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HTML

              
                <div id="canvaswrapper">
    <em>Click on the canvas to stop/restart the animation</em>
    <canvas id="democanvas" width="960" height="600">
        HTML5 Canvas is not supported by your browser.
    </canvas>
</div>

<script type="text/javascript">
    window.onload = () => {
        const canvas = document.getElementById('democanvas'),
            canvasBounds = {'left': 0, 'right': canvas.width,
                'top': 0, 'bottom': canvas.height};

        const ctx = canvas.getContext('2d');

        const numberOfBalls = 150,
            ballRadius = 15,
            minVelocity = 3,
            maxVelocity = 5,
            colors = ['#FF0000', '#00FF00', '#0000FF',
                '#FFFF00', '#FF00FF', '#00FFFF'];

        // Create the "World" and populate it with Balls
        let world = new World(new Bounds(canvasBounds));
        for(let i=0; i < numberOfBalls; i++) {
            world.addObject(new Ball(ballRadius, colors[i % colors.length])
                .setRandomPosition(canvasBounds)
                .setRandomVelocity(minVelocity, maxVelocity));
        }

        let useWorker = false,
            worker = undefined;

        if ( useWorker ) {
            let workerThreadUrl = 'collider-worker.js';  // 'KopOrq.js';
            worker = new Worker(workerThreadUrl);

            // Wait for message back from worker thread with updated World object
            worker.addEventListener("message", (evt) => {
                if ( evt.data.message === "draw") {
                    world = World.restoreFromData(evt.data.world);
                    world.draw(ctx);
                    if (runAnimation) {
                        requestAnimationFrame(animationStep);
                    }
                }
            });
        }

        // The animation loop
        let runAnimation = true;
        function animationStep() {
            if (useWorker) {
                 worker.postMessage(world);  //do world.move() in worker
            } else {
                 world.move();
                 world.draw(ctx);
                 if (runAnimation) {
                     requestAnimationFrame(animationStep);
                 }
             }
        }
        animationStep();

        // Click/tap in canvas to stop/restart animation
        canvas.addEventListener("click", (evt) => {
            runAnimation = ! runAnimation;
            if (runAnimation) {
                animationStep();
            }
        });
    };
</script>

              
            
!

CSS

              
                body {
    background-color: #afd9ee;
}
#canvaswrapper {
    margin-left: auto;
    margin-right: auto;
    margin-top: 40px;
    width: 960px;
    height: 600px;
}
#democanvas {
    border: 2px solid #555;
    background-color: #3a87ad;
}

@media screen and (max-width: 800px) {
    #canvaswrapper {
        margin-left: auto;
        margin-right: auto;
        width: 600px;
        height: 600px;
    }
}

@media screen and (max-width: 400px) {
    #canvaswrapper {
        margin-left: 10px;
        margin-top: 20px;
        width: 300px;
        height: 300px;
    }
}

              
            
!

JS

              
                'use strict';
/*jshint esversion: 6 */
/* global exports */
/* global console */

/**
 * Collider: simple demo of collision physics.
 * Bruce Wilson, 1/29/2018
 */
class World {
    constructor(bounds) {
        this.bounds = bounds;
        this.animationStep = 0;
        this.displayList = [];
        this.moveDuration = 0;
        this.drawDuration = 0;

        this.options = {};
        // Clear before drawing: if not, an interesting canvas-filling
        // graphic is created
        this.options.clearBeforeDraw = true;
    }
    static restoreFromData(data) {
        // Restore from data that has been serialized to a worker thread
        const bounds = Bounds.restoreFromData(data.bounds),
            world = new World(bounds);
        world.animationStep = data.animationStep;
        world.displayList = [];
        world.moveDuration = data.moveDuration;
        world.options = data.options;
        data.displayList.forEach((obj) => {
            const ball = Ball.restoreFromData(obj);
            world.displayList.push(ball);
        });
        return world;
    }
    addObject(obj) {  // fluent
        this.displayList.push(obj);
        obj.id = this.displayList.length;
        return this;  // fluent
    }
    draw(ctx) {    // fluent, graphics
        const tstart = new Date().getTime(),
            drawingAreaBounds = this.bounds.getBounds();

