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From: Doug Anderson <ethelthelogremovethis@gmail.com>
Newsgroups: comp.sys.mac.system
Subject: Re: Any quick, easy way to compare two images?
Date: 23 Jan 2014 16:05:44 -0800
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Alan Browne <alan.browne@FreelunchVideotron.ca> writes:

> On 2014.01.23, 18:25 , Doug Anderson wrote:
> 
> > diff <file 1> <file 2>
> >
> > will return nothing if the files are identical (as binary files) and
> > will return a message telling you they differ (if they differ).
> 
> Useless for sequential (or series of) frames that are "identical" from
> a human vision POV as there will be differences due to noise or hardly
> visible illumination changes/variance.

Correct.  

But useful for what I understand the poster to be asking for (a way to
determine if the _files_ were identical, not to determine whether the
images would be identical to the casual observer).