Category Archives: Video Evidence Pitfalls

Color Fidelity: It’s Hard to Objectively Evaluate Colors on Video

Welcome to this week’s Video Evidence Pitfalls post, dear friends! Today we’re dealing with the reliability of colors in CCTV footage (and with any kind of digital images in general). Colors are an essential part of human perception, but can often be misleading in surveillance footage. Keep reading to find out why!

Issue: Colors Are Often Unreliable in CCTV Footage

As part of an ATM fraud robbery investigation, you are reviewing footage from two cameras in the same area. This is the image of the suspect as he walks away from the ATM.

He’s suspected of having visited another ATM of a bank nearby. So you review the footage of that camera as well, but… you don’t find anyone with a blue jacket and beanie hat. After syncing the clocks and checking frame by frame, you realize that it could be him:

But you also realize you will have a hard time convincing anyone about it. Why are the colors completely different?

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Playback Speed: If the Video Plays Too Fast or Too Slow, It Can Affect the Interpretation of the Events

Dear friends, welcome to our weekly Video Pitfalls post! Last week, we talked about the importance of a video’s time resolution (recording frame rate). Today, we’re dealing with playback speed, which is tightly related and also very important. Keep reading to find out why!

Issue: the same thing looks different at a different playback speed

As part of a burglary investigation, you are reviewing some CCTV footage of a parking lot. At some point, there’s an orange car that seems to act a bit suspiciously — it runs in one direction, then possibly realizes there’s no exit, and rapidly turns back. You even manage to see that the driver at the wheel acts quickly like he/she’s in a rush. Could that car be worth focusing on?

Notice that we are essentially basing the decision on how much of a rush the driver seems to be in. But what if the video played like this?

Everything is the same except for the speed: now the driver seems to be someone who just took a wrong turn and quietly turns back. And so the question now is: which of the videos (if any!) is an accurate representation of the actual event?

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Low Frame Rate: If You Don’t See It, It Doesn’t Mean It Didn’t Happen

Dear friends, welcome to this week’s video pitfalls post! When thinking of “video resolution”, we normally recall possible pixel matrix sizes (640×480, 1920×1080, and so on). This is the spatial resolution. A video however has a temporal dimension as well, with the ‘tempo’ relating to the speed at which the frames are captured and then presented. In many cases, this can be more important than spatial resolution. For this reason, we will dedicate this and the next post to this subject. Today, we deal with the recording frame rate, while next week we’ll talk about the playback speed.

Issue: If the recording frame rate is low, you are missing information

You are called to a bar where a scuffle just took place. Upon arrival, you listen to all the various witness and victim accounts and then the bar owner shows you the CCTV. This is what you view (we’re using a video from the “UnarmedcombatOnline” YouTube channel).

No matter how many times you play the video, you hardly understand how the guy with a black sweatshirt turns from standing to laying on the ground, and it does not match some eye-witness accounts. You decide to acquire the evidence (in a forensic manner) utilizing the device’s USB Export function.  Back at the station, or on your laptop, you view the evidence in Amped Replay. Carefully navigating the frames allows you to check thoroughly all the available information in the recording. By scrolling the gallery below, you’ll notice that in frame 2 the guy is standing, and in frame 3 he’s grounded. What happened in between? We don’t know.

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Perspective: Size Comparison May Be Tricky

Welcome, dear friends, ready for this week’s video evidence pitfall? Today we’re dealing with one video issue that often gets overlooked: perspective. It’s something we’re all familiar with, but in video analysis, it can play nasty tricks on you.

Issue: you shouldn’t compare the size of things at different depths

A robbery takes place and you get called. When you reach the shop, the owner runs at you yelling a classic statement: “I got him! I have a video!”. Indeed, there’s a video on the CCTV system and the suspect is there, as he walks into the shop (we’ve simulated a balaclava in the picture below). The face is disguised, but there’s good information about clothes. Then you think: well, I’m here in the shop, the suspect is close to the door, so why don’t I measure the height of the door to find out the height of the suspect?

Well, certainly measuring the door is an excellent step towards height analysis, but don’t take it too easy: that would just be the first step of a longer process. Indeed, you can’t directly compare the size of things at different depths because perspective makes farther things look smaller!

And it’s not just a matter of measurements: perspective affects the human view of an event. What you think you see may be affected by perspective: two planes in the sky may appear to be about to crash into each other when they are actually hundreds of feet apart. For CCTV, it may look as if an action or movement is one thing, but it is not in reality. Moreover, the perspective of the camera must be considered, and this is often very different from a witness perspective. Many cameras are high up and angled down, whereas witnesses will be at ground level and looking perhaps from a different angle. What one person sees may be very different from the camera’s view. 

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Lens Distortion: Cameras May Change the Shape of Things

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:

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Aspect Ratio: Be Sure Your Image Is Not Stretched in Either Direction

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.

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Infrared: Don’t Trust the Colors of Most CCTV Footage at Night

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.

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Introducing “Video Evidence Pitfalls: Because You Don’t Know What You Don’t Know”

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:

  1. 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.
  2. 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!
  3. 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.
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