Crowd simulation is the process of simulating the movement of a large number of objects or characters, now often appearing in 3D computer graphics for film.

The need for crowd simulation arises when a scene calls for more characters than can be practically animated using conventional systems, such as skeletons/bones.

Animators typically create a library of motions, either for the entire character or for individual body parts. To simplify processing, these animations are sometimes baked as morphs. Alternatively, the motions can be generated procedurally - i.e. choreographed automatically by software.

The actual movement and interactions of the crowd is typically done in one of two ways:

• Particle Motion: The characters are attached to point particles, which are then animated by simulating wind, gravity, attractions, and collisions. The particle method is usually inexpensive to implement, and can be done in most 3D software packages. However, the method is not very realistic because it is difficult to direct individual entities when necessary, and because motion is generally limited to a flat surface.

• Crowd AI: The entities are given artificial intelligence, which guides the entities based on one or more of sight, hearing, basic emotion, energy level, aggressiveness level, etc.. The entities are given goals and then interact with each other as members of a real crowd would. They are often programmed to respond to changes in environment, enabling them to climb hills, jump over holes, scale ladders, etc. This system is much more realistic than particle motion, but is very expensive to program and implement.

The most notable examples of AI simulation can be seen in New Line Cinema's The Lord of the Rings films, where AI armies of many thousands battle each other. The crowd simulation was done using Weta Digital's MASSIVE software.

Crowd simulation can also refer to simulations based on group dynamics and crowd psychology, often in public safety planning. In this case, the focus is just the behavior of the crowd, and not the visual realism of the simulation.

External links

• NetLogo (http://ccl.northwestern.edu/netlogo), a free software for multi-agent modeling, simulation, and the like.
• MASSIVE (http://massivesoftware.com), the software used in The Lord of the Rings films.