*CarR Setup

Owned by Tom Polman
Last updated: Sept 28, 2023 by Christian van Surksum
6 min read

Configuration Article | CA-20201130-TP-08

VDG Sense | Setup | Profiles | CarR

CarR is the realtime ANPR or license plate recognition of VDG Sense. It can be enabled in the Profiles setup tab and is only available if the server has a license for one or more CarR cameras. This article explains how to configure a camera for fast ANPR. For more information on camera placement check the VCA pages about CarR.

With CarR activated the following two events can occur:

License Plate Found: License plate is scanned

License Match: License plate is matched / recognized which means the license plate is known in the Sense Database.

CarR Setup

CarR is enabled and configured per camera in the CarR subtab of the Profiles menu, see below:

CarR has different modes of operation which can be selected after CarR is enabled.  To enable CarR, check the checkbox ‘Activate CarR’. CarR has different modes of operation which will become available after CaR is enabled:

  • Recorded frames: in this mode only recorded frames are processed. How many frames are processed depends on how the recording settings are done in the General, Storage and Motion tab.

    • General: Determine the framerate of the camera.

    • Storage: Record on motion or continuous recording. If using continous recording all frames are processed.

    • Motion: Configure motion detection, it is recommended to decrease the sensitivity of motion detection and increase the motion (recording) threshold.

  • Macro: in this mode a single frame is processed by using the ’GrabLicensePlate’ in a macro.

  • Pos: in this mode the processing of frames is determined by the POS events ’Nozzle Out or ’Filling Token’ which are received from a POS server.

For best performance it is recommended to follow these steps in configuring a camera for CarR:

  • Follow the guidelines for CarR camera placement, read here.

  • Configure parameters in General, Storage and Motion tab to ensure only frames are processed in which a license plate is visible (before it is scanned); less frames is faster performance.

  • Determine which countries need to be scanned, this is done in the Server tab and the profile Countries parameter

  • Fine-tune CarR profile parameters Min/Max Character size, Confidence and Max process time (explanation below)

  • Determine how many events are generated for the same license plate use Match Distance and License Plate Found Distance parameters (explanation below)

Send processed frames

By enabling this setting only the processed frames are shown in the video panel. This will provide insight in what the impact of changes are in any of the settings related to CarR. It will also render a yellow rectangle around the license plate together with the license plate, confidence level and character size of the scanned license plate. This setting should only be used for finetuning the CarR settings. BE AWARE, by enabling this setting, the framerate of the displayed videostream can be lower than the configured framerate.

Confidence Level
Only show license plates with a higher confidence level than the specified value.

System Complexity
Obsolete, this setting can be ignored.

Min Character Size
The minimum size of the characters in the license plate. Advised is to use a minimum of 20 to ensure a higher confidence level. This value should be changed depending on the resolution of the camera and the (physical) distance of the license plate to the camera.

Max Character Size
The maximum size of the characters in the license plate. This value should be changed depending on the resolution of the camera and the (physical) distance of the license plate to the camera.

Max Process Time
The maximum process time in milliseconds to find a license plate in the video frame. Lower values result in faster response times. For faster response times it is advised to configure the values of min and max character size near to the actual size of the license plate. Normally a value of 250ms is enough to find a license plate in a FullHD image. With a setting of 250ms the CarR enginge will process 4 frames a second.

CarR License Plate Found Distance
The minimum time difference in seconds between two recognitions of the same licenseplate. If the same license plate is scanned within the set time it will not generate an event.

CarR Match Distance
The minimum time difference in seconds between two matches of the same licenseplate. If the same license plate is matched within the set time it will not generate an event.

CarR Countries
This field displays the activated countries. A double-click on the field shows a dialog with available countries. Check countries on or of to enable these for this profile. The activated countries can be configured in the ’Servers’ tab. For more information, check the Installed CarR Countries page.

 

Perspective Correction

LPR perspective correction is a method of changing the image of a CarR device in such a way that the angle of the field of view benefits the License Plate Recognition (LPR) algorithm. The perspective correction can be utilized in scenarios where due to the position of the CarR device, license plates are being misidentified or even missed and where the external conditions are often good and constant. For example, vehicles don’t drive too fast, which makes it easier to capture the vehicle at a fixed position. The best recognition is achieved if the guidelines for CarR cameras are followed, the guidelines can be found here: CarR Guidelines, 

However, in some situations, setting up a CarR device in such a way is not possible, for example due to architectural difficulties or enforcement of certain limitations. In these situations, the LPR perspective correction may help to create a viable image that the LPR algorithm can use. An example of an image that is taken at the entrance of a parking lot is shown in below:

Though the image would probably be recognized without specifying the perspective distortion, much better recognition rates are obtained if the distortion is specified. To this purpose we put a so called reference frame in front of the camera. This should be a frame of at least one meter by one meter with square angles. The frame should be located at the expected position of the front of the car at an image trigger. The frame in the image below is 1500mm x 1000mm.

Sense Configuration

To enable the perspective correction, check the checkbox ‘Perspective Correction’ in the CarR tab located in the Profile settings:

In the right column, the perspective correction can be configured using the following steps:

  1. Enable Perspective Correction by clicking the checkbox.

  2. Use the green rectangle in the reference image to identify the area in which license plate will appear.

  3. Set the actual size of the area in millimeters.

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