How to Create a Gravity Effect in the HTML5 Canvas

Mar 27, 2022 | O. Wolfson
canvas javascript

Create a gravity effect with javascript and HTML5 canvas.

Start by setting up some constants.

var gravity = .75;
var friction = 0.85;
const circleArray = [];
let ballArray = [];
var ball;

Add a Ball class

function Ball(x, y, dx, dy, radius, color) {
    this.x = x;
    this.y = y;
    this.dy = dy;
    this.dx = dx;
    this.radius = radius;
    this.color = color;

    this.update = function () {
        if (this.y + this.radius > canvas.height) {
            this.dy = -this.dy * friction;
        } else {
            this.dy += gravity;
        }
        if (this.x + this.radius + this.dx > canvas.width || this.x - this.radius < 0) {
            this.dx = -this.dx
        }
        this.x += this.dx;
        this.y += this.dy;
        this.draw()
    }

    this.draw = function () {

        ctx.beginPath();
        ctx.arc(this.x, this.y - this.dy, this.radius, 0, Math.PI * 2, false);
        ctx.fillStyle = this.color;
        ctx.strokeStyle = 'orange'
        ctx.fill();
        ctx.stroke();
        ctx.closePath();
    }
}

The init function builds an array of balls.

function init() {
ball = new Ball(canvas.width / 2, canvas.height / 2, 2, 30, 'red')
console.log(ball)
for (let i = 0; i < 300; i++) {
    let radius = Math.random() * 6 + 1;
    let x = Math.random() * (canvas.width - radius) + radius;
    let y = Math.random() * (canvas.height - radius * 4 - 100) + radius;
    let dx = (Math.random() - 0.5) * 2;
    let dy = (Math.random() - 0.5) * 2;

    ballArray.push(new Ball(x, y, dx, dy, radius, 'red'))
}
    console.log(ballArray)
}

The animate function runs the update method on each ball instance.

function animate() {
    requestAnimationFrame(animate)
    console.log('animate')
    ctx.clearRect(0, 0, innerWidth, innerHeight)
    ctx.fillStyle = 'white'
    ctx.fillRect(0, 0, canvas.width, canvas.height);
    for (var i = 0; i < ballArray.length; i++) {
        ballArray[i].update()
    }
    //ctx.clearRect(0, 0, innerWidth, innerHeight)

}

Download the app here. View page source to see the code.

This project was inspired from the Chris Course tutorial linked here.