JPEGs with Alpha Channels?!?

Posted on by Jack Turner

I wanted a reasonably sized photographic image with a 24-bit alpha channel. So I used a JPEG for what JPEGs are good for and a PNG for what PNGs are good for...

I combined them using an HTML5 canvas element and then inserted into the DOM. The results look the same as using a normal 24-bit PNG but are one-half to one-sixth the size. In one case we got a 573KB 24-bit PNG down to a 49KB JPEG with a 4KB PNG alpha-mask!

On a recent project I did this using jQuery and scanning the CSS for background-images, but this could easily be achieved without any special CSS using valid HTML5 syntax by referring to a normal JPEG throug the src of an img element and adding a new data-alpha-src attribute with the URL of a 24-bit PNG that is just a mask.

<img src="image.jpg" data-alpha-src="alpha.png" />

This little demo1 works in the newest versions of Firefox, Chrome, and Safari (including Safari for iOS). For Internet Explorer support, I used the wonderful FlashCanvas2. It’s fine in IE8 but it’s inconsistent in IE7. This might be solved by waiting for DOM to be ready. At the time of this writing the IE9 beta does not support globalCompositeOperation, so we’ll have to wait and see.

Recently we’ve taken on two different projects that involve lots of large sprites being animated around landscapes. Normally sites like this (we typically see them done in Flash) necessitate painful loading screens. By using this JPEG-alpha trick we should be able to keep load times to a minimum.

Here is the JavaScript code from the demo:

;(function() {

  var create_alpha_jpeg = function(img) {

    var alpha_path = img.getAttribute('data-alpha-src')
    if(!alpha_path) return

    // Hide the original un-alpha'd
    img.style.visiblity = 'hidden'

    // Preload the un-alpha'd image
    var image = document.createElement('img')
    image.src = img.src
    image.onload = function () {

      // Then preload alpha mask
      var alpha = document.createElement('img')
      alpha.src = alpha_path
      alpha.onload = function () {

        var canvas = document.createElement('canvas')
        canvas.width = image.width
        canvas.height = image.height
        img.parentNode.replaceChild(canvas, img)

        // For IE7/8
        if(typeof FlashCanvas != 'undefined') FlashCanvas.initElement(canvas)

        // Canvas compositing code
        var context = canvas.getContext('2d')
        context.clearRect(0, 0, image.width, image.height)
        context.drawImage(image, 0, 0, image.width, image.height)
        context.globalCompositeOperation = 'xor'
        context.drawImage(alpha, 0, 0, image.width, image.height)


  // Apply this technique to every image on the page once DOM is ready
  // (I just placed it at the bottom of the page for brevity)
  var imgs = document.getElementsByTagName('img')
  for(var i = 0; i &lt; imgs.length; i++)


In the head element I linked to FlashCanvas:

<!--[if lte IE 8]><script src="flashcanvas.js"></script><![endif]-->

... and I threw in this to avoid a flicker of the un-alpha’d JPEG:

<style>img[data-alpha-src]{visibility: hidden;}</style>
  1. Plastics trucker hats are available from Americorp. []
  2. This technique uses the globalCompositeOperation operation, which requires FlashCanvas Pro. Free for non-profit use or just $31 for a commercial license. []