Signing Images Not So Useful Now

[tcmay@netcom.com (Timothy C. May)]

I don't think signing digital images is especially useful right not.
This is not to say we shouldn't discuss it, or that someone out there
won't make some bucks on such a product.

Some points:

* signing at the digital camera level is the most unlikely, currently,
as most digital images are currently made by _scanning_ conventional
slides or prints. (Slide scanners are readily available, with >1200
dpi, and even higher, resolution.)

* as others have noted, almost all digital images are further massaged
in PhotoShop or the like. Even Nick Szabo's "picture distance" idea is
of marginal use, as many common filter operations introduce very large
picture distances and/or Hamming distance. Determining "alikeness"
metrics of an image and a suspected forgery or copyright violation
will be a challenge. (Could be a nice little niche business for a 
"expert witness" on such things, though.)

* most ripped-off images are of course done by using scanners. All
those "Playboy" and "Penthouse" images you see on the Net are done
with scanners. (Both Playboy and Penthouse offer on-line services, so
I suppose it's possible some of the bootleg images are in fact digital
copies. In these cases, a signature might reveal the copyright
violation. A simple Gaussian blur in PhotoShop would of course erase
most conceivable signatures. There may be some way to introduce
something that can't be easily blurred away, though...)

* the most common means of detecting forgeries and copyright
violations is of course the human recognition of such images, the
recognition that an image came from a well-known source.


--Tim May

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