# Imagineering

In the real world, it’s quite common to have to read and understand someone else’s code and even learn some new library along the way. Let’s do just that.

Suppose that you’ve been hired as a Software Engineer Imagineer at Pixar Animation Studios to work on a sequel to Finding Nemo and Finding Dory, which were animated films starring fish. Your team is interested in developing a model for fish movement, especially for scenes in which schools of fish will be swimming in the background.

Suppose that a previous imagineer created this program in JavaScript to simulate birds flying and “flocking” together, which seems like a reasonable starting point for modeling fish swimming together. (Click the program’s run button to run the code.) Your job is to figure out how their program works and, ultimately, propose how to adapt it for fish! No need to understand every line of code, though. We’ll guide you through some of it!

Note that the code uses p5.js, a JavaScript library for animation “that empowers artists, designers, students, and anyone to learn to code and express themselves creatively on the web.” You might find its reference helpful!

Take a look at the imagineer’s first function, setup:

let boids = [];

function setup() {
createCanvas(720, 400);

// Add an initial set of boids into the system
for (let i = 0; i < 100; i++) {
boids[i] = new Boid(random(width), random(height));
}
}


Notice that the previous imagineer commented, “Add an initial set of boids into the system”. A “boid” is a bird-like object. (Try saying it aloud.)

1. (1 point.) In no more than one sentence, explain how you could change how many boids are in the system.
1. (2 points.) In no more than three sentences, explain what the line of code below does. You might find it helpful to read up on JavaScript Object Constructors and then take a look at the imagineer’s constructor function (aka method).

 boids[i] = new Boid(random(width), random(height));


Next, take a look at the imagineer’s draw function:

function draw() {
background(51);

// Run all the boids
for (let i = 0; i < boids.length; i++) {
boids[i].run(boids);
}
}


Notice, in particular, that the previous imagineer commented, “Run all the boids”.

1. (3 points.) In no more than three sentences, what does it mean to “run” a boid? Be sure to look at the code for run and any functions it calls.
1. (3 points.) Suppose that, on slower computers, it might be painfully slow to run too many boids at once. In no more than three sentences, how could you change draw to only run some of the boids?
1. (2 points.) Notice how draw calls background, a function that comes with p5.js, per its reference, with an argument of 51, which makes the flock’s background gray. With what line of code could you instead change the flock’s background to the color that browsers know as Aqua, as though it’s swimming in water?

Finally, take a look at the imagineer’s run function (aka method),

run(boids) {
this.flock(boids);
this.update();
this.borders();
this.render();
}


as well as the imagineer’s flock function (aka method),

flock(boids) {
let sep = this.separate(boids); // Separation
let ali = this.align(boids);    // Alignment
let coh = this.cohesion(boids); // Cohesion
// Arbitrarily weight these forces
sep.mult(2.5);
ali.mult(1.0);
coh.mult(1.0);
// Add the force vectors to acceleration
this.applyForce(sep);
this.applyForce(ali);
this.applyForce(coh);
}


both of which are defined in the Boid class.

1. (3 points.) Notice how run calls this.flock(boids), wherein this refers to a boid object. In no more than three sentences, why does flock need access not only to this boid but also the whole array of boids?
1. (2 points.) Notice how flocking behavior is a function of three other behaviors: separation, alignment, and cohesion. See the comments atop separate, align, and cohesion, respectively, for definitions of each. Suppose that your team wants to create a single school of fish swimming tightly together. In no more than three sentences, explain how you could change flock to achieve that behavior. Again, no need to understand every line of code. See what you can glean from some trial and error! Note that if you check Auto-refresh atop the program, you can make changes to the code and see the effects in real time.