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Removing image background by two image comparison

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Benjamin Howard
Removing image background by two image comparison
on Feb 4, 2010 at 10:16:18 pm

I have two images: one with a subject against a detailed background of trees, and one of the exact same background without the subject. I would like to remove the tree background and be left with the the subject against a transparent background. This is exactly what apple photobooth and ichat do for background replacements. I'm sorry if this problem has been addressed previously, but part of my problem is that I don't know what to call this process for searching.

I know exactly how this feature should work; I just don't know how to do it in photoshop. The computer should compare the top layer against the bottom layer; if the pixels are the same (or within a range) the output should be transparency. If pixels are different, the output should be the corresponding pixel from the top layer. I'm two seconds away from trying to code this relatively simple process myself, but I thought I'd try here first over a shifty homemade fix.

I've tried a difference matte, but it has been unsuccessful for the following reasons. From the difference matte I get an output that is black where the background is the same, and a sort of combination of the subject and the background for where it is different. This is problematic because then if I try to remove the black, all of the black that was intended to be on the subject is also turned transparent, and there's also the problem of the subject being semitransparent.

Thanks for your help! (and a link to a previous post that addresses this issue directly would be equally appreciated)


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Paul Benson
Re: Removing image background by two image comparison
on Feb 5, 2010 at 1:40:33 am

I think you are on the right track. It is called 'Difference Keying' in after effects, and it essentially looks at the differences in pixels.

Using the Difference blend mode, as you are, will give you black in the exact matches, and then shades of gray for those 'partial' matches. When you get that image, 'Copy Merged' and paste it into a new layer. Then use the levels adjustment to change all of the grays to white, leaving the black alone. That should provide the mask you want.

Pauley



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Benjamin Howard
Re: Removing image background by two image comparison
on Feb 5, 2010 at 11:42:52 pm

Ah, it didn't even occur to me to turn THAT image into a mask. Good catch!

I guess all that needs to be done past that is that the brightness of the two images has to be increased before applying the difference key so that the naturally black parts of the image are maintained.

Solved.


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Paul Benson
Re: Removing image background by two image comparison
on Feb 6, 2010 at 7:26:44 pm

Glad I could provide that 'aha' moment. Use Levels to move all the grays to white.

Pauley



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Scott Roberts
Re: Removing image background by two image comparison
on Feb 6, 2010 at 6:09:39 am

If the photos were shit using a tripod with no camera movement, you can bring both images into one file, i'd put the image of the tree only on the bottom layer, i'd put the layer with your subject in a layer above, and add a layer mask to the top layer and use an airbrush to remove the subject and reveal the layer below - the layer without your subject.

http://www.youtube.com/graphicsdump


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