Animation with Skeletal Draw

Animation takes full advantage of the picture and deformation abstraction capability provided by skeletal strokes. The Skeletal Draw program has also been built with facilities to generate animation sequences. Traditional key-frame technique is used by Skeletal Draw. To create the key-frames, the user directly manipulates the characters in the scene. The system then calculates the in-betweens by interpolating Catmull-Rom splines. After the keyframes have been laid out, the program can do a quick playback using fewer in-betweens and substituting any skeletal strokes with rectangular (possibly deformed) ones. This quick preview is mainly to let the user to have a feel for the timing and smoothness of the motion. The in-betweens can be recorded in the same formats as the key-frames for inspection or further refinement. Because we are interpolating only the skeletons and they are usually of very simple forms, ensuring a proper interpolation of them is trivial. Furthermore, the shapes of the flesh are totally controlled by the skeletons, no interpolation of the individual points on the flesh is performed. Therefore the ambiguity of the shape blending problem[41,42] on the flesh would not arise at all.

The use of general anchoring is not limited to character creation. Since an arbitrary deformation can be recorded in one single stroke, a skeletal stroke can be viewed as an encapsulated unit of motion. These 'canned' motions can be applied conveniently at any time in a sequence. By redefining the sub-strokes, these motions can be used to animate other objects.

Stylized and complex drawings like those by Maurice Sendak[43] and Dr. Seuss[44] are known to be difficult to animate. It took Oscar winning animator Gene Deitch five years of experimenting before he found a way to transfer Maurice Sendak's drawing techniques to the screen[18]. With skeletal strokes' compact abstraction, complicated hatching or stipples can be condensed into simple units with which to further build up characters.

We have created a 1 minute animation sequence, The Cat in Skeletal Strokes, based on Dr. Seuss' The Cat in the Hat[44] character using the Skeletal Draw program. Two man days were used to create the sequence, which could have been even shorter

had the bugs which appeared then been fixed earlier. Taking into consideration that our system was run on a 486 PC, this efficiency compares even more favourably than the Inkwell system[32]. The most time consuming part in the course of production is actually in the generation of antialiased frames.

Figure 18: A rasterized frame from The Cat in Skeletal Strokes. The length and direction of the speed lines changes automatically with object movements.

Figure 19: A picture after Javier Mariscal's Spain, the New Rising Star (1990). A few strokes have been specially defined for this picture, e.g. the olive-green wiggly deco on the roof top and the hotel in the backdrop.

Figure 19: A picture after Javier Mariscal's Spain, the New Rising Star (1990). A few strokes have been specially defined for this picture, e.g. the olive-green wiggly deco on the roof top and the hotel in the backdrop.

Figure 20: After the colour-lithograph Loge with Golden Mask (Detail) by Henri de Toulouse-Lautrec (1864-1901).

morphing control lines on sampled images, we could build up photorealistic pseudo-3D models of images instead of vector based pictures. With the hierarchical control over various parts of the strokes, we could start to make Michael Jackson's eyes blink and his head turn independently instead of our cartoon character's.

Figure 21: The width and shear angle of the yellow 'sausage' stroke (detail of the picture in Figure 19) is being modified.
Javier Mariscal Easiest Drawings
Figure 22: Where on Earth is SIGGRAPH94. After a 'Cobi' poster by Javier Mariscal. Notice the fractal strokes at the lower part of the picture.

There are also published techniques which are useful for high level specification of animation sequences. For example, skeleton based 3D animation techniques based on physical deformations of complex 3D models[16,23]. While these may be useful for realistic deformations, they are not quite applicable to stylish figures, which do not even have a 3D realizable form. Furthermore, physical simulation is often unnecessary in stylish animation, which is the field we have been focusing on.

Techniques like gait control[22], constraint based methods [7,29,36,40,51,52] and dynamics simulation[13,46] are certainly important for creating highly realistic and complex physically based motions. Incorporation of these methods into our system is worth exploring in future.

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