AUTOMATIC LICENSE PLATE READING METHOD IMPLEMENTED BY A LICENSE PLATE READING DEVICE

Abstract

The invention relates to a method for authenticating license plates implemented by an authentication device (2) comprising a camera (4) positioned opposite parking spaces (5): identification of the plates and acquisition of an image by the camera (4); transmission of said image to a processing unit (6) of the device (2); detection of a string of characters on each image; parallax correction on each image; reconstruction of the identified string of characters; storage of the strings of characters; the method being characterized in that the parallax correction applied is a function of an angle of incidence varying for the correction of each character. Abstract figure: Figure 1

Description

Title of the invention: METHOD FOR AUTOMATICALLY READING LICENSE PLATES IMPLEMENTED BY A LICENSE PLATE READING DEVICE Technical field

[0001] The present invention relates to the field of vehicle presence detection equipment, associated with a system for counting and guiding users in a parking lot, while taking into account the flow of traffic of vehicles leaving or looking for a parking space in said parking lot.

[0002] The present invention also relates to the field of vehicle authentication devices associated with such detection, counting and guidance equipment. STATE OF THE ART

[0003] Known in the state of the art are systems for counting and authenticating by license plate recognition as described in document US6553131B1. These devices are generally known to those skilled in the art by the acronym "LAPI" for "Automatic Number Plate Recognition System" or "ANPR" for "Automatic Number Plate Recognition System". These devices use an optical character recognition method, known to those skilled in the art by the acronym "OCR". On average, the recognition rate of the characters of the license plates by OCR is greater than 98% in the case where the acquisition devices such as cameras are correctly positioned in order to avoid errors. In a parking lot, these devices are advantageously positioned at crossing points where vehicle authentication is necessary, for example at an entrance to the parking lot.

[0004] It is also known from the state of the art, in particular from document WO2021144617, to use simultaneously an ANPR device, for example positioned at the entrance to a parking lot and an acquisition device such as a camera facing a plurality of parking spaces. Such a device allows the recognition of the plates of vehicles parked on each side of a traffic lane, but has a strong limitation in the number of vehicles that can be authenticated from a single acquisition device. The number of license plates that can be recognized and authenticated is technically limited to the angle of the camera shot due to the parallax error, i.e. the error related to image distortion due to the change in the observation incidence. When the camera identifies one or two license plates, the known OCR methods are functional and present almost no error when authenticating the characters of the license plate(s). It has been observed that beyond two license plates visible from the camera's shooting angle, the angle of incidence of a plate relative to a reference axis of the camera is such that the recognition rate during authentication decreases significantly to be approximately equal to 80%. Such a rate is insufficient to correctly authenticate a vehicle, for example due to confusion between two similar characters, for example the letter of the Latin alphabet "O" and the number "0".It is then necessary to increase the number of cameras when the number of parking spaces increases, to reduce the shooting angles from each parking space, which necessarily represents a significant financial investment.

[0005] The invention aims to resolve all or part of the drawbacks of the state of the art, by proposing a method for automatically reading plates making it possible to reduce the number of acquisition devices necessary to cover all the parking spaces in a car park. PRESENTATION OF THE INVENTION

[0006] More specifically, the subject of the invention is a method for authenticating license plates implemented by a device for authenticating license plates, said device comprising at least one camera arranged on a traffic lane of a parking lot, opposite a plurality of parking spaces, so that license plates of at least two vehicles parked in adjoining parking spaces are visible to the camera, each license plate comprising a first lateral end and a second lateral end, the first end of each plate forming a first angle relative to a reference axis of the camera and the second end of each plate forming a second angle relative to the reference axis of the camera, the first angle being greater than the second angle, the method comprising the following steps: • the identification of each of the license plates by the device's camera and the acquisition of an image of each of the license plates by the camera; • the transmission by the camera of said image of each of the license plates to a processing unit of the license plate authentication device; • the detection on each license plate image of a string of alphanumeric characters of the license plate; • correction of parallax by the device's processing unit on each of the license plate images transmitted; • the reconstruction of the alphanumeric character string identified on each image by optical character recognition; • storage in memory by the device's processing unit of the reconstructed alphanumeric character strings.

[0007] The method is remarkable in that the parallax correction applied to each of the license plate images is a function of an angle of incidence, the angle of incidence being for each license plate image between the first angle and the second angle relative to the reference axis of the camera, the angle of incidence varying for the correction of a parallax of each character detected on each license plate image.

[0008] The term "identification" means the act of identifying the presence of a license plate in the field of view of the device's camera.

[0009] The term "authentication" means the act of constituting a string of alphanumeric characters from an image of a license plate, the reconstructed string corresponding to the characters visible on said image.

[0010] The term "detection" means the act of individually delimiting each alphanumeric character on a license plate image, without authenticating said character by character recognition.

[0011] Thanks to such a combination of characteristics, such a method makes it possible to authenticate a greater number of vehicle registration plates using the same acquisition equipment, in particular cameras. Also, such a method makes it possible to significantly reduce errors due to the parallax effect induced by the angle of incidence of the characters of a registration plate with the camera, in particular by applying a correction specific to each character, which promotes superior quality optical character recognition.

