As Amped Authenticate users hopefully learned during our training courses, authenticating a digital image means much more than attaching a fake/real label to it. In some cases, you may be asked whether the integrity of a questioned digital image is preserved (or broken). In such a case, forgery localization tools should not be your first choice from Authenticate’s powerful arsenal.
Indeed, proving that the integrity of an image is “broken” means demonstrating that the image file is not the original file produced by the acquisition device; instead, it has been processed after acquisition. “What” happened during the processing may even not be of interest, because in some cases broken integrity alone is enough to discard a potential evidence.
That’s why we always stress the importance of tools under Amped Authenticate’s File Analysis category: they are the best way to screen image properties, metadata and coding details looking for unexpected or suspicous elements.
In this post, I’ll share with you a tip that could prove important in your cases: check for un-updated Exif image resolution tags! Let’s take this nice picture from a Sony Xperia XA1 smartphone (formally called G3112), and let’s imagine we are asked to validate its integrity: is this an original file, untouched since acquisition?
Did you know the Amped FIVE interface can be customized to fit your preferences? Perhaps you have two monitors and want to adjust the panels, or prefer a darker application theme? No problem!
My personal preference is to use this layout, with the “darker” theme, as I tend to use only one monitor when using FIVE, but if you do want to re-organize the interface so it is more suited to your work style, each of the panels within the user interface can be moved or hidden from view.
Clone detection (aka “copy-move detection”) is a very important image authentication task. Clones are a special case of image manipulation where part of an image is copied, possibly resized, rotated, sheared, etc., and then pasted to another region of the same image. The two main applications of cloning are:
creating multiple (fake) copies of an object through copy-paste;
removing an object from the scene by covering it with a cloned portion of the background.
This is explained with a very simple example in the image below.
The image forensics research community worked hard to develop techniques for clone detection, and two main approaches have been invented: block-matching and keypoint-matching. As suggested by their names, they are based on two different strategies, briefly explained below.
Split the image in overlapping blocks;
Compute a digest (“descriptor”) for each block, possibly robust to rotation, scaling, compression, etc.;
Search for clusters of matching descriptors.
Detect keypoints (SIFT, SURF, BRISK, etc.) from the image;
Compute keypoint local descriptors;
Search for (clusters of) matching keypoints.
Which one is better? It depends, and we try to explain why with the table below:
So, if your question was: “Do I need a block- or a keypoint– based algorithm for my analysis?”, the answer is: you need both!
That’s why Amped Authenticate features both algorithms under the Local Analysis category: Clones Keypoints and Clones Blocks. Let’s compare their output on the sample image we used in this article:
We see that the cloned seagull (top row) is detected by the Clones Keypoints despite the strong down-scaling applied to the cloned object; such a geometrical transformation is too strong to be detected by Clones Blocks. On the other hand, Clones Blocks successfully detects the cloned background (bottom row), that is not detected by Clones Keypoints because the cloned area is just too flat and it does not contain enough keypoints.
We hope you enjoyed this quick tip! Stay tuned and don’t miss our next #ampedtiptuesday post!
We are present in many industry and partner events worldwide, but unfortunately, we can’t always meet our global customers face to face to show you how to get the most out of your Amped solutions. So we’ve created a new blog series called Tip Tuesday (#ampedtiptuesday). We hope this year will be full of useful tips and tricks for all our solutions.
We will teach you things in Amped FIVE like how to organize and customize the panels, how to use the Assistant, how to use the concatenation in Covert DVR, and the Screen Capture tool.
For Amped Authenticate, you will learn about differences between Clones Blocks and Clones Keypoints, how to find reference images for your case, how to use PRNU on images gathered from social media, and how some minor metadata may tell more than it seems.
And much much more!
If you have suggestions for things you would like to learn about, send them our way!
Our first tip will be published next Tuesday – don’t miss it!