Artificial Animals in Realistic Virtual Worlds Demetri Terzopoulos Department of Computer Science, University of Toronto, Toronto, Canada, M5S 1A4 Abstract This paper describes a virtual marine ecosystem inhab- ited by realistic artificial life that emulates the appearance, movement, and behavior of real fishes. Each artificial fish is an autonomous agent in a simulated physical world. It has (i) a three-dimensional body with internal muscle ac- tuators and functional fins, which deforms and locomotes in accordance with biomechanic and hydrodynamic prin- ciples, (ii) sensors, including eyes that can image the envi- ronment, and (iii) a brain with motor, perception, behavior, and learning centers. Artificial fishes exhibit a repertoire of piscine behaviors that rely on their perceptual awareness of their dynamic habitat. Furthermore, they can learn how to locomote through practice and sensory reinforcement. Introduction The modeling of animals is an important theme in the field of artificial life. 1 In this paper, I present research about virtual artificial animals. Analogous to natural animals, artificial animals are endowed with functional bodies and brains, and are the autonomous inhabitants of a simulated physical world. We have created realistic artificial animals in the form of fishes. Imagine a virtual marine world inhabited by a variety of artificial fishes. In the presence of underwater currents, the fishes employ their muscles and fins to swim gracefully around immobile obstacles and among moving aquatic plants and other fishes. They autonomously explore their dynamic world in search of food. Large, hungry predator fishes stalk smaller prey fishes in the deceptively peaceful habitat. Prey fishes swim around contentedly until the sight of predators compels them to take evasive action. When a dangerous predator appears in the distance, similar species of prey form schools to improve their chances of survival. As the predator nears a school, the fishes scatter in terror. A chase ensues in which the predator selects victims and consumes them until satiated. Some species of fishes seem untroubled by predators. They find comfortable niches and feed on floating plankton when they get hungry. Driven by healthy libidos, they perform elaborate courtship rituals to secure mates. To simulate worlds with the level of complexity of the one depicted above, we have taken a bottom-up, composi- tional modeling approach. We model the form, appearance, 1 For an engaging survey of the field, see, e.g., S. Levy, Artificial Life (Pantheon, 1992). Journals such as Artificial Life and Adaptive Behavior document the state of the art. F c o e u s r s intention Intention Generator Behavior Routines Habits Behavior Perception Motor sensory data control param- eters sensory data filtered Sensors Motor Controllers Actuators / Muscles Physical Model Learning Optimization Brain / Mind Figure 1: Overview of an artificial fish. The functional body harbors a brain with motor, perception, behavior, and learning centers. and basic physics of the animal and its environment, the animal’s means of locomotion, its perceptual awareness of its world, its behavior, and its ability to learn. I review the artificial fish model in the next section before present- ing some of the results of our research (see [1, 2, 3, 4] for further technical details and a review of relevant prior work). An Artificial Fish The artificial fish is an autonomous agent with a realistic deformable body actuated by internal muscles, eyes, and a brain that includes behavior, perception, and motor cen- ters. Fig. 1 presents a functional overview of the model. Through controlled muscle actions, artificial fishes are able to swim through simulated water in accordance with sim- plified hydrodynamics. Their functional fins enable them to locomote, maintain balance, and maneuver in the water. Thus the artificial fish captures not just the 3D form and appearance of real fishes, but also the basic physics of these animals in their environment. Though rudimentary com- pared to their natural counterparts, artificial fish minds are nonetheless able to learn some basic motor functions and carry out perceptually guided motor tasks. In accordance with their perceptual awareness of the virtual world, their