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Can I use Photoshop to detect or find out with 100% certainty if an image is altered or not?

When an image is doctored or manipulated, is there metadata embedded in the altered image so that one can know it is a manipulated image? Or is there some other tool or method that can assist?

I was reading some news about fake UFO photos and this question came to my mind.

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I kind of doubt it, because if I remember correctly the US military has been fooled by photoshop a few times. –  ekaj Jun 27 '12 at 21:43
    
No way to know for certain. Unless the person doing the editing made a reasonably obvious boo-boo, about all you can do is estimate the likelihood of modification. And even reasonably legit photos are often modified to remove objects in the background, etc. –  Daniel R Hicks Jun 27 '12 at 21:44
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There may be some metadata left be hind by some photo manipulation programs, but they are not guaranteed to be there or be correct. Any person working to do the forgery well may strip out metadata or put in fake metadata that the file is genuine. After all if they can fake the photo why can't they fake the metadata? –  Scott Chamberlain Jun 27 '12 at 21:46
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(I always edit my photos to remove the UFOs in the background -- the clutter things up so badly.) –  Daniel R Hicks Jun 27 '12 at 21:46
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What you want is not possible because you want 100% certainty. –  Ramhound Jun 28 '12 at 15:02
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It depends on what you want to detect. If you want to detect any manipulation at all, then there are some techniques that you can use somewhat effectively. If you want to detect fakes like with bigfoot and Nessie photos, then the options are more limited. While there are actually more signs of a doctored photo, the problem is that they tend to give a lot of false-positives.

When an image is doctored or manipulated, is there metadata embedded in the altered image so that one can know it is a manipulated image?

It's possible, but if someone were going to fake a photo (at least with enough skill for it not be an obvious fake), I would think they would be smart enough to disable or strip any metadata. Even so, the metadata would only indicate that the image had been saved by some program or another, not what was done. For example, someone may open a photo in a program, then save it as another format, compress/optimize it, or resize it. If the program adds metadata to the file, it does not indicate "doctoring", or even necessarily manipulation, only that the program created it (the file, not necessarily the image).

Or is there some other tool or method that can assist?

There are a small handful of programs/plugins that can perform some level of analysis on a photo to look for abnormal artifacts like incorrect edges (as when an item is artificially inserted into a photo), or improper blending, etc. The problem is that most of these can be caused by standard manipulations and so are not reliable ways to detect actual forgeries.

Can I use Photoshop to detect or find out with 100% certainty if an image is altered or not?

You can use Photoshop to manually search for most of the artifacts that the programs/plugins look for. For example, try zooming in to see if you can find image artifacts from the image being re-saved/converted/etc., but that could very well lead to false positives since a non-docotred image would exhibit similar artifacts when saved, compressed, etc.

I was reading some news about fake UFO photos and this question came to my mind.

When it comes to things like this the best and most effective tool is ultimately your own mind. Use common-sense to detect some fakes like photos of Obama eating an alien while riding a unicorn. For more realistic fakes, use your mind to detect things like incorrect shadows or "Photoshop fails":

enter image description here enter image description here

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You're missing this example :P –  Bob Jun 28 '12 at 15:36
    
Could you elaborate on what "handful of tools" you're referring to? –  Griffin Nov 24 '12 at 20:44
    
@Griffin, most image and quality analysis tools can be used to highlight suspicious artifacts (especially if you have another/original copy). Check out routines like VQMT, SSIM, and PSNR. –  Synetech Nov 25 '12 at 2:44
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Some people (me not included) would claim that Error Level Analysis can prove photo manipulation:

enter image description here

“Error level analysis (ELA) works by intentionally resaving the image at a known error rate, such as 95%, and then computing the difference between the images. If there is virtually no change, then the cell has reached its local minima for error at that quality level. However, if there is a large amount of change, then the pixels are not at their local minima and are effectively original.”
-Neal Krawetz, Ph.D. http://www.hackerfactor.com

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This does not meet the author's 100% certainty requirement..Just Saying :-) –  Ramhound Jun 28 '12 at 15:04
    
the service in the first link has been taken down. It looks like hackerFactor.com links to fotoforensics.com though. –  Griffin Nov 24 '12 at 20:08
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Curious; Krawetz’ ELA seems to more perform an edge-detection than differential analysis. @Griffen, errorlevelanalysis.com does still work at this time, and the second link is just part of the quote from Neal Krawetz who wrote a JPEG-quality analysis function which is the basis for his fotoforensics service (and which was used in the ELA site). –  Synetech Nov 25 '12 at 2:43
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There is no way of doing this with 100% certainty. If you're being theoretical, people have been manipulating photos since photos started to exist. The pre-war portraits and Civil War photos of Matthew Brady were staged. Shots in the 1840's and 1850s were often composed of multiple shots, since th e film at the time could not handle the brightness level differences - the equivalent of HDR photography. Gustave Le Gray, a photo master in the 1850s, was one of the first to be able to take a sea scene with a single shot. The photo of the Loch Ness monster you mentioned was not manipulated, but was staged. Does that make it more or less real? HDR phots are manipulated, are they more or less real? If you're into the meta thought relationship, check out Errol Morris who talks about photography and realness in depth.

If you're being mostly practical, it's difficult if not impossible to determine this with 100% accuracy. Most photo manipulation software will not leave any metadata as to mark manipulation. There are various ways to run some digital processing on an image, and you may detect manipulations with error level. But if I know the algorithm to detect this, I could fake the data somehow too.

But What is manipulation, what is "normal"? If I boost saturation to match what say, old saturated Slide film would do, is that manipulation or just processing? Making an image B/W is manipulation. Realize that all photography is processing. Even something that seems as simple as a crop can be controversial

Late edit:

Slashdot just posted an article claiming that the World Press Photo Winner was photoshopped, combining multiple files to one image. If you read the linked articles, they explain some of the analysis in depth.

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