Algorithm:
There are seven primary algorithms that the software requires for identifying a license plate:
- Plate localization : responsible for finding and isolating the plate on the picture.
- Plate orientation and sizing : compensates for the skew of the plate and adjusts the dimensions to the required size.
- Normalization : adjusts the brightness and contrast of the image.
- Character segmentation : finds the individual characters on the plates.
- Optical character recognition.
- Syntactical/Geometrical analysis : check characters and positions against country-specific rules.
- The averaging of the recognised value over multiple fields/images to produce a more reliable or confident result. Especially since any single image may contain a reflected light flare, be partially obscured or other temporary effect.
Note : The complexity of each of these subsections of the program determines the accuracy of the system. During the third phase (normalization), some systems use edge detection (like canny) techniques to increase the picture difference between the letters and the plate backing.
Tips to improve:
- A camera that makes use of active infrared imaging (with a normal colour filter over the lens and an infrared illuminator next to it) benefits greatly.
- A Median Filter may also be used to reduce the visual noise on the image.
Dificulties:
- Poor file resolution.
- Blurry images, particularly motion blur.
- Poor lighting and low contrast due to overexposure, reflection or shadows.
- An object obscuring (part of) the plate, quite often a tow bar, or dirt infront of the camera.
- A different font, popular for vanity plates (some countries do not allow such plates, eliminating the problem).
- Circumvention techniques (Round or mixed characters fonts,using a plate cover or a spray).
- Lack of coordination between countries or states. Two cars from different countries or states can have the same number but different design of the plate.
Still having perfect situation, there could be misreading due to
- Number plate partly out of field of view
- Fixings (inappropriately located,damaged or rusting)
- Mud
- Cracks
- Illegal font (or handwritten)
- Illegal character spacing
- Illegal background
- Illegal plate layout
- Different reads in visible and InfraRed images
- Character appearance alterations (black tape, IR-absorbing tape, ink etc.)
Customers experience only 90% to 94% success, even with new equipment under perfect conditions. Early systems were reportedly only 60% to 80% reliable.
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