Understanding the basic flow of the ANPRThresholdAxon

1.All recognition results are displayed on the 'live threshold list' (the right corner of the settingspanel).

2.The hit counter on the 'live threshold list' shows how many hits a license plate had so far.

3.A license plate can be promoted to the 'threshold passed list' (on the left of the settingspanel) when hit count passes the threshold.

4.The hit count of a license plate will still increase, but the license plate will not be promoted again after the threshold is met.

5.After each hit the cleanup period restarts.

6.After meeting the threshold and the license plate being promoted, all new license plates that meet the 'max character difference' criteria will not be promoted during the cleanup period of the promoted license plate.

7.In the live threshold list 'passed' will not be checked for new license plates even when it reaches the threshold.

8.Meanwhile recognition continues. The cleanup period restarts every time a license plate is recognised.

9.When a license plate gets out of scope. Cleanup period will end. The license plate will vanish from the 'live threshold list'.

10.The cycle will start over with a new license plate.

11.The history of passed license plates is seen in the 'Passed threshold' list.

Threshold

Threshold settings need to be fine-tuned based on location conditions. If dealing with a situation where accuracy is a top priority, for instance a car-park with access control that deals with a white list, the threshold can be kept higher than in a situation where cars pass at higher speed and only a few frames are available.

Max character difference

Max character difference is kept low (value = 0) when cars are passing at high speed.

Max character difference is set higher (value = 2) in a parking, a car wash or other venue, where cars are standing still for a longer period.  In order to stop slightly different recognitions of the same license plate to be passed on to a POS or other management system, max character difference can be set to 2. This setting is especially meant to handle license plates that are difficult to read or with cars waiting in a queue where partially obscured license plates occur.

Cleanup period

Cleanup period can be low when cars pass at higher speed.

Cleanup period can be longer for stationary cars. A longer period makes sure a license plate is not passed on during the cleanup period to an output Axon twice.

For instance at a car wash this can be a good choice. Personnel will cover a license plate to prewash the front of a car. As soon as the plate is visible again it will be recognized again. With a longer cleanup period the license plate will not be passed.

Min license plate width (px)

This setting defines the minimum width (in pixels) that a recognized license plate must have in order to be counted as a valid hit.

•If the detected license plate is smaller than the specified value, it will not be counted as a hit.

•Plates that meet or exceed the width value are processed normally and can contribute to reaching the threshold.

This helps to filter out very small or distant detections that are unlikely to be reliable.

Min character height (px)

This setting defines the minimum width (in pixels) of a recognized character on a license plate for it to be counted as a valid hit.
 

•If the detected characters are narrower than the specified value, the plate will not be counted as a hit.

•Plates with characters that meet or exceed the minimum width are processed normally and can contribute to reaching the threshold.
 

This helps filter out very small or unclear detections that are unlikely to provide reliable recognition.

Repeat period

When a license plate reaches the threshold, it is promoted from the live threshold list (right side of the settings panel) to the threshold passed list (left side of the settings panel). Promotion means that the license plate has been recognized often enough to meet the threshold value, and is therefore marked as an event of interest. At this point, it is passed on to the next Axon in the pipeline, where an action may follow (for example, granting access or triggering an output).

Normally, once a license plate is promoted, it will not be promoted again — even if new recognitions continue to increase its hit counter.

The repeat period defines after how much time a promoted license plate can be promoted again.

As soon as the repeat period has passed, the license plate is treated as if it were new: if it reaches the threshold again, it will once more be promoted to the threshold passed list.

This is useful for detecting whether a car is still present after some time. For example, if a vehicle remains in view, it can trigger a new promotion once the repeat period has expired.