily replaceable.

Figure 2: A stylish figure after Tomio Mohri[35]. (a) With a constant thickness stroke. (b) With 5 types of skeletal strokes.

Figure 2: A stylish figure after Tomio Mohri[35]. (a) With a constant thickness stroke. (b) With 5 types of skeletal strokes.

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