For a few boids more - boids in Rust (Part 2)

We give life to a school of fish by implementing the classic boids algorithm in Rust.

For a few boids more - boids in Rust (Part 2)

Hello again. In part 1, we began our ambitious experiment of simulating a school of fish. We set up a scene and added some basic animation for a moving cone. Those were baby steps. You can download the project at this stage from this tagged version.

In this part, we will,

  1. Generate boids and give them a random velocity
  2. Add the first rule to give boids life

At the end of it our boids will become a little intelligent and behave like this.

The tranquil pleasure of obstacle avoidance

A pinch of randomness

In this section, we'll generate many boids with and give them a random velocity. The rand crate will help us, so add rand = "0.7.3" to project dependencies. We'll also use the UnitSphere distribution from rand_distr crate, so add rand_distr = "0.2.2" to the dependencies as well.

Let's create the spawn_boids function in At the top of the file, add the following imports and constant values.

use rand::{thread_rng, Rng};
use rand_distr::{Distribution, UnitSphere};

// Scaling factors
const TIME_SCALE: f32 = 1.0;
const MAX_VEL: f32 = 4.0;
const MIN_VEL: f32 = 1.0;

--- clipped ---

impl Boid {
	--- clipped ---
   	pub fn spawn_boids(c: &[f32; 3], r: f32, n: usize) -> Vec<Boid> {
        let centre: Vector3<f32> = Vector3::from_row_slice(c);
        let mut boids: Vec<Boid> = Vec::new();
        let mut rng = thread_rng();

		--- in next section ---
spawn_boids in

This function spawns boids at random points inside a sphere. It takes the following parameters:

  1. c - The coordinates for the centre of the sphere
  2. r - The radius of the sphere or the bounds inside which boids can be spawned
  3. n - The number of boids to spawn

thread_rng() initializes a system seeded random number generator. We also intialise a Vec<Boid> to collect all the instances we generate.

Here's the next section, where we generate random points for a boid's starting position.

        --- clipped ---
        for _ in 0..n {
            // create position by random offset from centre within given radius
            let off_value = r * rng.gen_range(-1.0, 1.0);
            let coords: [f32; 3] = UnitSphere.sample(&mut rng);
            let offset: Vector3<f32> =
            let pos: Point3<f32> = Point3::from(centre + offset) * off_value;

			--- in next section ---
Randomness to the rescue

Here's what how it works:

  1. off_value is any value between positive and negative radius limits
  2. UnitSphere is a random distribution which picks a random point on the surface of a unit sphere. It generates a unit vector which represents the offset direction.
  3. We create the Point3 position of the boid by adding the offset vector to the centre vector. The offset vector is derived by multiplying offset value with offset direction.

Then we generate random velocities.

            // create random velocity with magnitude between MIN_VEL and MAX_VEL
            let vel_value: f32 = if rng.gen_bool(0.5) {
                rng.gen_range(MIN_VEL, MAX_VEL)
            } else {
                rng.gen_range(-MAX_VEL, -MIN_VEL)
            let coords: [f32; 3] = UnitSphere.sample(&mut rng);
            let vel: Vector3<f32> = Vector3::<f32>::from_row_slice(&coords) * vel_value;

            // add boid to Vector
            boids.push(Boid { pos, vel })

Give it some velocity

It is similar to generating random positions. A random magnitude and a random direction vector to creates a random velocity. Using the position and velocity, we create a Boid and push it into boids. To use spawn_boids the following changes to are necessary.

  1. Create an equivalent for spawn_boids namely spawn_cones
  2. Modify the render function to handle a lists of boids and cones
use three::{Object, Mesh};

mod boid;
use boid::Boid;

const BACKGROUND_C: u32 = 0xF0E0B6;
const SPAWN_CENTRE: [f32; 3] = [0.0, 0.0, 0.0];
const SPAWN_RADIUS: f32 = 3.0;
const SPAWN_NUMBER: usize = 10;
Make constants forspawn_boids arguments 

Change the imports to freely use Boid and Mesh struct without the absolute path.

fn spawn_cones(win: &mut three::Window) -> Vec<Mesh> {
    let mut cones: Vec<Mesh> = Vec::new();
    for _ in 0..SPAWN_NUMBER {
        let cone = {
            let geometry = three::Geometry::cylinder(0.0, 1.0, 1.5, 12);
            let material = three::material::Wireframe { color: three::color::BLACK };
            win.factory.mesh(geometry, material)


Create a number of cones

The roles of spawn_cones is to:

  1. Create SPAWN_NUMBER of cone meshes
  2. Add them to the Window
  3. Push them into a Vec<Mesh>
  4. Return the list of meshes

