Animating fracture

The task of specifying the motion of even a simple animated object like a bouncing ball is surprisingly difficult. This difficulty is due, in part, to our highly developed human skill of observing movement and quickly detecting motion that is unnatural or implausible. Moreover, the motion of many objects is complex, and specifying their movement requires the generation of a great deal of data. For example, cloth can bend and twist in many ways, and the breaking bunny statue in Figure 1a involves hundreds of individual shards. Animators use three main techniques to generate synthetic motion: keyframing (manually specifying motion); motion capture (using data recorded from actors); and procedural methods (using computer algorithms to generate motion). Keyframing and motion capture both require that motion be specified by some external source. In contrast, procedural methods compute original motion automatically. Many procedural methods are based on informal heuristics, but a subclass known as “physically based modeling” employs numerical simulations of physical systems to generate synthetic motion of virtual objects. With the introduction of simulated water in the 1998 feature film Antz [7] (see Foster and Metaxas’s “Modeling Water for Computer Animation” in this section) and clothing in the 1999 Stuart Little [8], physically based modeling was clearly demonstrated to be a viable technique for commercial animation. Physically based modeling is especially effective for animating passive objects, because these objects are inanimate and lack an internal source of energy. The advantage of using simulation is not surprising, as these objects tend to have many degrees of freedom, making keyframing or motion capture difficult. Moreover, while passive objects are often essential to the plot of an animation and to its appearance or mood, they are not characters and do not require the same control over the subtle details of their motion. Therefore, simulations in which motion is These fracture patterns propagate arbitrarily in 3D solid objects as they break, crack, or tear realistically. a