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(, , 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(, , width, height, true) + ')' });
Changelog
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