Hidden digital signature in a graphic?

["Hart, Geoff" <Geoff-H -at- MTL -dot- FERIC -dot- CA>]

Steve Hudson, writing concerning hidden digital signatures in graphics,
replies: <<Another, slightly more technical way is to include a CHUNK of
text, marked as text, after the bitmap chunk. With any luck, folks won't
realise its even included in the binary file, giving you a signature that
travels with it.>>

Long before the commercial products for embedding signatures came out, I'd
suggested something similar to various friends concerned about protecting
their art: Create a copyright statement using your software's text tool
(e.g., "This image copyright (c)2001 by Geoff Hart (geoff-h -at- mtl -dot- feric -dot- ca).
All rights reserved"). You then have two options:

First and easiest, you can scale the text down to the smallest point size
permitted by the software, leave it as text (rather than converting it to a
bitmap), then fit the text to a curve so it doesn't seem to be text. At a
typical zoom level, nobody will ever notice it's there--and particularly so
if you're using layers and place the signature behind other graphics. Of
course, a sophisticated artist (in contrast with the typical devil-may-care
thief) might check the layers and see that the graphic contains a text
layer, so it might be worthwhile converting the text to a bitmap and
layering it into the rest of the graphic. For example, if you're editing the
"value" (black content) of a color layer, you could easily pick an area of
black that contains (say) 90% black, and drop the signature into that layer
at 100% black intensity. A casual glance at the black area in the composite
graphic won't reveal the signature, but anyone who examines the black area
can see the signature clearly enough by enhancing the contrast within that
area.

Second and far more sophisticated, create something that resembles a
signature and embed it in the image, adjusting color etc. so that it hides
quietly in the image; if you really want to get subtle, you can do this
using the channels option, and only embed the signature in one color channel
so that it's only easily visible if the software users display that channel.
As noted above, fitting the text to a curve makes it harder to recognize as
text, particular if the curve follows other curves in the image (e.g., run
it vertically amidst a picture of a stand of bamboo). Michael Whelan, one of
the finer current cover illustrators for science fiction and fantasy novels,
uses the signature "MW", with the bottom of the stems of the M blending into
the top stems of the W so the two letters run together into something
resembling a sideways 8; it's become something of a game among the
cognoscenti to find where he's hidden this signature in each piece of art.

--Geoff Hart, FERIC, Pointe-Claire, Quebec
geoff-h -at- mtl -dot- feric -dot- ca
"User's advocate" online monthly at
www.raycomm.com/techwhirl/usersadvocate.html

"Signatures? We doan need no steenkeen' signatures!"

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