Canvas Mask Utility

Use HTML5 Canvas to apply an alpha mask to an image element. Compatible with any browser supporting Canvas.

Tested with Chrome 11 and Firefox 3.6+

Demo

Simple animation demo can be found here: http://playground.benbarnett.net/canvas-mask/

What it does

The function will use HTML5 Canvas to read alpha data from a PNG-24, and apply this to any image element. I have often used this technique to take advantage of the smaller file sizes with JPEG, and then a 2 colour PNG-24 to apply the transparency.

Example images:

JPEG Image: http://playground.benbarnett.net/canvas-mask/img/octagon.jpg

PNG-24 Mask: http://playground.benbarnett.net/canvas-mask/img/alpha.png

Usage

Simply pass in two image elements (pre-loaded, as shown on demo), the width and the height and you're done.

By default the function will return Canvas pixel data which you can then use to draw on to a Canvas. For example:

output.putImageData(
	applyCanvasMask(<BASE IMAGE>, <MASK IMAGE>, width, height), 0, 0, 0, 0, width, height
);

Alternatively, you can specify a 5th parameter to have the function return the masked image as a Base64 encoded data URL. This is useful for applying to background images. For example:

$('body').css({
	'background-image': 'url(' + applyCanvasMask(<BASE IMAGE>, <MASK IMAGE>, width, height, true) + ')'
});

Changelog

0.2 (10/01/2013):

• Merge pull request #3 - allow two transparent pngs to mask

0.1 (02/06/2011):

• Initial release.

Credits

• Author: Ben Barnett - http://www.benbarnett.net - @benpbarnett
• Jake Archibald for originally demonstrating this type of pixel manipulation - http://jakearchibald.com/scratch/alphavid/ - @jaffathecake