Greetings,

I am trying to write an algorithm in MATLAB for detecting modifications in an image. Specifically, in the image I have to process, there was a person who was removed (using photoshop) and the space was filled with background pixels (which is a white wall). I was trying to detect reapeated patterns (using background blocks) but this method is not efficient. Do you guys have any ideas on how to do this in MATLAB? Thanks in advance. 

Answers:

Forensic image analysis is a fairly big research field, with huge applications ranging from law enforcement to show-biz. It's a huge (but very complicated) problem with lots of parameters, so don't be surprised if you don't find a lot of code examples available.

Before you even think about the technology you're going to use to implement it (e.g. to MATLAB or not to MATLAB), you should take a step back and think about the actual algorithm. You should also do your homework and perform a research survey using a site like Google Scholar.

Here's a couple of points to get you started:

• One of the biggest guys in image forensics is Hany Farid. Check out his website. Read his papers, read the papers that he cites, and the papers that cite him. Be sure to watch the videos there too.
• Dealing with compressed images actually helps image forensics. Read about blocking artifacts in JPEG images (most common image compression format). This link is a starting point, don't be shy to put a bit of effort in and look it up elsewhere, like Google Scholar.
• Think about how editing the image alters the artifacts -- does it destroy them, replace them, modify them in some detectable way?
• Read about fourier analysis -- it is a useful tool for image forensics

Be prepared to easily spend days or weeks on researching this problem.