I think comparing edges only may be the key (excellent link, JoshuaD). The edges should capture the features of the coins being compared. Simply comparing images will not work - here's why.


The actual (raised) features of the coin - a capital "I" followed by a capital "T". 
++++++++++++++++++++
++++++++++++++++++++
++######++++######++
++++##++++++++##++++
++++##++++++++##++++
++++##++++++++##++++
++######++++++##++++
++++++++++++++++++++
++++++++++++++++++++

[download]

Here's how the coin would photograph lit from the lower left. Legend: "+" for the metal's "normal" color, "_" for highlights, and "#" for shadows. 
++++++++++++++++++++
++######++++######++
+_++++++#++_++++++#+
++__++__++++__++__++
+++_++#++++++_++#+++
++##++##+++++_++#+++
+_++++++#++++_++#+++
++______++++++__++++
++++++++++++++++++++

[download]

Here's how the coin would photograph lit from the lower right. 
++++++++++++++++++++
++######++++######++
+#++++++_++#++++++_+
++__++__++++__++__++
+++#++_++++++#++_+++
++##++##+++++#++_+++
+#++++++_++++#++_+++
++______++++++__++++
++++++++++++++++++++

[download]

And here's how it would photograph lit dead on. 
____________________
__######____######__
_#______#__#______#_
__##__##____##__##__
___#__#______#__#___
__##__##_____#__#___
_#______#____#__#___
__######______##____
____________________

[download]

The edges for all three images would (I think) all look like the last image.

In reply to Re: Re: comparing two images and detecting differences by bean
in thread comparing two images and detecting differences by moof1138

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