[magick-users] identify: What is signature, and can it be used to recognize similar images?

[Glenn Randers-Pehrson]

At 07:14 PM 3/13/03 -0500, Michael Greisman wrote:
>I am interested in
>a) recognizing duplicate images and
>b) recognizing similar images.
>
>I noticed that ImageMagick's Identify command produces a signature 
>for each image. Can you please tell me where I can find out how the 
>signature is generated, and to what degree it identifies the image? 
>I'm wondering if I can use the signature for recognizing duplicate 
>and similar images.

Two images with identical pixel data, even if stored in different
formats, will produce the same signature.  It is the SHA digest
computed over the pixels in a canonical form.

It is of no use in recognizing similar images; two images differing
only in one pixel will have entirely different signatures.

Glenn