How to troubleshoot license plates not being detected

Symptom

You have LPR running on a camera and license plates are not being detected and recognized.

Verification

1. Open the latest LPR log file found in %APPDATA%\Senstar\AILogs\lpr_YYMMDD_##.txt
2. One or more license plates show a Confidence value of 0
3. One or more license plates show a low Confidence value (relative to the Reporting Confidence value set for the system where Low = 0, Medium = 50%, High = 100%, and the values increment by 10 in between)

Example of an LPR log where a Syrian License plate is found but with a Confidence of 0.

10:35:29.940  4336 <BasicInf> Found: 434882, Confidence: 0, Region: 0
10:35:30.440  4336 <BasicInf> Found: 434882, Confidence: 0, Region: 0
10:35:32.112  4336 <BasicInf> Found: 434882, Confidence: 0, Region: 0
10:35:59.425  4296 <BasicInf> Found: 434882, Confidence: 0, Region: 0

Potential Causes

• Server CPU speed
• High CPU load on the server

Solution

To improve license plate detection, you can configure the LPR system to process each image for a longer period of time.

The Analysis Timeout setting available on the Analytics Configuration tab under Advanced Settings configures the maximum amount of time the algorithm spends attempting to detect a license plate in a frame.

When you increase the Analysis Timeout value, the LPR system has a better chance of detecting and verifying plates with confidence, though the algorithm can take longer to process each frame and decrease the number of frames that can be processed overall.

After adjusting the Analysis Timeout value, when verifying the LPR logs, you will see higher confidence values and potentially a region value.

08:35:56.858 25976 <BasicInf> Found: 434882, Confidence: 35, Region: 206001
08:35:57.219 25976 <BasicInf> Found: 434882, Confidence: 44, Region: 206001
08:35:57.941 25976 <BasicInf> Found: 434882, Confidence: 24, Region: 206001

Notes

By default, the Analysis Timeout value is set to 250 ms. Typically the system takes less than the specified maximum time to process a frame.

• Decreasing the Analysis Timeout value from the default can result in a higher number of false positives. because the system returns a “best guess” value
• Increasing the Analysis Timeout value from the default can result in an increased number of missed license plates.

For example, if you configure the LPR engine to do analysis at 5 frames per second (FPS), setting the Analysis Timeout value to 1000 ms (i.e., 1 second) configures the algorithm to takes one second to process a single frame. In that second, the algorithm will not process the other four frames, which means potentially missing a plate that showed up in one of the four frames that were skipped.