Dear loyal readers welcome to this week’s video pitfall post! Today we continue our mission of shedding doubts on whether the aspect of things in your video is actually trustworthy. Last week, we showed the “stretching effect” that is often introduced by various aspect ratio issues. Today, we’re dealing with optical distortion.
Issue: Can you trust the shape of objects in a video?
Imagine you’re looking for a missing person. You ask the family for a recent picture, and they tell you, “Look, we have a video from our home camera. It shows her just before she gets out for the last time”. Hey, that could be useful, perhaps she still wears the same clothes and that would help the search. “Okay, we’ll take that video!”, you say. And here it is:
Dear friends, welcome to a new video pitfall post! Among the many visual clues, the shape and size of objects are quite important ones. However, we’ll see that in surveillance videos, they can quite often be misleading. Hold tight and keep reading!
Issue: Can you trust the proportions of objects in a video?
You get an urgent call. “They’ve just kidnapped a girl and fled in a car. We have a video from the pub next door!”. While you rush to the place, you’re already drafting the plan: you’ll look at the video and immediately start a search for the car based on the license plate… or at least the model!
Once you reach the place, they pass you a USB drive with a video file in it. You plug it in your laptop and play the video. Here’s the car.
Dear friends, ready for this week’s Video Evidence Pitfall? Today we’re talking about infrared (IR) images, how they could be misleading, and some potential IR-related issues that may involve even “normal” videos. Keep reading!
Issue: you can’t trust any color in infrared images
How many times have you seen this kind of “washed out” image?
These are typical infrared images, and, as we will see, you should not use them to get any color information. To convince you, let me show you a couple of pictures. On the left, you have the visible light version of a shirt, on the right the IR version of the same shirt in the same scene and same everything.
Dear friends, I’m so glad to introduce this new blog series! Every Tuesday, for several weeks, we’ll walk together to discover some delicate or even dangerous aspects that you may easily encounter when dealing with images and videos during investigations. And of course, we’re not belittling your skills when we write “because you don’t know what you don’t know”! It’s just something that comes from our experience: we talk with investigators on a daily basis, and we’ve noticed that, sometimes, there’s a tendency to treat images and videos as “something everyone knows about”. All in all, we have them on our smartphones, we share them on social media, perhaps we also edit them with some consumer app or software from time to time, and with nice results.
Alas, my friends, it’s not that simple, for many reasons:
Videos you deal with in forensics often come from CCTV surveillance systems. The acquisition and processing lifecycle of such videos is very different than what goes on in a smartphone. Smartphones have never dealt with analog video, while many CCTV systems still work with analog cameras connected to a DVR. And what about compression? One minute of video on my Google Pixel 3a is worth hundreds of megabytes, while it would probably be <10 MBs in a CCTV system.
Video encoding and playback is a complex topic, and this is especially true when it comes to proprietary video formats, that are used by most surveillance systems. Those working in the field know that most of the time, the original video extracted from a DVR just won’t play in standard computer players, it is normal. How do we “convert” it to a playable video? There’s a whole world inside, and investigators must at least know that such a world… exists!
Remember that a shallow interpretation could steer a whole investigation in the wrong direction. Want an example? Take a look at this infrared picture of a shirt.