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

Sigbjorn Finne sigbjorn.finne at gmail.com
Sun Dec 6 22:18:53 PST 2009

On 12/5/2009 13:24, Franz Buchinger wrote:
> 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
> process.
> 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?
Hi Franz,

ImageData is cloneable via postMessage(), so you could perform some
forms of image processing that way.

--sigbjorn <sof at opera.com>

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