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The ANPRThresholdAxon is responsible to define the conditions, that have to be met, before a license plate is seen as a valid recognition result. Only then the Axon will pass a recognition result to the next Axon.

It is one of the most complex of all Axons, because of the interaction between the settings. When used in a proper way it is a powerful tool to optimise recognition results.

In CortexClient the images received by the ANPRThresholdAxon are displayed in its viewpanel.

 

1. The ANPRthresholdAxon settings- and viewpanel

 

CC_ANPRThresholdAxon

 

 

2. Settings

 

ANPR_threshold_axon_settings

1.ANPR threshold
The threshold is set for the number of times a license plate needs to be recognised in the same way before it is promoted to the 'passed threshold' list. Only then it is seen as a valid recognition result.
The higher the threshold the higher the chance for a correctly recognized license plates. A threshold of 1 lets all recognized license plate pass; a threshold of 10 needs 10 equally recognized license plates before promoting it. The ANPRThresholdAxon can be seen as a recognition optimizer. Especially in difficult recognition circumstances the tool has proven it worth.
 

2.Cleanup period
A hit is a recognized license plate. All hits are administered in hit count per recognition and are displayed on the 'live threshold list' in the settingspanel. When hit count reaches the threshold a license plate is promoted to the 'passed threshold' list.
Cleanup period is the period a license plate stays on the 'live threshold list' before is it cleaned up. After each hit 'cleanup period' will restart. Cleanup period prevents the same license plate to be promoted to the 'passed threshold' list twice.
A long cleanup period is used in a situation where cars are not moving and chances are the license plate can be temporarily covered, for example by crossing people at a gas station. A short cleanup period is recommended in situations ANPR is used with moving cars.  
 

3.Max character difference
After a license plate has reached the threshold and is promoted to the 'threshold passed list', the ANPRThresholdAxon uses 'max character difference' to decide to promote a new recognition result or not. When set to two, more then two characters have to differ relative to the promoted license plate before this new recognition result is promoted.
Max character difference can be set to loosen or tighten the conditions license plates are promoted or not. In situations where lighting is poor and circumstance extremely difficult for recognition this can be used to optimize even for these situations.
 

4.Minimum license plate width
The minimum width at which a recognition result is counted to reach the threshold. License plates with a width smaller than the setting will not be counted as a hit and will not add up to the threshold.

 

 

In some cases, it may be necessary to fine-tune the threshold settings for a more strict or lenient selection.

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.

 

 

3. Live threshold list

 

ANPR_threshold_axon_live_list

 

The live threshold list diplays all the license plates that are currently detected. Per detection the amount of hits, pixel width of the license plate and the recognition result can be seen.

When hit count reaches the threshold, a license plate is promoted to the 'passed threshold' list. It is now seen as a valid recognition. On that moment the ANPRThresholdAxon adds a messageinfo 'threshold passed' to the received message and forwards the message . This will trigger behavior of sequential axons. A recognition result is only promoted once.

 
When a license plate is no longer recognised (because it is out of scope) for the amount of time set in the 'cleanup period' setting, a license plate will be removed from the live threshold list.  When a row highlights red, it is going to be removed. When a row is green a new license plate just reached the threshold.

 

1.Passed
The check mark in the passed' column indicates whether a license plate has reached the threshold and is promoted to the passed threshold list and to the next axon.

2.Last hit
The time of the last recognition of a license plate

3.License plate
The license plate recognized by the ANPR axon

4.Hits
The number of times the license plate is recognized

5.Min-Max-Avg
The minimum width of the license plate at which it was recognized.
The maximum width at which the license plate was recognized
The avg width at which the license plate was recognized.
The red color means the license plate width is too wide or to small for recognition. When configuring ANPR this is an important setting to take into account.

 

 

 

 

 

 

4. Passed threshold

 

ANPR_threshold_axon_passed_threshold

 

The passed threshold list diplays all the license plates that passed the threshold. Per detection the moment of recognition, pixel width of the license plate and the recognition result can be seen.

 

By selecting a license plate in this list, the matching image will be displayed (see first screen shot of this page).

Scrolling down the list previous recognitions can be inspected.