When we began our design of 'Menagerie', we wanted to create a playful virtual world that was dynamically alive and interactively confrontational.
The central elements of the encounters in 'Menagerie' are behavioral simulations of abstracted animal forms. Five different species of animals enter and exit the visible space through 'portholes' and 'doorways' that materialize near the user as she/he roams the infinite ground plane beneath. The animals appear to come from an invisible world behind the doors, beyond the virtual world explored by the user. 'Menagerie' is structured as a series of events comprised of different combinations of animals that emerge and interact with the user according to varying behavioral scenarios.

Through interaction, the character of the animals is revealed. For example, in one event, as the user moves toward a group of birds gathered on the ground, they take off and land at a slightly further location, baiting the user to advance again, eventually disappearing into a porthole that closes as the last bird flies in.
The motion of the individual animals is modeled with algorithms that simulate the physical qualities of movement. Many of the techniques employed are inspired by the robotics field. Legged animals respond to simulated gravity as they walk and run in various gaits. They are able to spontaneously plan footholds on the ground so that they appear to be dynamically balanced. Gait shifting algorithms were developed that minimize the dynamic perturbation of the body as legs undergo a phase shift during the gait transition period. Birds and other flying creatures accelerate when flapping their wings and bank realistically into turns. (Michael Girard/Susan Amkraut)