JPH07114689A - Method for recognizing vehicle registered number - Google Patents

Abstract

PURPOSE:To provide the method for fast vehicle registered number recognition although geometrical conversion for obtaining a front facing image by correcting geometrical distortion of an image of a license plate is included by solving the problem points that it is difficult to detect the edge of the license plate when the license plate and a vehicle body are in colors of the same system as a result of the geometrical conversion and processes are complicated and require a long time. CONSTITUTION:The shape when the circumscribed quadrilateral of the character part of a license plate is viewed from the front is stored as a standard quadrilateral (S0), an image of a vehicle travel path is picked up by a video camera to input an image including the front or rear scene of the traveling vehicle (S2) corresponding to vehicle detection (S1), and the characters of the license plate are cut (S3), the circumscribed quadrilateral of the cut character part of the license plate part is found (S4), and a coordinate conversion parameter set is so determined (S5) so that the circumscribed quadrilateral is similar to the standard quadrilateral. Coordinate conversion is performed by using this coordinate conversion parameter set to obtain a front facing image of the license plate part (S6), and the respective characters are recognized (S7) from the front facing image.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a vehicle registration number recognition method, and more particularly to a vehicle registration number recognition method including a geometric transformation process for correcting geometric distortion of an input image.

[0002]

2. Description of the Related Art A device for recognizing a vehicle registration number by image-processing a video signal from a video camera is used. For example, a vehicle registration number recognition device for an entering vehicle as shown in FIG. 5 is used for automation even at a gate of a parking lot.

In FIG. 5, reference numeral 10 denotes a vehicle registration number recognition device installed diagonally forward of the vehicle travel path 5 at the parking lot gate. The vehicle registration number recognition device 10 includes a video camera 1, an image processing unit 11 that performs image processing on an output video signal of the video camera 1 to recognize a vehicle registration number, an illuminator 3, an illumination controller 4, and a vehicle head sensor. The vehicle head sensor is used for the projector 2 installed on one side of the vehicle road 5.
A, the projector 2A and the light receiver 2B that are installed so as to face each other with the vehicle traveling path 5 in between.

The vehicle head sensor (light receiving side) 2B is an image processing unit.
It is connected to 11 and detects that the passing vehicle 7 blocks the irradiation light from the floodlight 2A, and captures the foreground including the license plate 6 of the passing vehicle. At this time, the illuminator 3 and the illumination controller 4 emit strobe light in synchronization with the output of the on-vehicle sensor light receiver 2B to obtain a clear image indoors, outdoors and day and night.

In this type of image processing apparatus, generally, a video signal including a vehicle obtained by an image input unit is stored in an image storage unit as a still image, and then a license plate extraction unit extracts a license plate corresponding portion to extract characters. The cutout portion extracts the character equivalent portions, and the character recognition portion recognizes the characters corresponding to these character equivalent portions.

Various devices for extracting such a license plate have been proposed, for example, the applicant of the present invention, Japanese Patent Application No. 4-1377873 (image processing device). A license plate extraction method has also been proposed, for example, by the present applicant as Japanese Patent Application Laid-Open No. 5-143888 (Japanese Patent Application No. 3-332552).
Regarding the method of recognizing the vehicle registration number, for example, Japanese Patent Application No. 4-137874 (a method of recognizing numbers etc.) by the applicant
I have a suggestion.

In such a vehicle registration number recognizing device, the installation position of the image input unit (video camera) is generally restricted. For example, in the case of FIG. 5, it is installed on the roadside, and the image is obtained obliquely from the front. The processed image is the target of image processing. An example of an image of the license plate thus obtained is shown in FIG.

By the way, in applications such as recognition of a vehicle registration number, the binary pattern of the character equivalent portion of the license plate is compared with a character pattern (dictionary) stored in the apparatus to determine which character each. It has become. Therefore, as shown in the image example of FIG. 4A, a character having geometric distortion may not be recognized or may be erroneously recognized because each character portion does not match the character pattern dictionary.

In order to deal with this, arithmetic processing is performed based on an image in which geometric distortion has occurred, and after the geometrical conversion to a front-facing image (an image corresponding to the case of being imaged from the front: see FIG. 4B), Image processing to improve recognition accuracy. Pseudo-affine transformation is well known as the above-mentioned method of geometric transformation. For example, as shown in FIG. 2, each coordinate transformation parameter (a, b, c, d, e,
f, g, h) is obtained, and the geometric transformation of each coordinate is performed using this coordinate transformation parameter set (a, b, c, d, e, f, g, h). By the conversion, for example, the coordinates (X, Y) are obtained corresponding to the coordinates (x, y).