We have not manipulated the position or transformation of the cone in anyway. We'll do that inside the game loop.

fn main() {
	--- clipped ---
    // create boid
    let mut boids: Vec<Boid> = Boid::spawn_boids(&SPAWN_CENTRE, SPAWN_RADIUS, SPAWN_NUMBER);
    let cones: Vec<Mesh> = spawn_cones(&mut win);

    // render scene
    while win.update() && !win.input.hit(three::KEY_ESCAPE) {
    	--- clipped ---

        // compute new boxy velocity and set it
        boids.iter_mut().for_each(|b: &mut Boid| b.frame_update(win.input.delta_time()));
        boids.iter().zip(cones.iter()).for_each(|(b, c)| c.set_transform(b.pos_array(), b.rot_array(), 1.0));

		--- clipped ---
Game loop renders all the cones

We create a list of boids and cones each. Inside the game loop we update the objects on each frame. This section is perfectly suited for iterator syntax, so bear with me if it's not immediately obvious.

  1. Iterate over each Boid in boids and call frame_update on it. Since frame_update mutates the position, the reference must be mutable. iter_mut allows this by creating an iterator of mutable references in boids.
  2. Next the updated boid is used to set the cone's position and orientation a.k.a transform. set_transform does not require a mutable reference so just iter will do. However, each cone needs information from its respective boid. To achieve this, zip the boid and cone iterators together. The iterator yields a tuple containing the related boid and cone, which can then be used in set_transform.

Finally cargo run, shows this cool animation of rockets firing in all directions.

Firing rockets

We've created most of the graphical parts to simulate a flock of birds. However, we are lacking the most important components that gives life to these boids - the rules to interact with boundaries and each other.

tagged version

Finding the right direction

Right now, the boids simply ignore the boundaries of the box. In this section, we will add the logic to keep boids confined inside an enclosed space. It is a bit involved so pay close attention.

To do this, we need to create obstacles and a mechanism to detect them. The boids will only move in the unobstructed direction. The ncollide3d library will help us here. For obstacles, we'll use shapes that implement the RayCast trait. The trait contains the intersects_ray method which the boid will use to find an unobstructed direction.

intersects_ray in RayCast trait

The first obstacle

We encounter our first problem when creating a list of obstacle shapes. Let's try to define its type. Suppose we specify a Vec that can contain any struct that implements RayCast.

let mut obstacles: Vec<T: RayCast<f32>> = Vec::new();
Won't work

There are many distinct structs that implement the RayCast trait. For example, we have the Cylinder, the Plane, and the Cuboid among others. It won't work because a Vec can only contain objects of the same type and hence, size. T: RayCast does not tell the compiler anything about memory required by Cylinder and Cuboid. They might require different amounts of memory. The solution is to use dynamic dispatch, which is analogous to virtual functions from languages like Java and C++.

let mut obstacles: Vec<Box<dyn RayCast<f32>>> = Vec::new();
Will work

This way, we tell the compiler that each element is a fixed-size pointer i.e. Box which points to a struct implementing the RayCast trait. dyn RayCast tells the compiler that it will have to (dynamically) find out which type of shape is calling intersect_ray at run time.

We are not done yet. Shapes are always created at origin. If we want to compute collision with a cylinder at (10, 0, 0), we'll have to translate it before checking for ray collision. So we have a Vec containing the pointer to a shape and a related Isometry i.e. offset/translation/rotation to be performed.

let mut obstacles: Vec<(Box<dyn RayCast<f32>>, Isometry<f32>)> = Vec::new();

We'll use this definition to write out a function that creates some obstacles.

fn create_obstacles() -> Vec<(Box<dyn RayCast<f32>>, Isometry<f32>)> {
    // create obstacles
    let mut obstacles: Vec<(Box<dyn RayCast<f32>>, Isometry<f32>)> = Vec::new();
        Isometry::translation(-15.0, 0.0, 0.0)
        Isometry::translation(15.0, 0.0, 0.0)
        Isometry::translation(0.0, -15.0, 0.0)
        Isometry::translation(0.0, 15.0, 0.0)
        Isometry::translation(0.0, 0.0, -15.0)
        Isometry::translation(0.0, 0.0, 15.0)
        Box::new(Cylinder::new(25.0, 3.0)),
        Isometry::translation(-10.0, 0.0, 0.0)

Lot of obstacles

We create 6 planes that together create an enclosed cubical space. The cylinder is added just for fun. Note that obstacles are just logically entities that we will use for calculations. To show them in the scene, we will create Mesh objects that are similar in shape to obstacles. We'll refactor the code that adds meshes, by moving it to a dedicated function.

