Image analysis – Applications – Vehicle or traffic control
Reexamination Certificate
2000-11-28
2004-06-22
Johns, Andrew W. (Department: 2621)
Image analysis
Applications
Vehicle or traffic control
Reexamination Certificate
active
06754369
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to license plate reading apparatus and method for recognizing a license plate by an image of a vehicle photographed on a road. More particularly, the invention relates to license plate reading apparatus and method for automatically detecting a running vehicle from information of an image photographed by one television camera and reading characters on the license plate at a high precision.
2. Description of the Related Arts
In recent years, a system for recognizing a license plate of a vehicle running a road has socially played an important role. Therefore, there has been developed a system in which a TV camera is put on a road and a license plate of a vehicle is recognized from a photographed image. A license plate reading apparatus has to satisfy two requirements such that, first, all running vehicles can be detected without missing a detection, and second, the license plate of the detected vehicle is correctly read. As a conventional technique for this purpose, for example, there is an apparatus disclosed in JP-B-2893814. In such an apparatus, a head of a running vehicle is detected by extracting an edge of an image. A license plate is read by a method of binarizing the image by a background difference.
However, the method of detecting the vehicle head of the running vehicle by extracting the edge of the image has a drawback such that the vehicle head is easily erroneously discriminated as a shadow of the vehicle, a reflection of headlights, or the like. Also a method of reading a license plate by binarizing an image by a background difference has a problem such that it is difficult to avoid an adverse influence on conditions of external illuminance and a reading precision is low. In order to read the license plate of the vehicle at a high precision, it is necessary to raise a resolution of characters. However, to keep the high resolution, it is necessary to use a plurality of cameras or use a camera of a high resolution, so that the costs become very high. Therefore, in case of using an ordinary TV camera, a passing position such as a gate or the like of the vehicle is extremely limited and the system is limited to an application in which the position of the license plate has been fixed on the image. Therefore, it is difficult to read the license plate of the vehicle which is running a road at a high precision by using the ordinary TV camera.
SUMMARY OF THE INVENTION
According to the invention, there are provided license plate reading apparatus and method for certainly detecting a running vehicle from an image of a relatively low resolution photographed by one camera and reading a license plate from the detected image at a high precision.
A license plate reading apparatus of the invention comprises: an image pickup unit (TV camera) which is set above a road and photographs a running vehicle at a predetermined frame period; an image storing unit for successively storing two images of a present frame image and a previous frame image which were photographed by the image pickup unit; a vehicle detecting unit for detecting a motion of a license plate in a head portion of the running vehicle on the basis of a difference between the present frame image and the previous frame image which were stored in the image storing unit; a dictionary in which character images of all characters used for the license plate have been registered; and a character recognizing unit for recognizing all of the characters of the license plate from an image in a license plate area detected by the vehicle detecting unit by correlating with the registered character images in the dictionary. Therefore, even for a video image photographed by one camera by a sight of about two times as wide as a width of an ordinary vehicle, the license plate can be recognized at a high precision.
The vehicle detecting unit comprises: a block dividing unit for dividing each of the present frame image and the previous frame image into blocks of a predetermined pixel size; a vector calculating unit for obtaining a pixel position, as a block moving position, where a sum of differences between each pixel in the block of the previous frame and all pixels in the blocks of the present frame is equal to a minimum value bg, thereby calculating a motion vector of the block; and a vehicle head detecting unit for scanning all of the blocks, searching a group of blocks in which sizes and directions of the motion vectors are almost the same in an area of a rectangular size corresponding to the license plate, detecting the vehicle on the basis of the presence or absence of the group of blocks which satisfy such conditions, and at the same time, detecting a license plate area of the vehicle head. Since the vehicle detection is performed by paying attention only to the motion of the license plate in the image of the running vehicle as mentioned above, the vehicle can be accurately detected without being influenced by an image other than the license plate.
Further, in order to improve the detecting precision of the vehicle, the vehicle detecting unit can also have: a block dividing unit for dividing each of the present frame image and the previous frame image into blocks of a predetermined pixel size; a luminance level threshold value setting unit for always measuring a luminance of a predetermined area on the road in the frame image, discriminating statistical external illuminance state by obtaining an average value of a predetermined time, and obtaining and setting a luminance level threshold value (bg_th) which monotonously increases in accordance with the illuminance state by a predetermined simple increasing curve; a vector calculating unit for obtaining a pixel position, as a block moving position, where a sum of a difference between each pixel in the block of the previous frame and all pixels in the block of the present frame is equal to the minimum value (bg), thereby calculating a motion vector of the block; a reliability amount calculating unit for calculating a motion vector reliability amount (bgm) by subtracting a sum (bg0) of a difference between each pixel in the block of the present frame and all pixels in the block of the previous frame from the minimum difference value (bg) calculated by the vector calculating unit; and a vehicle head detecting unit for scanning all blocks, searching a group of blocks in which sizes and directions of the motion vectors are almost the same and the reliability amount (bgm) is equal to or larger than the luminance level threshold value (bg_th) in an area of a rectangular size corresponding to a license plate, detecting a vehicle on the basis of the presence or absence of the group of blocks which satisfy those conditions, and at the same time, detecting a license plate area at the head of the vehicle. As mentioned above, by always measuring the luminance level of a part of the image, setting the difference of a correlation result between the image and the position where the motion is equal to zero to the reliability amount of the vector, and discriminating the motion from a shadow and noises of the vehicle, the vehicle and its license plate area can be detected accurately from the image.
The character recognizing unit comprises: a low resolution recognizing unit for recognizing a character at a position of the character having the largest size on the license plate with respect to the frame image of a low resolution obtained by thinning out the original image as a target; and a high resolution recognizing unit for returning the resolution of the frame image to the original resolution, recognizing the character at the character position recognized by the low resolution recognizing unit, and recognizing a character at a position of another small character on the license plate in which a relative position has previously been known on the basis of a character recognition result. As mentioned above, by performing the recognition of two stages such that the character recognition of the license plate is performed with respect to
Fujitsu Limited
Johns Andrew W.
Nakhjavan Shervin
Staas & Halsey , LLP
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