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Last active Jun 7, 2021
An example of 2D collisions using JavaScript Canvas
 class State { constructor(display, actors) { this.display = display; this.actors = actors; } update(time) { /** * provide an update ID to let actors update other actors only once * used with collision detection */ const updateId = Math.floor(Math.random() * 1000000); const actors = this.actors.map(actor => { return actor.update(this, time, updateId); }); return new State(this.display, actors); } } class Vector { constructor(x, y) { this.x = x; this.y = y; } add(vector) { return new Vector(this.x + vector.x, this.y + vector.y); } subtract(vector) { return new Vector(this.x - vector.x, this.y - vector.y); } multiply(scalar) { return new Vector(this.x * scalar, this.y * scalar); } dotProduct(vector) { return this.x * vector.x + this.y * vector.y; } get magnitude() { return Math.sqrt(this.x ** 2 + this.y ** 2); } get direction() { return Math.atan2(this.x, this.y); } } class Canvas { constructor(parent = document.body, width = 400, height = 400) { this.canvas = document.createElement('canvas'); this.canvas.width = width; this.canvas.height = height; parent.appendChild(this.canvas); this.ctx = this.canvas.getContext('2d'); } sync(state) { this.clearDisplay(); this.drawActors(state.actors); } clearDisplay() { // opacity controls the trail effect set to 1 to remove this.ctx.fillStyle = 'rgba(255, 255, 255, .4)'; this.ctx.fillRect(0, 0, this.canvas.width, this.canvas.height); this.ctx.strokeStyle = 'black'; this.ctx.strokeRect(0, 0, this.canvas.width, this.canvas.height); } drawActors(actors) { for (let actor of actors) { if (actor.type === 'circle') { this.drawCircle(actor); } } } drawCircle(actor) { this.ctx.beginPath(); this.ctx.arc(actor.position.x, actor.position.y, actor.radius, 0, Math.PI * 2); this.ctx.closePath(); this.ctx.fillStyle = actor.color; this.ctx.fill(); } } class Ball { constructor(config) { Object.assign(this, { id: Math.floor(Math.random() * 1000000), type: 'circle', position: new Vector(100, 100), velocity: new Vector(5, 3), radius: 25, color: 'blue', collisions: [], }, config ); } update(state, time, updateId) { /** * if slice occurs on too many elements, it starts to lag * collisions is an array to allow multiple collisions at once */ if (this.collisions.length > 10) { this.collisions = this.collisions.slice(this.collisions.length - 3); } /** * this is the most stable solution to avoid overlap * but it is slightly inaccurate */ for (let actor of state.actors) { if (this === actor || this.collisions.includes(actor.id + updateId)) { continue; } /** * check if actors collide in the next frame and update now if they do * innaccurate, but it is the easiest solution to the sticky collision bug */ const distance = this.position.add(this.velocity).subtract(actor.position.add(actor.velocity)).magnitude; if (distance <= this.radius + actor.radius) { const v1 = collisionVector(this, actor); const v2 = collisionVector(actor, this); this.velocity = v1; actor.velocity = v2; this.collisions.push(actor.id + updateId); actor.collisions.push(this.id + updateId); } } // setting bounds on the canvas prevents balls from overlapping on update const upperLimit = new Vector(state.display.canvas.width - this.radius, state.display.canvas.height - this.radius); const lowerLimit = new Vector(0 + this.radius, 0 + this.radius); // check if hitting left or right of container if (this.position.x >= upperLimit.x || this.position.x <= lowerLimit.x) { this.velocity = new Vector(-this.velocity.x, this.velocity.y); } // check if hitting top or bottom of container if (this.position.y >= upperLimit.y || this.position.y <= lowerLimit.y) { this.velocity = new Vector(this.velocity.x, -this.velocity.y); } const newX = Math.max(Math.min(this.position.x + this.velocity.x, upperLimit.x), lowerLimit.x); const newY = Math.max(Math.min(this.position.y + this.velocity.y, upperLimit.y), lowerLimit.y); return new Ball({ ...this, position: new Vector(newX, newY), }); } get area() { return Math.PI * this.radius ** 2; } get sphereArea() { return 4 * Math.PI * this.radius ** 2; } } // see elastic collision: https://en.wikipedia.org/wiki/Elastic_collision const collisionVector = (particle1, particle2) => { return particle1.velocity .subtract(particle1.position .subtract(particle2.position) .multiply(particle1.velocity .subtract(particle2.velocity) .dotProduct(particle1.position.subtract(particle2.position)) / particle1.position.subtract(particle2.position).magnitude ** 2 ) // add mass to the system .multiply((2 * particle2.sphereArea) / (particle1.sphereArea + particle2.sphereArea)) ); }; const isMovingTowards = (particle1, particle2) => { return particle2.position.subtract(particle1.position).dotProduct(particle1.velocity) > 0; }; const runAnimation = animation => { let lastTime = null; const frame = time => { if (lastTime !== null) { const timeStep = Math.min(100, time - lastTime) / 1000; // return false from animation to stop if (animation(timeStep) === false) { return; } } lastTime = time; requestAnimationFrame(frame); }; requestAnimationFrame(frame); }; const random = (max = 9, min = 0) => { return Math.floor(Math.random() * (max - min + 1) + min); }; const colors = ['red', 'green', 'blue', 'purple', 'orange']; const collidingBalls = ({ width = 400, height = 400, parent = document.body, count = 50 } = {}) => { const display = new Canvas(parent, width, height); const balls = []; for (let i = 0; i < count; i++) { balls.push(new Ball({ radius: random(8, 3) + Math.random(), color: colors[random(colors.length - 1)], position: new Vector(random(width - 10, 10), random(height - 10, 10)), velocity: new Vector(random(3, -3), random(3, -3)), })); } let state = new State(display, balls); runAnimation(time => { state = state.update(time); display.sync(state); }); }; collidingBalls();