fn add_objects_to_scene(win: &mut three::Window) {
	--- clipped axes and other mesh code ---

    let mbox = {
        let geometry = three::Geometry::cuboid(30.0, 30.0, 30.0);
        let material = three::material::Wireframe { color: three::color::GREEN };
        win.factory.mesh(geometry, material)
    mbox.set_position([0.0, 0.0, 0.0]);

    let mcylinder = {
        let geometry = three::Geometry::cylinder(3.0, 3.0, 30.0, 12);
        let material = three::material::Wireframe { color: three::color::GREEN };
        win.factory.mesh(geometry, material)
    mcylinder.set_position([-10.0, 0.0, 0.0]);
Add mesh for obstacles

Here we simply use a cuboid to visually represent 6 planes. We did not use a cuboid for creating the obstacles because of performance reasons. Ray casting inside solid object like a cuboid is slower.

fn main() {

	--- clipped ---
	// add objects to scene
    add_objects_to_scene(&mut win);

    // create obstacles
    let obstacles = create_obstacles();
    // create boid
    let mut boids: Vec<Boid> = Boid::spawn_boids(&SPAWN_CENTRE, SPAWN_RADIUS, SPAWN_NUMBER);
    let cones: Vec<Mesh> = spawn_cones(&mut win);
    --- clipped ---
modified section for

main uses add_objects_to_scene and create_obstacles functions. cargo run should show something like this.

A glass prison

I have a ray gun

The boids don't care about the obstacles yet. They need to detect the obstacles and then find the closest unobstructed direction. To do this, a boid will fire off rays in all directions, like the image shown below.

Image from Sebastian Lague's video on boids

The directions are not random, they are equally spaced on the surface of a sphere. The technique to generate the directions is also taken from Sebastian Lague's code.

const RAY_NUMS: usize = 100;

use lazy_static::lazy_static;

lazy_static! {
    static ref RAY_DIRS: [Vector3<f32>; RAY_NUMS] = {
        let mut ray_dirs = [Vector3::new(0.0, 0.0, 0.0); RAY_NUMS];
        // initialize ray angles
        let golden_ratio: f32 = 1.618;
        let angle_increment = 3.1415 * 2.0 * golden_ratio;

        for i in 0..RAY_NUMS {
            let t: f32 = i as f32 / RAY_NUMS as f32;
            let inclination: f32 = (1.0 - 2.0 * t).acos();
            let azimuth: f32 = angle_increment * i as f32;

            let x = inclination.sin() * azimuth.cos();
            let y = inclination.sin() * azimuth.sin();
            let z = inclination.cos();
            ray_dirs[i] = Vector3::new(x, y, z);

creating a global, read-only array in

For the purpose of this experiment, you don't need to understand this completely. Just that the expression samples equally spaced points from the surface of a sphere and stores it in an array. You can check out the mathematics behind it here, and explain it to me sometime.

What's more important is that, we've used a macro called lazy_static. Add lazy_static = "1.4.0" to project dependencies. We are trying to create a static array of ray directions. These are some things to consider.

  1. The array of ray directions does not change once initialized.
  2. This array does not belong to any one Boid instance but can be considered a "class property" from other languages.
  3. Rust forbids const variables from containing heap allocated data and Vector3 is a heap allocated value.
  4. Rust does not allow for loops when declaring a static variable. This is to ensure that the amount of memory allocated is known at compile time.

The conclusion is that, we're in a pickle. Rust's strict rules do not allow the common pattern of declaring a static array with dynamically allocated data. lazy_static solves this by creating a one-off type that can be intialised only once. The variable still behaves like an array but it is only allocated when accessed for the first time. This solves our problem.

Non-aligned ray cast sphere and boid velocity

There is one flaw in the directions we created. The first value RAY_DIRS[0] is Vector3::new(0.0, 0.0, 1.0) i.e. positive z-axis. Rays get equally spaced starting from the positive z-axis. The above image shows a boid with a velocity not aligned with the axis of ray cast sphere. We will need to orient these rays along the boid's velocity before firing them.

Next, we implement the function that does the collision checking. It will perform the following steps:

  1. Check for obstruction. If not obstructed, skip the following steps.
  2. Iterate over all ray directions
  3. Correct the ray direction as per orientation
  4. Construct a Ray for the corrected orientation
  5. Iterate over all obstacles and check for intersection with ray
  6. Store and return unobstructed ray direction
  7. Change the boid's velocity

A ray consists of a point and a direction (somewhat like a Boid).