        if (this.options.clearBeforeDraw) {
            ctx.clearRect(drawingAreaBounds.left, drawingAreaBounds.top,
                drawingAreaBounds.right, drawingAreaBounds.bottom);
        }
        this.displayList.forEach((obj) => {
            obj.draw(ctx);
        });

        const duration = (new Date().getTime() - tstart);
        if (duration > 0) {
            this.drawDuration = duration;
        }

        this.drawDurationText(ctx);

        return this;  // fluent
    }
    drawDurationText(ctx) {  // debugging
        const x = 10, y = 20;
        ctx.font = '16px sans-serif';
        ctx.fillStyle = '#e0e0e0';
        ctx.fillText("Move: " + this.moveDuration + " msec, Draw: " +
            this.drawDuration + " msec", x, y);
    }
    move() {  // fluent
        this.animationStep += 1;

        const tstart = new Date().getTime();
        this.checkForCollisions();
        const duration = (new Date().getTime() - tstart);
        if ( duration > 1) {
            // 60 fps => 16.7 msec per frame
            // console.log('%d Check for collisions: %d msec', this.animationStep, duration);
            this.moveDuration = duration;
        }

        let self = this;
        this.displayList.forEach((obj) => {
            self.bounds.checkInBounds(obj);
        });

        this.displayList.forEach((obj) => {
            obj.move();
        });
        return this;  // fluent
    }
    checkForCollisions() {
        const listLength = this.displayList.length;
        for (let i=0; i < listLength; i++) {
            const obj1 = this.displayList[i];
            for (let j=i + 1; j < listLength; j++) {
                const obj2 = this.displayList[j],
                    minDist = obj1.getRadius() + obj2.getRadius(),
                    dist = obj1.distanceTo(obj2);

                // Check distance > 0 for degenerate case where balls overshoot
                if ( dist > 0 && dist < minDist ) {
                    // console.log("Collision: i=%d, j=%d, dist=%.3f, minDist=%.3f", i, j, dist, minDist);
                    // This reverses the direction of movement of each object
                    const newVelocity1 = obj1.bounceOff(obj2),
                        newVelocity2 = Vector.negate(newVelocity1);
                    // console.log("vel1: " + JSON.stringify(newVelocity1)); // debugging
                    // console.log("vel2: " + JSON.stringify(newVelocity2));
                    this.separateOverlappedBalls(obj1, obj2);
                    obj1.setVelocity(newVelocity1);
                    obj2.setVelocity(newVelocity2);
                }
            }
        }
    }
    // TODO: try an alternate "separate" method that checks dot products
    separateOverlappedBalls(ball1, ball2) {
        const vdiff = Vector.subtract(ball2.getPosition(), ball1.getPosition()),
            radiusSum = ball1.getRadius() + ball2.getRadius(),
            dist = ball1.distanceTo(ball2),
            pos1 = ball1.getPosition(),
            pos2 = ball2.getPosition();

        // Adjust each ball's position by half the difference
        // of the sum of the radii and the distance
        const adjustDist = radiusSum - dist,
            adjust = Vector.divide(Vector.setMagnitude(vdiff, adjustDist), 2.0);
        ball1.setPosition({x: pos1.x + adjust.x, y: pos1.y + adjust.y});
        ball2.setPosition({x: pos2.x - adjust.x, y: pos2.y - adjust.y});
    }
    ballsOverlap(ball1, ball2, dist) {
        return dist < ball1.getRadius() + ball2.getRadius();
        // return dist < Math.min(obj1.getRadius(), obj2.getRadius());
    }
}

class Bounds {
    constructor(bounds) {
        this.bounds = { top: bounds.top, right: bounds.right,
            bottom: bounds.bottom, left: bounds.left };
    }
    static restoreFromData(data) {
        return new Bounds(data.bounds);
    }
    getBounds() { return this.bounds; }
    checkInBounds(obj) {
        const objBounds = obj.getBounds();

