1、According to the guideline for vehicle inspection 9, all license plates must be rectangular and have the dimensions and have all 7 characters written in a single line. Under practical environments, the distance from the camera to vehicles and the inclined angles are constant, so all characters of th
2、e license plate have a fixed width, and the distance between the medium axes of two adjoining characters is fixed and the ratio between width and height is nearly constant. Those features can be used to locate the plate and segment the individual character.B. Color collocation of the plateThere are
3、four kinds of color collocation for the Chinese vehicle license plate .These color collocations are shown in table I. TABLE ICategory of license plateColor collocationsmall horse power plateblue background and white charactersmotor truck plateyellow background and black charactersmilitary vehicle an
4、d police wagon plateblack background and the white charactersembassy vehicle platewhite background and black charactersMoreover, military vehicle and police wagon plates contain a red character which belongs to a specific character set. This feature can be used to improve the recognition rate.C. Lay
5、out of the Chinese VLPSThe criterion of the vehicle license plate defines the characters layout of Chinese license plate. All standard license plates contain Chinese characters, numbers and letters which are shown in Fig.1. The first one is a Chinese character which is an abbreviation of Chinese pro
6、vinces. The second one is a letter ranging from A to Z except the letter I. The third and fourth ones are letters or numbers. The fifth to seventh ones are numbers ranging from 0 to 9 only. However the first or the seventh ones may be red characters in special plates （as shown in Fig.1）. After segme
7、ntation process the individual character is extracted. Taking advantage of the layout and color collocation prior knowledge, the individual character will enter one of the classes: abbreviations of Chinese provinces set, letters set, letters or numbers set, number set, special characters set.（a）Typi
8、cal layout（b） Special characterFig.1 The layout of the Chinese license plateIII. THE PROPOSED ALGORITHMThis algorithm consists of four modules: VLP location, character segmentation, character classification and character recognition. The main steps of the flowchart of LPR system are shown in Fig. 2.
9、 Firstly the license plate is located in an input image and characters are segmented. Then every individual character image enters the classifier to decide which class it belongs to, and finally the BP network decides which character the character image represents.Image acquisitionPlate locationChar
10、acters segmentation segmentationclassifierChinese characterLetterLetter or numberNumberSpecial characterCharacters recognitionFig.2 The flowchart of LPR systemA. Preprocessing the license plate1） VLP LocationThis process sufficiently utilizes the color feature such as color collocation, color center
11、s and distribution in the plate region, which are described in section II. These color features can be used to eliminate the disturbance of the fake plates regions. The flowchart of the plate location is shown in Fig. 3.Characters edge detectionBinary image segmentingCandidate image detectionVehicle
12、 plate extractionFig.3 The flowchart of the plate location algorithmThe regions which structure and texture similar to the vehicle plate are extracted. The process is described as followed: （1） （2）Here, the Gaussian variance is set to be less than W/3 （W is the character stroke width）, so gets its m
13、aximum value M at the center of the stroke. After convolution, binarization is performed according to a threshold which equals T * M （T MaxFFor each character segmentsCalculate the medium point For each two consecutive medium pointsCalculate the distance Calculate the minimum distanceMerge the chara
14、cter segment k and the character segment k +1NF = NF - 1End of algorithm where Nf is the number of character segments, MaxF is the number of the license plate, and i is the index of each character segment.The medium point of each segmented character is determined by: （3）where is the initial coordina
15、tes for the character segment, and is the final coordinate for the character segment. The distance between two consecutive medium points is calculated by: （4）Fig.6 The segmentation resultsB. Using specific prior knowledge for recognition The layout of the Chinese VLP is an important feature （as desc
16、ribed in the section II）, which can be used to construct a classifier for recognizing. The recognizing procedure adopted conjugate gradient descent fast learning method, which is an improved learning method of BP neural network10. Conjugate gradient descent, which employs a series of line searches i
17、n weight or parameter space. One picks the first descent direction and moves along that direction until the minimum in error is reached. The second descent direction is then computed: this direction the “conjugate direction” is the one along which the gradient does not change its direction will not
18、“spoil” the contribution from the previous descent iterations. This algorithm adopted topology 625-35-N as shown in Fig. 7. The size of input value is 625 （25*25 ） and initial weights are with random values, desired output values have the same feature with the input values.Fig. 7 The network topologyAs Fig. 7 shows, there is a three-l
