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Cross-Line Detector (advanced Visitors Counter, or Tripwire)

Generally, security is particularly concerned with entrance and exit points for the guarded area, since those are easier to keep under surveillance than an entire perimeter and provide the most traffic. A particularly wide-spread method is setting lines across doorways or narrow corridors to control the number of people crossing them (or across driveways and lanes for cars) – a simple traffic counter. The method allows to both protect the area from unwanted visitors and collect statistical data, which is what makes it so popular. We’ve mentioned the topic while dealing with “Visitors Counter”, because it functions in a similar fashion. However, there are more intricate ways the same method can be applied, where this module may lack the ability to distinguish directions of line crossing. One has to admit that more often than not someone entering the area demands more attention that somebody leaving. This is where another module comes into play – “Cross-Line Detector”.

 

This module is sometimes also called a ‘tripwire’ for it basically does the same – detects someone crossing a line.

The main principle behind its work is that it gets information about objects that fit the designated size in the camera’s field of view or looks for them itself. “Cross-Line Detector” checks whether any of those objects crossed any of the lines in the right direction.

Depending on the types of objects that you’d need to detect crossing a virtual line, the ‘Cross-Line Detector’ can work in combination with various modules, and its detection accuracy will also depend on what module gives it the information about objects.
1. ‘Cross-Line Detector’ has embedded tracker of most popular objects – people and cars – so if you need to track their crossing the line, you can use ‘Cross-Line Detector’ by itself and connect it to Universal Camera and to destination modules with no intermediary filter modules.
2. If you’re looking for people or cars that cross the drawn lines but need more accuracy, you can put a “Face Recognition” module into the chain before “Cross-Line Detector” that will detect and send to ‘Cross-Line Detector’ only episodes with people in sight, – or an “ANPR” module that will only push video stream with cars to ‘Cross-Line Detector’.
3. If you’d like to tripwire other types of objects, connect an “Object Recognizer” module that can recognize birds, animals, drones, etc too before the ‘Cross-Line Detector’. The latter will then receive information from the former and will only react if the selected objects types are trespassing the line.

4. If it is the arrival of a truck with a certain number in the unloading area that you’d like the system to recognize, you can use the “Cross-Line Detector” with the “Text Recognition” module, in the settings of which it is selected to respond to certain numbers or letters.

5. The “Cross-Line Detector” is also combined with the “QR Code Recognition” module, so you can, for example, count incoming people with QR code badges, or keep track of the number of products on the conveyor that have barcodes on the label.

6. And if you’d like to get reaction to any types of objects crossing the lines you’ve drawn, place the “Object Detector” there, so the simplest chain looks like this:

Here are the detector’s settings:

• Counter ID sets the module’s name, this will come in handy later when it comes for logging. Note that several counters can have the same ID.
• Interval of resetting the counter shows how often the traffic counter will start tallying anew. If no reset is required, drag the slider to the far left.
• Show count of visitors makes “Cross-Line Detector” indicate the results on the preview in the lower right corner of the frame.
• The drawing screen allows you to create lines by clicking and dragging. A single detector can work with as many lines as you need.
• Selected line chooses one of the lines to apply the following 2 options:
• Change direction switches the direction for the chosen line (bidirectional tallying works too).
• Remove selected line deletes the chosen line.
• Remove all lines deletes any and all lines present on the drawing screen.
• Save data in CSV report creates a log-file that will keep the info on all the times the lines were crossed, including date, time, the counter’s name and the tally. You can give several detectors the same name (e.g. if they control the same area with different entrances and exits) to sum up their results.

Now let us look at this module’s practical applications.

“HTTP Marking” will enable you to search the archive for the times the lines were crossed. Here is how it works: when the “Cross-Line Detector” tripwire module triggers, it activates “HTTP Request Sender”, which, in turn, instructs “HTTP Marking” to start the event. Once the object is done crossing the line, another “HTTP Request Sender” stops the event. The same system applies for the other detector, so that you could search the archive separately for different lines. When viewing the archive you will be able to go Archive menu → Search by HTTP Marking and choose the line you need – the timeline will have the right intervals highlighted. This part serves as a people counter with a smart archiving system.
At the same time, “Detector of abandoned objects” checks if anything is left motionless for too long, and if it does react to something – “Sending Email” notifies you.

This chain accounts for different times of day. The top “Scheduler” works during daytime and allows “Cross-Line Detector” to count cars entering and exiting the lot. “Object Detector” is set to react only to car-size objects, as we don’t want a people counter here. “ANPR” is connected to an external database with license plates belonging to cars known to infringe regulations – those are given special attention and reported by “HTTP Request Sender”.
The bottom “Scheduler” works during nighttime and enables the “Motion Detector”, which, in turn, uses “Sound Alarm” to notify security personnel and keeps a separate archive for detected motion.

A smart traffic counter yields a lot of benefits and works as a reliable part of security.

April 16, 2018, updated May 15, 2023

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