        // Bounce if crossing a boundary
        if ( objBounds.left < this.bounds.left ) {
            obj.velocity.x = - obj.velocity.x;
            // Make sure ball is within bounds: counteract overshoot
            obj.position.x = this.bounds.left + obj.radius;
        }
        if ( objBounds.right > this.bounds.right ) {
            obj.velocity.x = - obj.velocity.x;
            obj.position.x = this.bounds.right - obj.radius;
        }
        if ( objBounds.top < this.bounds.top ) {
            obj.velocity.y = - obj.velocity.y;
            obj.position.y = this.bounds.top + obj.radius;
        }
        if ( objBounds.bottom > this.bounds.bottom ) {
            obj.velocity.y = - obj.velocity.y;
            obj.position.y = this.bounds.bottom - obj.radius;
        }
    }
}

class CircularBounds {
    constructor(bounds) {
        this.bounds = { top: bounds.top, right: bounds.right,
            bottom: bounds.bottom, left: bounds.left };
        const width = bounds.right - bounds.left,
            height = bounds.bottom - bounds.top;
        this.center = {x: width / 2.0, y: height / 2.0};
        this.radius = Math.min(width / 2.0, height / 2.0);
    }
    static restoreFromData(data) {
        const bounds = new Bounds(data.bounds);
        bounds.radius = data.bounds.radius;
        return bounds;
    }
    getBounds() { return this.bounds; }
    checkInBounds(obj) {
        const objBounds = obj.getBounds();

        const vDistToCenter = Vector.subtract(obj.getPosition(), this.center),
            distToCenter = Vector.magnitude(vDistToCenter);

        if ( distToCenter + obj.getRadius() > this.radius) {
            // bounce off boundary
        }
    }
}

class Ball {
    constructor(radius, color, position={x: 0, y: 0}, velocity={x: 0, y: 0}) {
        this.id = 0;
        this.radius = radius;
        this.color = color;
        this.position = position;
        this.velocity = velocity;
    }
    static restoreFromData(data) {
        // Restore from data that's been serialized to a worker thread
        const ball = new Ball(data.radius, data.color);
        ball.position = data.position;
        ball.velocity = data.velocity;
        return ball;
    }
    getPosition() { return {x: this.position.x, y: this.position.y}; }
    setPosition(p) { this.position = {x: p.x, y: p.y}; return this; }  // fluent method
    getVelocity() { return {x: this.velocity.x, y: this.velocity.y}; }
    setVelocity(v) { this.velocity = {x: v.x, y: v.y};  return this; }  // fluent method
    getRadius() { return this.radius; }
    getBounds() {
        return {top:(this.position.y - this.radius), right:(this.position.x + this.radius),
            bottom:(this.position.y + this.radius), left:(this.position.x - this.radius)};
    }
    move() {
        // Move according to the current velocity
        this.position.x = this.position.x + this.velocity.x;
        this.position.y = this.position.y + this.velocity.y;
    }
    draw(ctx) {    // graphics
        const FULLCIRCLE = 2 * Math.PI;  // TODO
        ctx.beginPath();
        ctx.arc(this.position.x, this.position.y, this.radius, 0, FULLCIRCLE, true);
        ctx.fillStyle = this.color;
        ctx.fill();
        ctx.strokeStyle = '#202020';
        ctx.strokeWeight = 0.25;
        ctx.stroke();
        // this.drawVelocity(ctx);  // debugging
    }
    drawVelocity(ctx) {  // for debugging
        const multiplier = 5;
        ctx.beginPath();
        ctx.moveTo(this.position.x, this.position.y);
        ctx.lineTo(this.position.x + multiplier * this.velocity.x,
            this.position.y + multiplier * this.velocity.y);
        ctx.strokeStyle = '#202020';
        ctx.strokeWeight = 2.0;
        ctx.stroke();
    }
    bounceOff(otherBall) {
        // There's no consideration of mass or conservation of momentum
        // in this simple example - each ball retains its velocity, it
        // just moves off in a different direction
        // console.log("bounce " + this + " off " + otherBall);
        const normalVector =
                Vector.unitVector(Vector.subtract(otherBall.getPosition(), this.getPosition())),  // TODO
            incidentVector = Vector.unitVector(this.getVelocity()),
            scalarSpeed = Vector.magnitude(this.getVelocity()),
            ndotI = - 2.0 * Vector.dotProduct(normalVector, incidentVector);
        let newVelocity = Vector.multiply(normalVector, ndotI);

