On Sun, 16 Sep 2007 10:56:53 -0300, "Lucas Prado Melo" <[EMAIL PROTECTED]> wrote: > How can I identify (using gimp) if a photograph has been faked? >
There are different techniques that can be applied. Some of the focus on detecting if the image has been modified (detecting suspicious patterns in the pixels after loading the image) and some others focus on analyzing the file, not the pixels. You already got several replies mentioning the first part (changes in the image), so let me give you a few hints about the second part (changes in the file). For example, if you have a JPEG image to analyze, then the first thing you can do is to see if the image has been saved directly by a digital camera or if it came from GIMP, Photoshop or any other image editing software. If the EXIF metadata tells you that the image was saved by Photoshop, then you can already be sure that it did not come straight out of the camera. This does not necessarily mean that the contents of the image have been modified, but at least you know that the image may be different from what was taken by the camera. And even if the JPEG file does not contain any EXIF block or if some clever hacker has replaced the EXIF block by the original one from the camera, then you can look at the JPEG quantization tables and check if these match the tables that are used by some cameras, or if these are the tables used by Photoshop or any other program. This is a bit harder to fake and most of those who create fake images and go through the trouble of replacing the EXIF block ignore the fact that the quantization tables in the JPEG file can betray them. Note that I recently added a feature in GIMP that allows you to preserve the quantization tables from the original image, so that could in theory be used to "improve" some forgeries. However, there are other details in the JPEG file layout can can reveal if the file came straight out of the camera or if it was processed by some other software. I do not want to say too much about that because I do not want to give too many ideas to the crooks, but let's say that the way some things are split or ordered in the JPEG file is usually different between digital cameras and image editing software. -Raphaël _______________________________________________ Gimp-user mailing list Gimp-user@lists.XCF.Berkeley.EDU https://lists.XCF.Berkeley.EDU/mailman/listinfo/gimp-user
