Video Motion Detection
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Basic Information

Video Motion Detection is a form of Motion Detection that uses some form of video camera or digital camera, wired to a computer. The computer constantly compares the current image to the a control image that was recorded of the area when it was safe and unoccupied. Any variation beyond a certain number of pixels is automatically flagged. Depending on the system, it may be set to trigger an alarm when that happens.

The main drawback to a VMD system is false alarms (see also False Posititive). If the system is too sensitive, it may be triggered by birds, by wind blowing tree branches or debris, by objects falling over, by shadows or nightfall, dark current or camera flicker, etc. The easiest and cheapest remedy, of course, would be to set the system to require a larger number of divergent pixels to trigger the alarm. Of course, that devalues the effectiveness of the system in detecting intrusion. Somewhat more expensive would be having human back-up available to track down false alarms and review tapes any time something was triggered.

Hooking the system up in parrallel to an infra-red or thermal imaging camera is also a viable solution - much like a more advanced version of a domestic PIR this means that the system needs both motion and a significant heat signature to alarm, cutting false positives down to large mammals and enormous birds.

The technology for this has been on the market since the late 1980s. Lately, it's been improved to be a lot more accurate, produce fewer false alarms, and detect more than just motion. See Facial Recognition System and Video Analytics.


Game and Story Use

  • Sometimes a computer can be fooled by something that wouldn't trick a person.
    • Careful review of recordings from the night of the break-in reveal a very short person crawling slowly down hallways, dressed in a body suit of the same color as the carpet. The variation per frame was too small to notice.
    • Vampires don't show up in mirrors, and some settings extend that to film or video. See Our Vampires Are Different.
  • Sometimes a computer can catch something that a person never would.
    • The intruder had mind-control or mind-clouding powers, and strolled right past the security guards. Maybe even stopped to talk with them, even though they don't remember it.
    • Some sort of Stealth Technology or camoflague breaks up the silhouttes enough that people don't recognize it as human. The computer still registers it as motion, though.
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