Topics: Boids II

Topics showcase applications of the core concepts, in this case arrays and making choices.

Behavioral animation is an animation and simulation technique where simple behaviors are combined to yield complex individual and group behavior.

These exercises are a chance to explore some of those combinations and to see how one might write a program to simulate complex group behavior; the focus is on combining behaviors to get interesting results rather than implementing the steering behaviors themselves. The Boids I exercises utilize the three patterns for boid behavior: a single behavior, multiple behaviors active at once, and choosing between behaviors or sets of behaviors. These exercises bring all of those elements together in a simulation of a small ecosystem (shown).

Important prerequisite: these exercises build on the Boids I exercises — complete those first!

Your task is to create a sketch which simulates an ecosystem where prey boids forage for food and try to escape predators, as shown in the demo at the top of this page. The red boids are the prey which run from the predator and the blue boid is the predator which chases the prey. Over time the color of red boids fades as they become hungry. The black dot is a food source; its colors fades as it becomes depleted.

To create this sketch:

Evade and Pursue

The boids library contains the following function for this behavior:

// compute the evade steering vector 
//  pos, vel - position and velocity of boid
//  maxspeed - boid's max speed
//  pursuerpos, pursuervel - position and velocity of pursuer
PVector computeEvade ( PVector pos, PVector vel, float maxspeed, PVector pursuerpos, PVector pursuervel ) { ... }

// compute the pursue steering vector - pursues the nearest quarry within the boid's neighborhood
//  pos, vel - position and velocity of boid
//  radius, angle - define this boid's neighborhood
//  maxspeed - boid's max speed
//  quarrypos, quarryvel - positions and velocities of potential quarries
PVector computePursue ( PVector pos, PVector vel, float radius, float angle, float maxspeed, PVector[] quarrypos, PVector[] quarryvel ) { ... }

These functions return a steering vector with a magnitude of 0 if there is nothing in sight.

Exercise 2

In this exercise you'll create your own sketch which demonstrates an interesting combination of behaviors. Here's your chance to experiment! For full credit your sketch must include the following elements:

• Name the sketch advboids2.

• There must be some sort of visible and describable individual or group behavior that arises from a combination of behaviors. For example, you might use offset pursuit to have a group of boids fly in formation, or you might have a game of tag where a boid pursues another that tries to hide behind obstacles. Describe the result — e.g. "fly in formation" or "game of tag" — and include the description in a comment in your sketch. (If you can only describe what happens in terms of the behaviors it is built from, it's not a describable behavior.)

• Do something substantially different from what was implemented in Exercise 1 and in the Boids I exercises. Simply combining two or more exercises doesn't count as "substantially different" — add to them.

• Use at least one behavior not used in Exercise 1 and the Boids I exercises. There are additional behaviors in the boids library file — look through that, focusing on the comments and the function headers. (You can look at how the behaviors are implemented in the function bodies if you are curious, but it isn't necessary to do that to use them.)

• Demonstrate all three patterns for combining behaviors: a single active behavior, multiple behaviors active at the same time, and choosing between behaviors. These can overlap, for example, a boid which seeks the mouse when the mouse is nearby and otherwise flocks with other boids covers all three patterns.

To do this:

For extra credit, create an especially elaborate or nifty sketch and/or implement and use one or more of your own steering behaviors. Stop by office hours to discuss possibilities if you are interested in implementing new steering behaviors. (Some familiarity with working with vectors and/or geometry would be helpful.)