Snow with HTML Canvas

On the HTML5 Rocks webite they've added snow to the main banner area.

It's very interesting to see how they've done it, you can get some ideas for your next project.

Canvas to the rescue

Yep, this black magic snow uses HTML canvas to create the area where the snow is gonna fall. So first of all we need a container:

<div class="snowContainer" style="height: 500px;"></div>

That's it, next thing...

Sweet JavaScript

No libraries, frameworks or any dirty code. Just plain and simple JavaScript. You'll probably want some information about Canvas first so here it is:

• Canvas and HTML, Mozilla Developer Network
• Posts about canvas on HTML5 Rocks
• Canvas - Dive into HTML5
• requestAnimationFrame tutorial by Paul Irish, you'll need it.

Now ready or not here comes the code:

(function () {

  // Will it be a storm or peaceful?
  var COUNT = 300;

  // Get our cotaniner
  var snowContainer = document.querySelector('.snowContainer');

  // Create the canvas element
  var canvas = document.createElement('canvas');
  var ctx = canvas.getContext('2d');

  // Get the size of the container, that's why we defined the height in the HTML
  var width = snowContainer.clientWidth;
  var height = snowContainer.clientHeight;
  var i = 0;
  var active = false;

  function onResize() {
    width = snowContainer.clientWidth;
    height = snowContainer.clientHeight;
    canvas.width = width;
    canvas.height = height;
    ctx.fillStyle = '#FFF';

    requestAnimFrame(update);
  }

  var Snowflake = function () {
    this.x = 0;
    this.y = 0;
    this.vy = 0;
    this.vx = 0;
    this.r = 0;

    this.reset();
  }

  // You can set up the 
  Snowflake.prototype.reset = function () {
    this.x = Math.random() * width;
    this.y = Math.random() * -height;

    // More speed? Change this
    this.vy = 1 + Math.random() * 3;
    this.vx = 0.5 - Math.random();

    // Bigger snow?
    this.r = 1 + Math.random() * 2;

    this.o = 0.5 + Math.random() * 0.5;
  }

  canvas.style.position = 'absolute';
  canvas.style.left = canvas.style.top = '0';

  var snowflakes = [],
    snowflake;
  for (i = 0; i < COUNT; i++) {
    snowflake = new Snowflake();
    snowflake.reset();
    snowflakes.push(snowflake);
  }

  function update() {

    ctx.clearRect(0, 0, width, height);

    for (i = 0; i < COUNT; i++) {
      snowflake = snowflakes[i];
      snowflake.y += snowflake.vy;
      snowflake.x += snowflake.vx;

      ctx.globalAlpha = snowflake.o;
      ctx.beginPath();
      ctx.arc(snowflake.x, snowflake.y, snowflake.r, 0, Math.PI * 2, false);
      ctx.closePath();
      ctx.fill();

      if (snowflake.y > height) {
        snowflake.reset();
      }
    }

    requestAnimFrame(update);
  }

  // shim layer with setTimeout fallback
  window.requestAnimFrame = (function () {
    return window.requestAnimationFrame ||
      window.webkitRequestAnimationFrame ||
      window.mozRequestAnimationFrame ||
      function (callback) {
        window.setTimeout(callback, 1000 / 60);
      };
  })();

  onResize();
  window.addEventListener('resize', onResize, false);

  snowContainer.appendChild(canvas);
})();

Demo

View Demo