13. EVENTS AND ANPR
This chapter explains the mechanism of analysing the video stream, segmenting it to image
sequences corresponding to one passing vehicle, then finding, validating, storing and publishing the
license plate of said vehicle.
The basic unit of the process is an event, typically consisting of one passing vehicle. In order to
segment the video stream into events, an external or internal trigger is needed. A trigger is a pair of
time coordinates: a start time stamp and an end time stamp, representing the duration of the event.
An internal trigger source may be:
• Vehicle Detection: Image processing algorithm designed to detect a vehicle in the given image
• Motion Detection: Image processing algorithm designed to detect motion in a sequence of
images
• Scheduler: Regular series of triggers, for example every 5 seconds
• Virtual Loop Trigger Module: Advanced 3D motion detection developed for license plate
recognition.
An external trigger source may be:
• GPI: A TTL level input to the camera (not all models)
• UART: A UART (RS232 or RS485 level) input to the camera (not all models)
• SW: Direct software-based trigger
The above trigger options cover a wide range of applications, for example:
• Induction loop linked to the GP input
• Radar communicating through the serial port of the camera
• A laser software trigger via Ethernet.
NOTE to users of previous (before 3.6r1.3787) releases: The 'Normal/Quick' and 'Post' stages
have been renamed to 'Preselection' and 'ANPR' respectively, which better reflect their
function. The 'Quick' engine has been phased out, the corresponding functionality is
implemented internally in the Preselection stage.
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