In most applications, the actual size of the recorded vehicle registration plate is not fixed. This is caused by variation of the distance between the license plate and the camera when an image is captured.

For some application domains, like parking, these variations are minimal. Other domains like law enforcement and especially mobile handheld applications exhibit a large spread in possible sizes of the characters on the license plate.

Intrada ALPR is capable of dealing with these variations and can recognize characters of any height. However, the user can inform the recognition process about the possible character sizes on valid license plates. Specifying the appropriate character size range will increase the recognition speed and at the same time deliver a larger amount of correctly read license plates.

Image set variation

French INTRADA HandHeld images: large character size interval: 20 - 100 pixels

Norwegian INTRADA ALPR roadside images: strict character size interval: 14 - 23 pixels

Norwegian Example

The analysis of a large sample set of Norwegian imagery showed a character height distribution as shown in the following graph.

The effects of three different character height ranges on the recognition performance of Intrada ALPR for this Norwegian data set is shown in the following graph. The error-correct performance curves are based on grayscale images with a size of 784x584 pixels.

Character height:
(red) 13-19 px - Too small: Low recognition rate, but fast: 0.28 sec per recognition
(blue) 10-70 px - Too large: High recognition rate, but slow: 0.44 sec
(green) 13-24 px - Correct: High and fast recognition: 0.31 sec

Specifying a character range that is too large will not result in additional correctly read license plates, but will consume more processing time, as more image locations are analyzed as possible license plates. A character range that is too small will cause some of the clearly readable license plates to be rejected as their character height will be well outside the specified range.

INTRADA ALPR contains tooling that helps the find the appropriate character range for the application at hand. Furthermore the actual size in pixels of the characters on a recognized vehicle registration plate is available as output of the recognition process and may be used to automatically fine-tune the character range.