Tainted canvases may not be exported
For security reasons, your local drive is declared to be "other-domain" and will taint the canvas.
(That's because your most sensitive info is likely on your local drive!).
While testing try these workarounds:
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Put all page related files (.html, .jpg, .js, .css, etc) on your desktop (not in sub-folders).
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Post your images to a site that supports cross-domain sharing (like dropbox.com or GitHub). Be sure you put your images in dropbox's public folder and also set the cross origin flag when downloading the image (
var img=new Image(); img.crossOrigin="anonymous"...) -
Install a webserver on your development computer (IIS and PHP web servers both have free editions that work nicely on a local computer).
In the img tag set crossorigin to Anonymous.
<img crossorigin="anonymous"></img>