The following method is known as a method of geometrically converting four points. (1) Hough transform is applied to the edge detected vehicle image, the boundary line of the license plate is detected from the extracted straight line group,
The vertex coordinates of the four corners of the license plate are obtained, and an image of the license plate viewed from the front is created by coordinate conversion.
References regarding this method include, for example, “Cui, Yasuiin, Nakajima;“ License plate area extraction method using adaptive parameter plane area limited Hough transform ”: IEICE Transactions '89 / 4 Vol. J72-D-II N
o. 4, pp597-604 ”.

(2) Using the character arrangement rule of the license plate, after detecting the lower right number that is always present in the license plate, a peripheral distribution in the vertical direction is created to obtain the inclination of the license plate character, Estimate the vertex coordinates of the four corners of the plate and create an image of the license plate viewed from the front by coordinate conversion. References regarding this method include, for example, “Kawashima, Tsuji, Suzuki;“ Vehicle number recognition of gate-passing vehicles ”: Traffic Engineering Vol. 23 No. 4, pp15-
24 1988 ”.

The above-mentioned conventional methods have the following problems. In the method (1), it is difficult to detect the edge when the color of the license plate and the color of the vehicle body are similar colors. In addition, it takes processing time. Method (2) is
The process is complex and time consuming.

[0013]

SUMMARY OF THE INVENTION The present invention has been made in view of such circumstances, and it is difficult to detect an edge when the license plate and the vehicle body have the same color, which is a conventional geometric conversion method. Alternatively, a method for recognizing a vehicle registration number with high accuracy by adopting a geometric conversion method that solves the problem of complicated and time-consuming processing and obtaining a facing image in which the geometric distortion of the license plate image is corrected is obtained. The purpose is to propose.

[0014]

In order to achieve the above object, in the vehicle registration number recognition method of the present invention, the shape of the circumscribed quadrilateral of the character portion of the license plate as viewed from the front is stored as a standard quadrilateral. Every (S0), image the vehicle traveling path with a video camera, capture the image including the foreground or the foreground of the traveling vehicle in response to vehicle detection (S1) (S2), and cut out the characters on the license plate (S3) , The circumscribed quadrilateral of the cut out license plate character part is obtained (S4), the coordinate transformation parameter set is determined so that this circumscribed quadrilateral becomes similar to the standard quadrilateral (S5), and this coordinate transformation is performed. Coordinate conversion is performed using the parameter set to obtain a facing image of the license plate portion (S6), and each character is recognized from the facing image (S6).
7).

[0015]

[Function] An image including a license plate is taken in, and first, the character portion of the license plate portion is cut out to obtain a circumscribed quadrangle of the character portion. It is only necessary to know the character part accurately, and it is possible to easily cut out the character even if the ground color of the license plate and the vehicle body color are of the same system. Next, a quadrangle circumscribing the character portion is obtained. When the shape of the circumscribed quadrilateral determined in this way is used for the predetermined coordinate conversion processing, the shape of the circumscribed quadrilateral of the known character part when viewed from the front (standard quadrilateral)
Then, a coordinate conversion parameter set that is at least similar to the above is determined, and coordinate conversion is performed using this coordinate conversion parameter set to obtain a facing image. Both processes are much simpler than the conventional coordinate conversion process, and the processing time can be shortened. Each character is recognized from the obtained facing image of the license plate.

[0016]

EXAMPLES The present invention will be described below based on examples.
FIG. 1 is a flowchart showing an embodiment of a vehicle registration number recognition method of the present invention. The vehicle registration number recognition device to which the method of the embodiment is applied may be the same as that shown in FIG. 5, and only the control process of each part is a control process suitable for the present invention.

The method of the present invention makes use of the fact that each character of the license plate is arranged based on a fixed character arrangement rule. For this purpose, the shape of a quadrangle (standard quadrangle Qs: see FIG. 3B) circumscribing the character portion when the license plate is viewed from the front, such as the angle formed by each side, is stored in advance ( Figure 1: S0).

During the actual vehicle registration number recognition operation, a light beam is constantly emitted from the light projector 2A toward the light receiver 2B (see FIG. 5). The light receiver 2B sends out a signal different from that at the time of light reception (normal state) in response to the light from the light projector 2A being blocked by the entering vehicle. This change (rise or fall) is recognized by the image processing unit 11 as a vehicle head detection signal, and it can be seen that the vehicle 7 has entered a fixed position (vehicle detection: S1).

