Re: Compare images in Cocoa
Re: Compare images in Cocoa
- Subject: Re: Compare images in Cocoa
- From: Graham Cox <email@hidden>
- Date: Fri, 31 Oct 2008 16:04:10 +1100
On 31 Oct 2008, at 2:21 pm, Pierce Freeman wrote:
Hi everyone. I am wondering if there is some way to compare two
images in
Cocoa, and then somehow spit out a percent of how similar they are.
The only
way I could think of is comparing every pixel, but this seems like
it would
take a long time, and even so I have no idea how to go about doing
that.
It's non-trivial. Comparing each pixel doesn't really work. You could
have two identical images but if one was shifted by just 1 pixel,
you'd have almost no match even though to the eye they'd look the
same. You can compare images for equality this way, but not similarity
(i.e. if they are absolutely identical in every way you can tell, but
the smallest difference means no match at all).
Researchers have been looking into this sort of thing for years. There
are ways to do it, but it requires some pretty heavy lifting in terms
of breaking down an image into "features" then finding whether those
same features can be found in the second image, regardless of how
those features might have been transformed in size, position or angle.
You can then come up with a figure for the number of feature matches
and how these are different, and so arrive at a figure for the overall
similarity of the two images. So, it can be done but you'll need to
look at some serious academic papers to get a flavour for what's
involved - it's almost verging on AI.
Good luck ;-)
cheers, Graham
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