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#1 Posted : Tuesday, April 8, 2014 11:06:01 AM(UTC)

Joined: 1/4/2013(UTC)
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It is imperative to configure LPR correctly to each specific environment as it requires configuration to run properly and cannot be expected to run "out of the box".

  • Make sure you have the needed camera resolution to attain the needed plate width requirement.

  • And a LPR camera is as a safe “rule of thumb” approximately 3-5 times as “heavy” on CPU load as a normal camera. As a result if your system is not configured correctly, running an LPR setup can easily result in heavy CPU load on your LPR server.

  • If not setup correctly it will produce “garbage” and “partial” hits and use more CPU than necessary.

And as a general guide please see the Administrators Manual

Resolution – Stroke Width

The term pixels per stroke is used to define a minimum requirement for LPR.
See the illustration on page 36 in the Manual
Because the thickness of strokes depends on country and plate style, measurements like pixels/cm or pixels/inch are not used.
The resolution for best LPR performance should be at least 2.7 pixels/stroke.

Resolution – Plate Width

  • For single line US plates (plate width 30.5 cm; stroke width around 0.7 cm) this typically means that the plate width in the image must be at least 130 pixels.

  • For single line European style plates (plate width 52 cm; stroke width around 1 cm) this typically means that the plate width in the image must be at least 170 pixels.

When vehicles are moving when recorded, and an interlaced camera is used, only a half of image can be used (only the even lines). This means that the resolution requirements almost doubles.

What is interlacing? Interlacing is a method determining how an image is refreshed when shown on a screen. With interlacing, the image is refreshed by first scanning every other line in the image, then scanning every opposite line, and so forth. This allows for a faster refresh rate because less information must be processed during each scan. However, in some situations, interlacing may cause flickering, or the changes in only half of the image’s lines for each scan may be noticeable.

Character height

These character size settings influence the recognition process as they partly determine the recognition time.
As a rule, the larger the difference between the minimum and the maximum character size:

  • the more complex the LPR process is
  • the higher the CPU load is
  • the longer you have to wait for the results

The minimum height could be “translated” to, the maximum distance to the car from the camera, where the detection engine is allowed to start analyzing the image. (This of cause has to follow the plate width requirement)

Recognition Area

The Recognition area should only cover the part of the image where the license plate is visible as the vehicle moves in and out of the image. And obviously the optimal images is when the car is closest to the camera.

If to many irrelevant items are included in the recognition area, this results in "noise" and too many resources spent on analyzing and detecting these irrelevant items instead of license plates.

  • Signs
  • Fences
  • Gates

These are all items that can "confuse" the LPR engine when detecting.

Recognition Area - Physical Environment

This plays in heavily when having to adjust the Recognition area – if it is a gate scenario then "where" the car is going to be located is pretty well defined which makes it easier to setup a area. But if it is a wide entryway with cars coming from both left and right then this can be difficult.

With todays cameras and resolutions in HD then every bit of image that can be excluded from analysis is "saved" CPU cycles, so do setup a area when possible.

Physical Positioning of Cameras

When mounting cameras for LPR, note the following:

Single-line rule: Mount the camera in such a way that you are able to take an image of a single line license plate recorded by the camera, and draw a horizontal line that crosses both the left and right edge of the license plate.

Plate in image center: Mount the camera in such way that an ideal image of the license plate is captured when the license plate is in the center of the recorded image. This allows the plate to be dead-on angle vise – but on the other hand you have worry about distance and the car is typically closer to the camera in the lower half of the image and thus achieving a bigger character size.

Vertical angle: The maximum vertical view angle of a camera used for LPR is 30°.

Horizontal angle: The maximum horizontal view angle of a camera used for LPR is 25°. In most systems the horizontal angle is somewhere between 15° and 20°.

Blocking objects: Avoid possible blocking objects in the view path of the camera, such as pillars, barriers, fences, gates, etc. Remember that barriers, gates, etc. are likely to be movable between different positions.

How to reduce CPU load in LPR

The following guidelines can help you reduce your CPU load to an acceptable level.

  • Set video resolution as low as possible but still keeping license plates clearly visible and adhering to the plate width requirement.

  • Reduce video frame rate to match the traffic speed. This is done by limiting the processing frame rate in the LPR camera configuration.

  • Set as small a recognition area as possible. And ensure that static high contrast areas (especially containing text) are not included in the recognition area. Use the Complex area option (see "Recognition Area tab" on page 15).

  • Minimize the difference between minimum and maximum character height. Use the Character Height tab.

  • Play around with the processing options and closely follow the effect of what you are doing on the CPU load of the relevant LPR machines. Use the Processing tab.

Kind Regards


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 1 user thanked Christian for this useful post.
Michael on 4/8/2014(UTC)
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