> Most of these
> will be an undistinguished 50% gray (if the bits are small enough
> in your field of view), but if there's even a small correlation,
> positive or negative, at some offset, then it will be visible as a
> light or dark horizontal band.
I read an account of an approach to card
marking that supposedly had professional
magicians bamboozled, and worked on this
principle.
Instead of being the small marks that the
other magicians were looking for, the card
would have a very large pattern, bleached
onto the back. When examining the cards,
the high-frequency signal of back pattern
dominated, so the marking wasn't obvious,
but by standing some distance away (and
possibly squinting) the normal card-back
pattern went to undistinguished evenness,
and the low-frequency marking showed up.
I think it was supposed to be done with
woven- or diamond-back cards, like these:
<http://www.kardwell.com/images/bee.jpg>
but I've gimp'ed up a few experiments on
them, and can't seem to make marks that
are both subtle and visible.
-Dave
