[whatwg] "offscreen canvas" /Access to canvas functionality from a worker

Franz Buchinger fbuchinger at gmail.com
Sat Dec 5 04:24:36 PST 2009

Gears introduced the concept of an "offscreen canvas" that doesn't draw
anything in the browser window, but can be used to manipulate images in a
web worker.

I used this functionality to implement a "resize-before-upload" feature in
my photo gallery uploader. Now I'm trying to port my uploader to HTML5 but
there seems no way to delegate the scaling work to a HTML5 web worker.
Surely I could use the DOM canvas to scale down the photos in the main
"browser thread", but this means that the UI gets blocked during this

If the user chooses more than a few multi-megapixel photos, the
CPU-intensive downscaling will likely freeze the browser and trigger a
script abortion warning.

Are there plans to introduce an "offscreen canvas" in html5? If not, which
workarounds are possible?

kind regards,

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