bg.js

const DENSITY = 100; // lower = more dense
const HUE_START = 120;
const HUE_END = 230;
const MAX_SIZE = 3;
const MIN_LIGHT = 33;
const MAX_LIGHT = 67;
const MIN_TRANSPARENCY = 10;
const MAX_TRANSPARENCY = 60;
const SATURATION = 100;
const START_ANGLE = 240;
const END_ANGLE = 300;
const MAX_SPEED = 20;

let canvas = document.getElementById('bg-canvas');
let ctx = canvas.getContext('2d');
let width = canvas.width;
let height = canvas.height;
let num_bits;
let bits = [];

class Bit {
  constructor() {
    this.x = randomRange(0, width);
    this.y = randomRange(0, height);
    this.size = randomRange(1, randomRange(1, MAX_SIZE));
    let hue = randomRange(HUE_START, HUE_END)
    let light = randomRange(MIN_LIGHT, MAX_LIGHT);
    let sat = SATURATION;
    let trans = randomRange(MIN_TRANSPARENCY, MAX_TRANSPARENCY);
    this.color = "hsla(" + hue + ", " + sat + "%, " + light + "%, " + trans + "%)";
    // We want to generate a random angle between 180 and 360
    // to make most of the bits go up
    this.angle = randomRange(START_ANGLE, END_ANGLE)
    this.speed = randomRange(1, MAX_SPEED) * 0.025;
  }

  move() {
    this.x = this.x + (this.speed * Math.cos(this.angle * Math.PI / 180));
    this.y = this.y + (this.speed * Math.sin(this.angle * Math.PI / 180));
    if (this.x < 0) {
      this.x = width;
    }
    if (this.x > width) {
      this.x = 0;
    }
    if (this.y < 0) {
      this.y = height;
    }
    if (this.y > height) {
      this.y = 0;
    }
  }
}

function generateBits() {
  for (let i = 0; i < num_bits; i++) {
    bits.push(new Bit());
  }
}

function randomRange(min = 0, max = 360) {
  return Math.floor(Math.random() * (max - min) + min);
}

function draw() {
  ctx.clearRect(0, 0, canvas.width, canvas.height);
  for (let i = 0; i < bits.length; i++) {
    let bit = bits[i];
    ctx.fillStyle = bit.color;
    ctx.beginPath();
    ctx.arc(bit.x, bit.y, bit.size, 0, 2 * Math.PI);
    ctx.fill();
    bit.move();
  }
}

function resizeCanvas() {
  canvas.width = window.innerWidth;
  canvas.height = window.innerHeight;
  width = canvas.width;
  height = canvas.height;
  num_bits = Math.floor(width * height / DENSITY / DENSITY);
  bits = [];
  generateBits();
}

// Event handler to resize the canvas when the document view is changed
window.addEventListener('resize', resizeCanvas, false);
resizeCanvas();
setInterval(draw, 20);