Use HTML5 Canvas to apply an alpha mask to an image element.
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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+


Simple animation demo can be found here:

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:

PNG-24 Mask:


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:

	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:

	'background-image': 'url(' + applyCanvasMask(<BASE IMAGE>, <MASK IMAGE>, width, height, true) + ')'


0.2 (10/01/2013):

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

0.1 (02/06/2011):

  • Initial release.