### GMartigny commented Aug 1, 2020

 Hi, I liked reading your article. It's well explained and has some real interesting detail. Your code is squeaky clean ! I too have done a lot of animation with `` and I though I will share with you what I learned too. The elephant in the room is the collision. Of course, it's the most difficult part. Lots of your issues comes from Euler physics (adding speed to position each loop) and can be solved with Verlet integration. It's more complex, but a lot more stable. Objects have a "position" and a "previous position" properties. Each frame, you move an object by the difference between the two. The magic trick is, you just need to change the "position" to change the velocity. In case of collision, both particles repeal each other by the distance they overlap multiplied by some ratio. Check out this pen I made using this technique. Finally, I would say that immutability is a great thing, but not always the best solution. Here, you're using a lot of memory by duplicating your objects each time you do any operation. I don't have any definitive answer. With a small amount of particles on an decent computer it shouldn't matter anyway. `` are awesome and you figured out all you need to know to do any 2D animation scene. Keep at it.

### cyrilf commented Aug 13, 2020

 Thanks! 👏 I really appreciate your article. 👌 There is a small typo left. Here's how to fix it. Solution 1: ```-const collidingBalls = ({ width = 400, height = 400, parent = document.body, count = 50 }) => { +const collidingBalls = ({ width = 400, height = 400, parent = document.body, count = 50 } = {}) => {``` Solution 2: ```-collidingBalls(); +collidingBalls({});```

 Cool

### joshuabradley012 commented Oct 12, 2020 • edited

 @GMartigny thank you so much for the kind words and advice! I will certainly be looking into Vertlet Integration and how it can improve my solution. @cyrilf I'm not sure I see the mistake? The code works correctly as far as I can tell, what error are you solving for?

### cyrilf commented Oct 13, 2020

 @joshuabradley012 if I copy/paste your gist into a codepen the following error appears: `Uncaught TypeError: Cannot read property 'width' of undefined` (line 219). It is due to the fact that the function `collidingBalls` is expecting an object as an argument but got undefined (so the deconstructing is failing).That's what my two solutions above are trying to solve.

### joshuabradley012 commented Oct 13, 2020

 @cyrilf got it! Thank you. Updating now.