5 ms·

He used PCA as well as ELA, making your first statement unfounded.

by drx 15y ago

greendestiny 15y ago
While the result of the PCA is quite well defined, there is nothing well defined about the inferences he draws from that. In the end it shows a marked difference in the suspicious area, but there is no proof that the technique will only show those results on tampered areas. If you can simply run filters until the suspicious area 'looks a bit different' and thats your success criteria, you haven't proven anything.
- Natsu 15y ago
  Neal specializes in this sort of thing. This blog post wasn't written to convince you that PCA (or ELA) work. If you've read his other posts on forensics, you'd see that he always starts with the common tests, then works through the most likely explanations, trying to see what you can rule out and where you should focus. A lot of the time, you can rule things out with common sense (the out-of-order numbers jumped out at me), though he always goes on to ferret out what manipulation was performed, because that's what the whole blog is about. Anyhow, people have done lots of tests with lots of different tools to see what they report in different circumstances. So this isn't the first time that someone has used ELA or something. You will notice if you read past explanations that he always tries to find the exact manipulations done, rather than just running a tool and declaring something to have been Photoshopped. I'm no expert, but I've read his blog for long enough to know that he knows an awful lot about the various quirks of many different image editing programs (Photoshop being by far the quirkiest). But if you still have questions, you can always email him. He was kind enough to reply with a lot of useful information when I asked him something a long time ago. Actually, I think he even blogged about it.
  - greendestiny 15y ago
    It doesn't matter how good your intentions are if you place human intuition into the loop you will fool yourself.
    - Natsu 15y ago
      That's true, but trivially so. Things that aren't infallible can still be useful, but understanding their limitations is important. If you read that blog, you can find plenty of discussion of those limitations, for example, how the absence of markers of digital manipulation does not prove that an image is genuine. After all, there are plenty of staged photos out there, images that were framed in a misleading way, etc.
      - greendestiny 15y ago
        I'm not making a trivial point though. I believe the tools, techniques and expertise this guy professes in image analysis are modern day snake oil. I just think that he has deluded himself instead of being deliberately dishonest.
        Natsu 15y ago
        How then do you explain the fact that his methods have worked? He has outed doctored photos that appeared in news stories and they have been retracted after investigation.
        greendestiny 15y ago
        That's not quite what I'm saying, I don't think his tests are scientific not that they show absolutely nothing (although probably fairly close to nothing with ELA). I also haven't doubted his ability to detect fakes but I do doubt the particular efficacy and explanation of his techniques. There is no doubt using PCA can highlight unusual changes in an image - but there are also other explanations. His famous suggestion that terrorist videos had books inserted into them could simply have been a slightly different coloured spot light.
        Natsu 15y ago
        If his techniques are unreliable, he should have some notable failures by now after having analyzed so many images publicly. Where can these failures be found?

He used PCA as well as ELA, making your first statement unfounded.

by drx 15y ago

He used PCA as well as ELA, making your first statement unfounded.

While the result of the PCA is quite well defined, there is nothing well defined about the inferences he draws from that. In the end it shows a marked difference in the suspicious area, but there is no proof that the technique will only show those results on tampered areas. If you can simply run filters until the suspicious area 'looks a bit different' and thats your success criteria, you haven't proven anything.

Natsu 15y ago
Neal specializes in this sort of thing. This blog post wasn't written to convince you that PCA (or ELA) work. If you've read his other posts on forensics, you'd see that he always starts with the common tests, then works through the most likely explanations, trying to see what you can rule out and where you should focus. A lot of the time, you can rule things out with common sense (the out-of-order numbers jumped out at me), though he always goes on to ferret out what manipulation was performed, because that's what the whole blog is about. Anyhow, people have done lots of tests with lots of different tools to see what they report in different circumstances. So this isn't the first time that someone has used ELA or something. You will notice if you read past explanations that he always tries to find the exact manipulations done, rather than just running a tool and declaring something to have been Photoshopped. I'm no expert, but I've read his blog for long enough to know that he knows an awful lot about the various quirks of many different image editing programs (Photoshop being by far the quirkiest). But if you still have questions, you can always email him. He was kind enough to reply with a lot of useful information when I asked him something a long time ago. Actually, I think he even blogged about it.
- greendestiny 15y ago
  It doesn't matter how good your intentions are if you place human intuition into the loop you will fool yourself.
  - Natsu 15y ago
    That's true, but trivially so. Things that aren't infallible can still be useful, but understanding their limitations is important. If you read that blog, you can find plenty of discussion of those limitations, for example, how the absence of markers of digital manipulation does not prove that an image is genuine. After all, there are plenty of staged photos out there, images that were framed in a misleading way, etc.
    - greendestiny 15y ago
      I'm not making a trivial point though. I believe the tools, techniques and expertise this guy professes in image analysis are modern day snake oil. I just think that he has deluded himself instead of being deliberately dishonest.
      - Natsu 15y ago
        How then do you explain the fact that his methods have worked? He has outed doctored photos that appeared in news stories and they have been retracted after investigation.
        greendestiny 15y ago
        That's not quite what I'm saying, I don't think his tests are scientific not that they show absolutely nothing (although probably fairly close to nothing with ELA). I also haven't doubted his ability to detect fakes but I do doubt the particular efficacy and explanation of his techniques. There is no doubt using PCA can highlight unusual changes in an image - but there are also other explanations. His famous suggestion that terrorist videos had books inserted into them could simply have been a slightly different coloured spot light.
        Natsu 15y ago
        If his techniques are unreliable, he should have some notable failures by now after having analyzed so many images publicly. Where can these failures be found?