        newVelocity = Vector.subtract(incidentVector, newVelocity);
        newVelocity = Vector.setMagnitude(newVelocity, scalarSpeed);
        return newVelocity;
    }
    distanceTo(otherBall) {
        const pos1 = this.getPosition(),
            pos2 = otherBall.getPosition(),
            dx = pos1.x - pos2.x,
            dy = pos1.y - pos2.y;
        return Math.sqrt(dx * dx + dy * dy);
    }
    setRandomPosition(bounds) {  // fluent method
        const x = Math.random() * (bounds.right - bounds.left) + bounds.left,
            y = Math.random() * (bounds.bottom - bounds.top) + bounds.top;
        this.position = {x: x, y: y};
        return this;  // fluent method
    }
    setRandomVelocity(minVelocity = 2, maxVelocity=5) {  // fluent method
        const velocityRange = Math.abs(maxVelocity - minVelocity);
        let direction = (Math.random() > 0.5) ? 1 : -1;
        this.velocity.x = direction * (velocityRange * Math.random() + minVelocity);
        direction = (Math.random() > 0.5) ? 1 : -1;
        this.velocity.y = direction * (velocityRange * Math.random() + minVelocity);
        return this;  // fluent method
    }
    toString() {
        const id = this.id || 0,
            precision = 1,
            xpos = (this.position.x).toFixed(precision),
            ypos = (this.position.y).toFixed(precision),
            vx = (this.velocity.x).toFixed(precision),
            vy = (this.velocity.y).toFixed(precision);
        return id + ", pos: (" + xpos + ", " + ypos + "), vel: (" + vx + ", " + vy + ")";
    }
}

window.Vector = (function(){
    "use strict";

    return {
        // Operates on vectors as object literals like '{x: ..., y: ...}'
        dotProduct: function(v1, v2) { return (v1.x * v2.x) + (v1.y * v2.y); },
        magnitude: function(v1) { return Math.sqrt((v1.x * v1.x) + (v1.y * v1.y)); },
        unitVector: function(v1) {
            const magn = Vector.magnitude(v1);
            return {x: v1.x / magn, y: v1.y / magn};
        },
        setMagnitude: function(v1, m) {
            // Assuming uv is a unit vector, the x and y components of uv
            // are the cosine and sine of the vectors
            let magn = Vector.magnitude(v1);
            return {x: m * v1.x / magn, y: m * v1.y / magn};
        },
        subtract: function(v1, v2) {
            // Subtract v2 - v1, gives a vector from v2 to v1
            return {x: (v2.x - v1.x), y: (v2.y - v1.y)};
        },
        negate: function(v) { return {x: -v.x, y: -v.y}; },
        multiply: function(v1, scalarValue) { return {x: (v1.x * scalarValue), y: (v1.y * scalarValue)}; },
        divide: function(v1, scalarValue) { return {x: (v1.x / scalarValue), y: (v1.y / scalarValue)}; },
        same: function(v1, v2, tolerance=0.00001) {
            return (Math.abs(v1.x - v2.x) < tolerance) && (Math.abs(v1.y - v2.y) < tolerance);
        },
        asString: function(v) { return "{x: " + v.x + ", y:" + v.y + "}"; }
    };
})();

if (typeof exports !== 'undefined') {
    exports.World = World;
    exports.Bounds = Bounds;
    exports.CircularBounds = CircularBounds;
    exports.Ball = Ball;
    exports.Vector = Vector;
}

              
            
!
999px

Console