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Glossary: Video Motion Detection (VMD)

Video Motion Detection (VMD) is a feature found in IP cameras, recording software and network video recorders and is used to trigger alarms by sensing physical movement in a given area.

Software driven, Video Motion Detection works by using a simple algorithm which continually analyses current live image data against a reference image from a previous frame. Any significant changes will trigger an alert from the camera. This alert can be used to initiate many actions such as emailing the current live image, moving a pan-tilt-zoom camera to a specified point, operating an external device such as turning on a light or sounding an alarm etc.

Sensitivity adjustment

The motion detection algorithm used by most cameras is very simple. It works by counting the number of pixels which change from one scene to another and if the number exceeds a set threshold it will trigger a positive result.

The downside of using such a simple method is that images can differ naturally throughout the course of a day due to factors such as varying lighting conditions, JPEG artifacts/image noise, movement from plants/trees etc. All of these factors can contribute to false alarm activity by the camera.

Luckily most cameras come with tools to help you adjust the sensitivity of motion detection. At their simplest they allow you to adjust threshold settings so that the speed or size of a moving target can be adjusted. Other tools allow the masking of areas from calculation, multi-window VMD gives a number of unique windows in which to base activity calculations or high-end intelligent video analytics found in DEPA supported Sony cameras provide far better motion detection algorithms which use many more reference frames in the calculation to reduce false alerts.

Motion detection settings in a Panasonic bl-c111
Motion detection settings in a Panasonic BL-C111 (firmware version: 3.10R02)

Axis 207W motion detection window shown over a door
Multi-window motion detection settings in an Axis 207W (firmware version: 4.40)

Published on June 10th, 2008 by Greg

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