A point with a direction

Firing a ray means extending a line from the origin for a specific length along the given dir. If the line intersects any shape, it registers a collision. With this information, we can implement the collided function.

const OBSTACLE_DIST: f32 = 5.0;

/// check if a ray collides with the given obstacles
fn collided(obs: &Vec<(Box<dyn RayCast<f32>>, Isometry<f32>)>, ray: Ray<f32>) -> bool {
        .any(|(shape, iso)| shape.intersects_ray(iso, &ray, OBSTACLE_DIST))
helper function in

This function iterates over obstacles and returns true if any one of them intersects the given ray . Here 5.0 is the length of the ray and the limit of our collision detection. The next function is unobstructed_dir. It finds the closes unobstructed ray direction.

impl Boid {
	--- clipped ---
    /// return unobstructed direction closest to current velocity
    fn unobstructed_dir(&self, obs: &Vec<(Box<dyn RayCast<f32>>, Isometry<f32>)>) -> Option<Vector3<f32>> {
    --- next section ---
unobstructed_dir type signature

Let's see how it works.

  1. It received the list of obstacles and their corresponding translations we created earlier.
  2. There is a very slim possibility that the boid is completely surrounded by obstacles. In this case, the function returns None. This is indicated by its return type Option<Vector3<f32>>.
impl Boid {
	--- clipped ---
    /// return unobstructed direction closest to current velocity
    fn unobstructed_dir(&self, obs: &Vec<(Box<dyn RayCast<f32>>, Isometry<f32>)>) -> Option<Vector3<f32>> {
        // create a rotation to orient ray directions along velocity
        let ray_axis: Vector3<f32> = Vector3::new(0.0, 0.0, 1.0);
        let rot = UnitQuaternion::rotation_between(&ray_axis, &self.vel).unwrap_or(
            UnitQuaternion::from_axis_angle(&Vector3::x_axis(), std::f32::consts::PI),

	--- next section ---
Create correct rotation

This snippet is similar to the one we used to rotate the cone in the direction of its velocity. Here we create a rotation to orient the rays with respect to the boid's velocity. The next section performs the actual intersection checking.

impl Boid {
	--- clipped ---
    fn unobstructed_dir(&self, obs: &Vec<(Box<dyn RayCast<f32>>, Isometry<f32>)>) -> Option<Vector3<f32>> {
        --- clipped ---

        let mut best_dir: Option<Vector3<f32>> = None;
        for dir in RAY_DIRS.iter() {
            let ray = Ray {
                origin: self.pos,
                dir: rot * dir,

            // if direction is unobstructed store it
            // after correcting it's orientation
            if !collided(obs, ray) {
                best_dir = Some(rot * dir);

Find the correct direction

We iterate over ray directions, orient them, and check for collision. The first unobstructed direction is returned. We'll use the returned value to update self.vel but only if its current direction is blocked.

imple Boid {
	--- clipped ---
    pub fn frame_update(&mut self, obs: &Vec<(Box<dyn RayCast<f32>>, Isometry<f32>)>, delta_time: f32) {
    	--- clipped ---

        let cur_ray: Ray<f32> = Ray{origin: self.pos, dir: self.vel.normalize()};
        if collided(obs, cur_ray) {
            if let Some(dir) = self.unobstructed_dir(obs) {
                self.vel = dir * self.vel.magnitude();
Set self.vel to correct value

This is the frame_update function we created earlier. These are the changes:

  1. We added an input parameter to the function. It will receive the obstacle list.
  2. We fire in the current direction and check if it is blocked
  3. If it is blocked, we find an unblocked direction and update self.vel. Only its velocity direction changes, with its magnitude staying the same.
  4. In case there is no unblocked direction, we do nothing.

The if-let idiom is used to perform steps 3 and 4 concisely. You can read about it here.

With these changes, we are 75% of the way there. The boids no longer escape the box. You can enjoy your serene animation over some soft music.

The tranquil pleasure of obstacle avoidance

a slightly refactored tagged version


In this part of the series, we:

  1. Generated boids and cones using rng (Random Number Generation)
  2. Added obstacle avoidance to make a pretty animation

In the next and last part (phew!) of this series, we'll get the boids to behave like a flock. We'll do this by adding rules to change a boid's velocity based on its neigbours. Till then, ciao!

All criticism and questions are welcome. File an issue at the repo or comment below with your GitHub account.

Next part


  1. Rust idiom, if-let
  2. Rust dynamic dispatch
  3. 3b1b explains Quaternions
  4. Understanding ray casting
  5. Evenly distributed points on a sphere surface
  6. lazy_static docs