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A major challenge of making WALLldrE was creating believable physics for human and robot crowds. To do this, Pixar technical directors combined a custom spring-physics system in the Massive software platform with traditional simulation methods. The performance was fast enough to scale for large crowds and maintain interactivity for previews. Computer animation is a developed, thriving field that has benefitted greatly from the techniques of traditional animation and its insights into what elements of motion create appealing characters. Pixar Animation Studios introduced the adaptation of such elements as squash, stretch, follow-through, and anticipation to the field, which guide some of the most successful computer animation to date. However, these elements are often neglected in crowd simulations, where rendering challenges, group behavior, and appearance variation are the dominant focus.