The image processing unit 11 correspondingly illuminates by the illuminator 3 in the near infrared region, and synchronously captures the foreground of the vehicle as a still image including the license plate 6 from the video signal (S2). Next, the license plate area in the still image is specified and the character portion is cut out (S3). Actually, it is sufficient to cut out the characters located at the four corners of the character area. Next, a quadrangle Qt (an example is shown in FIG. 3A) circumscribing the cut out character portion (each character at the four corners) is determined (S4).

The coordinate transformation parameter set (a, b, so that the circumscribed quadrangle Qt obtained in the above process becomes at least similar to the above-mentioned standard quadrangle Qs by the pseudo affine transformation.
c, d, e, f, g, h) is calculated (S5).

Then, using this coordinate conversion parameter set (a, b, c, d, e, f, g, h), coordinate conversion is performed for each point of the character portion by calculation of pseudo affine transformation, and the license plate character area is obtained. The front facing image P'of is obtained (S6).

Each character in the character area is recognized from the obtained facing image P '(S7). The vehicle registration number may be recognized by a method according to the above-mentioned Japanese Patent Application No. 4-137874 (a method for recognizing numbers etc.).

According to the method of the present invention, the coordinate conversion parameter set is required as long as the license plate characters can be cut out. Therefore, even if the license plate and the vehicle body are of the same color and the edge of the license plate frame is difficult to find, the coordinate conversion parameter set can be easily set. Can be obtained, and the processing time therefor can be shortened.

In addition to the pseudo affine transformation used in the embodiment, the present invention can be applied to a vehicle registration number recognition method using the same kind of geometric transformation using parameters, and when used for each geometric transformation, It suffices to obtain an appropriate coordinate transformation parameter set such that the circumscribed quadrilateral of the cut out text portion is transformed into a standard quadrilateral, and use this to perform a predetermined geometric transformation on each portion.

As a result, the operation is stable regardless of the background color of the license plate, and the process of determining the coordinate conversion parameter set for each event is simpler than that of the conventional one. The vehicle registration number can be recognized at sufficiently high speed.

[0026]

As described in detail above, the vehicle registration number recognition method of the present invention stores the shape of the circumscribed quadrilateral of the character portion of the license plate as a standard quadrilateral when viewed from the front (S0), Image the vehicle running path with a video camera, capture the image including the foreground or background of the running vehicle in response to vehicle detection (S1) (S2), and cut out the characters on the license plate (S2).
3), find the circumscribed quadrilateral of the cut out license plate character part (S4), and determine the coordinate conversion parameter set such that this circumscribed quadrilateral is similar to the standard quadrilateral.
(S5), coordinate conversion is performed using this coordinate conversion parameter set to obtain a facing image of the license plate portion (S6), and each character is recognized from the facing image (S7). Since it is configured, the vehicle registration number recognition is significantly faster than the conventional method even though it includes the geometric conversion processing for correcting the geometric distortion of the image of the license plate portion to obtain the front facing image. . There is also an effect that the license plate portion can be appropriately extracted and the vehicle registration number can be surely recognized even when the license plate and the vehicle body whose edges are difficult to detect have the same color.

[Brief description of drawings]

FIG. 1 is a flowchart showing an embodiment of a vehicle registration number recognition method of the present invention.

FIG. 2 is a diagram illustrating an example of geometric transformation according to the present invention.

FIG. 3 is a diagram illustrating an example of geometric transformation according to the embodiment of the present invention.

FIG. 4 is an example of an image including a license plate portion according to the present invention.

FIG. 5 is a plan view illustrating an arrangement of a vehicle registration number recognition device according to the present invention.

[Explanation of symbols]

 5 ... Vehicle running path, 6 ... License plate, 1 ... Video camera, 11 ... Image processing unit.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical display location G08G 1/04 D 7531-3H H04N 7/18 K

Claims (1)

[Claims] 1. The shape of the circumscribed quadrilateral of the character portion of the license plate when viewed from the front is stored as a standard quadrilateral (S0), and the vehicle running path is imaged by a video camera to detect the vehicle (S1). Corresponding to, the image including the foreground or background of the traveling vehicle is captured (S2), the characters on the license plate are cut out (S3), and the circumscribed quadrilateral of the cut out license plate characters is obtained (S4). A circumscribed quadrangle determines a coordinate transformation parameter set such that it has a shape similar to the standard quadrilateral (S5), and performs coordinate transformation using this coordinate transformation parameter set to obtain a front facing image of the license plate ( A vehicle registration number recognition method, characterized by including the steps of S6) and recognizing each character from the facing image (S7) in the order described above.