The ALPR matcher uses the "Number of character differences" technique to improve the plate read accuracy rate.
This technique allows for a difference in the number of characters between the plate read and the plate number in the hotlist. This allows the system to account for characters in the plate that cannot be read (dirt, bad camera angle, and so on), and for objects on the plate that might be mistaken for legitimate characters (screws, pictures, and so on).
You can configure how the ALPR matcher handles the "Number of character differences" by modifying the MatcherSettings.xml file. For more information, see MatcherSettings.xml file.
- OCR equivalence (see ALPR matcher technique: OCR equivalence)
- Common and contiguous characters (see ALPR matcher technique: Common and contiguous characters)
- There’s no exact match possible for plate reads AB123 and ABC0123 because the hotlist contains only six-character plates.
- Both plate reads AB123 and ABC0123 match the plate ABC123 because you allowed for one character difference. It doesn’t matter if it is one character more, or one character less than the matched plate.
- Both plate reads AB123 and ABC0123 match the plates A8C123, ABCI23, and ABC1Z3 because you allowed for one character difference, and one OCR equivalent character.
If you were using a permit list instead of a hotlist, none of the matched plates in the example would raise hits (you get a permit hit when a plate is not on the permit list).