can anyone explain how / why a canvas drawing should be unique? Doesn't that defeat the point of canvas?

edit:

So here is poc canvas fingerprint code [1]. The upshot seems to be (1) the process of turning the canvas drawing into an image (image processing, export, compression, etc) leaks information, and (2) anti-aliasing and sub-pixel rendering also leak information. But as near as I can tell, both of those are essentially equivalent to information available in the user-agent string: os version and browser version. There must be more here because, eg, for this to be useful as a ios fingerprinting solution there are so few device + os versions available this info is mostly useless. What am I missing?

[1] https://www.browserleaks.com/canvas

This seems like just one more thing to add to the list at broswerspy.dk, shown in practice by the EFF: https://panopticlick.eff.org/index.php?action=log

Not wanting to detract from linking to an interesting technique, I should say, but NoScript seems to block it just fine.

I suppose using an old browser without canvas support would work too. But then again you are opening yourself up to all sorts of other vulnerabilities.

http://caniuse.com/#search=canvas

or, you can get around it by using an ios device: there is roughly 1 hardware release / year plus a near-lockstep upgrading of the os + browser. Thus fingerprinting techniques are nearly useless because you have the same fingerprint as the other (rough guess) 40m US iphone-5 owners.