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From: rvdouder@ms.philips.nl (Rob van der Ouderaa)
Subject: Re: Navaho Thread
Message-ID: <1995Apr18.093634.20415@ms.philips.nl>
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Date: Tue, 18 Apr 1995 09:36:34 GMT
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Bob Mercier (mercier@news.cinenet.net) wrote:
: Frank Morgan (frank.morgan@prostar.com) wrote:
: :   Ever wonder how you tell a real Navaho rug from a fake? Probably not, but 
: : you look for the gray thread. After the weeks, months, if not years that it 
: : would take the artisan to make his creation he would always weave in a 
: : solitary, gray thread so that his spirit might find its way out.
: :   Frequently, I'll find myself in the middle of a file praying for much the 
: : same. Anyone got any ideas for a ray tracing equivalent...?

: If you were the author of a ray tracer, or any renderer producing a
: full-color image, it might be possible to subtly shift the color
: values in an image with a 2d pattern so that the pattern would only
: show up via a transform of the image, perhaps fft or something.
: The color shift would have to be small so that it was imperceptable to
: the naked eye.  Any of you image processing types know if this would
: be possible?

It is certainly possible. I read an article that described applications
to store a secret picture into a banknote image.
After some clever stuff with fft this
secret picture could be retrieved. 
They want to use this to prevent color copiers from copying banknotes.
Whenever you tried to copy a banknote the copy would show an additional
picture on the copied banknote.

It is also possible to encode a bitmap image or other information
into another bitmap image by using some of the lower bits of the colors
of the bitmap image. This is not visible for the naked eye.
