  Animal collective behavior is often modeled with self-propelled particles,
assuming each individual has ``omniscient'' knowledge of its neighbors. Yet,
neighbors may be hidden from view and we do not know the effect of this
information loss. To address this question, we propose a visual model of
collective behavior where each particle moves according to bio-plausible visual
cues, in particular the optic flow. This visual model successfully reproduces
three classical collective behaviors: swarming, schooling, and milling. This
model offers a potential solution for controlling artificial swarms visually.
