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I was wondering is there a way to know if a video has been edited or not? Like say someone take a video using their webcam or whatever device and send me and i want to be sure that it is not "fake", edited?

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Do you mean visually/content-wise or from a file-system/OS perspective? –  Synetech Aug 15 '11 at 4:21
    
@Synetech either way, as long as someone send me a video i can be sure that it is "unedited", "real", not modified –  Pacerier Aug 15 '11 at 5:06
    
You’ll need a more specific requirement than that. I could take a video, convert it to another format and send you the new file, and technically it is unedited. Would that satisfy your needs? –  Synetech Aug 15 '11 at 5:08
    
@Synetech heys take a look at the question edit –  Pacerier Aug 15 '11 at 5:09
    
Again, that’s not specific. Do you want to be sure that something is not CGI or other items inserted? Do you want to be sure that something has not been cut out? Frames removed? Something has not been blurred or otherwise graphically altered? “Faking” a video is a surprisingly vague concept these days. –  Synetech Aug 15 '11 at 5:13
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What you are touching on is the field of image forensics*. There is no simple way to determine whether or not a video has been edited (if it has been well done), but there are many subtle things that might give you a hint. There are both technical and visual giveaways. Technical giveaways are artifacts of the editing technique, for example, editing software use known mathematical formula to produce effects like blurring that do not match reality, so by closely examining a blurred edge (often with the help of mathematical analysis) you might be able to determine whether it is a natural blur or one created by an editing program.

You might know that this technique is also used on still images - still image forensics and motion image forensics use many of the same techniques, but they are more complicated when applied to video. Of course, there are also extra techniques that can be used on video, since you have motion to deal with ('faked' videos often seem to have fake-looking motion because of the difficulty of matching motion precisely and creating good random motion).

Visual analysis comes down to simply looking very closely to see that everything is consistent. For example, differences between a shadow and the object casting it, or the angle of a shadow through the video, might indicate that something has been changed. Of course, a competent video editor will be careful to insert proper shadows. You might have to look at much smaller clues.

Forensic image analysis is something that people must study and practice extensively in order to gain competence, and even for a highly skilled analyst it can be difficult to make an absolute determination, simply because there are also highly skilled video editors.

I was curious about the career path for image forensics, so I looked in to the title of such analysts a bit. Forensic Document Examiners, which are well established in courts, do not generally deal with photographs (FDEs, instead, specialize in things like typography, handwriting, signatures, etc). It looks like most image analysts have a background in Computer Science or Information Assurance, which makes sense because many analysis techniques have to do with the details of image compression algorithms. Some of the researchers in the area are mathematicians.

Here's a few articles I found that list some common techniques:

*Terminology note: image forensics seems to refer to the practice of analyzing an image or video to determine how (or if) it has been altered. video forensics generally refers to the practice of analyzing a video that is assumed to be unalterede in order to extract information from it (for example, enhancing a frame from surveillance footage to produce a clear image of a suspect). So, what you are asking about is image forensics on a video. This conflict of terminology is confusing and annoying.

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so basically you are saying it had to be manual and there's no way we can feed the file into a program and the program will tell us like say 90% probability this file is edited and stuff like that? –  Pacerier Aug 15 '11 at 7:32
    
No, I'm not aware of any software that attempts to make an automatic analysis, and any software that did would be very limited (i.e. only detect certain specific types or methods of editing). –  jcrawfordor Aug 16 '11 at 6:27
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By "video", presumably you mean a digital video file. By edit, presumably you mean that frames or scenes have been cut. By "edit" I assume you are not referring to image manipulation, like inserting a person into the scene, which could be detected by incorrect lighting or shadows etc.

Consider the simpler case of a text file. If you're provided a text file, could you tell if it has been "edited"? Probably not, unless you have the original file or a copy of it, with known and good provenance.

A digital video file is a data file just like a text file. You probably would not be able to tell if it has been edited if someone wants to deceive you.

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heys could you elaborate on the incorrect lighting shadows etc? –  Pacerier Aug 15 '11 at 5:08
    
This PBS Nova Science Now segment about digital forensics shows how some photos were exposed as fakes: pbs.org/wgbh/nova/tech/hany-farid.html This segment features the author of the 2nd link posted in jcrawfordor's excellent answer. –  sawdust Aug 15 '11 at 6:45
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Finding out if a file is altered after the fact is hard. However, when you create videos and want to make sure they arent altered, you can do a MD5 hash on the file. At a later date if you run the hash again on the file and it comes up different you know it has been altered.

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no i do not mean that, i mean for example someone sends me a video and i want to know if it is "real" or "fake" (edited) –  Pacerier Aug 15 '11 at 5:07
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all OSs can show last modification date when giving a directory listing.

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The timestamp can be altered. –  Synetech Aug 15 '11 at 4:21
    
true, but its just a simple test. –  Keltari Aug 15 '11 at 4:23
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AFAIK, the best method would be to put a transparent text or image overlay into the video. I don't think there is an easier way to provide digital signage to a video. . .

I'm, of course, assuming MPEG format.

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