[0012] Advantageously, the method comprises a step of searching for each reconstructed alphanumeric character string in a database of license plates acquired subsequently or previously. In such a configuration, each reconstructed character string authenticating a given vehicle is compared to a character string stored in a license plate database, making it possible, for example, to verify that the reconstructed string is indeed a valid and existing license plate number.

[0013] Advantageously, the database is made up of alphanumeric character strings each associated with a license plate image acquired by a camera of the device at an entrance to the parking lot, and / or images of license plates acquired by a camera of the device at an exit of the parking lot. In such a configuration, the search is carried out on a reduced sample of license plates, limiting the number of possible errors.

[0014] Advantageously, in the case where an alphanumeric character string stored in memory corresponds to an image of license plates from the database acquired by a camera of the device at the exit of the parking lot, said character string is erased from the memory storage. In such a configuration, the device only stores in memory the character strings that can correspond to license plates of vehicles that have entered and are circulating in the parking lot.

[0015] Advantageously, the parallax correction applied comprises an application of the Hough transform followed by a rotation of the image of each plate.

[0016] Advantageously, the parallax correction step is carried out by an artificial intelligence of the processing unit, based on deep neural networks operating by learning.

[0017] According to another aspect of the invention, it relates to a device for authenticating license plates of a parking lot configured to implement the method described above, comprising: • at least one camera arranged on a traffic lane of the parking lot, opposite a plurality of parking spaces, so that the license plates of at least two vehicles parked in adjoining parking spaces are visible to the camera; • a processing unit connected to the camera and comprising a storage memory, the processing unit being configured to process images acquired by the camera; • the authentication device being configured to correct a parallax induced by an angle of incidence between a license plate and a reference axis of the camera.

[0018] Advantageously, the license plate authentication device comprises at least two cameras arranged on a traffic lane of the parking lot, each camera facing a plurality of parking spaces on a given side of the traffic lane. In such a configuration, the cameras make it possible to acquire images of license plates located on either side of the traffic lane.

[0019] Advantageously, the license plate authentication device comprises a camera positioned at an entrance to the parking lot and / or a camera positioned at an exit from the parking lot.

[0020] Advantageously, the processing unit comprises artificial intelligence based on deep neural networks operating by learning. PRESENTATION OF THE FIGURES

[0021] The invention will be better understood on reading the following description, given solely by way of example, and referring to the appended drawings given by way of non-limiting examples, in which identical references are given to similar objects and in which:

[0022] [Fig.l] is a schematic representation of a parking lot comprising a plurality of parking spaces and equipped with a license plate authentication device according to a first embodiment of the invention;

[0023] [Fig.2] is a schematic representation of a plurality of parking spaces facing a camera of the authentication device;

[0024] [Fig.3] is a schematic representation of a detection of a license plate by the camera of the authentication device;

[0025] [Fig.4] is a schematic representation of license plate images according to a plurality of incidences relative to a reference axis of the camera of the device;

[0026] [Fig.5] is a schematic representation of a parallax correction and an optical character recognition according to a step of an authentication method according to another aspect of the invention of a parallax correction step.

[0027] It should be noted that the figures set out the invention in detail to enable the invention to be implemented; although not limiting, said figures serve in particular to better define the invention where appropriate. DETAILED DESCRIPTION OF THE INVENTION

[0028] [Fig. 1] represents a parking lot 1 comprising a plurality of parking spaces 5. The parking spaces 5 are each configured to accommodate a vehicle 3 of a user wishing to park his vehicle 3. Each parking space 5 is adjacent to at least one other parking space 5, so that the parked vehicles 3 extend longitudinally and parallel to each other. Advantageously, pluralities of parking spaces 5 extend on either side of a traffic lane so as to form rows of parking spaces 5.

[0029] According to a first aspect of the invention, the parking lot 1 is equipped with an authentication device 2 configured to authenticate license plates 7 of the vehicles 3 in the parking lot 1. The authentication device 2 comprises at least one camera 4 arranged on the traffic lane of the parking lot 1, opposite one of the pluralities of parking spaces 5. The camera 4 is positioned so that the license plates 7 of at least two vehicles 3 parked in adjacent parking spaces 5 are visible to the camera 4. In other words, at least two license plates 7 are included in a viewing angle of the camera 4, in a horizontal plane parallel to the ground of the parking lot 1 as illustrated in [Fig.2]. Said camera 4 has a reference axis A, preferably defined as the bisector of the viewing angle of the camera 4.

[0030] The authentication device 2 further comprises at least one processing unit 6. The processing unit 6 is configured to receive and process images 11 acquired by the camera 4. In the case where the device 2 comprises several cameras 4, each camera 4 is connected to the processing unit 6. The processing unit 6 may comprise sub-units, each associated with a camera 4 and configured to carry out local processing of the images 11 captured by the associated camera 4. In the embodiment described in [Fig.l], the cameras 4 are opposite the parking spaces 5 located on either side of the traffic lane of the parking lot 1.

[0031] The remainder of the description focuses on describing a method for authenticating license plates 7 implemented by the authentication device 2 according to another aspect of the invention. The term "authentication" means the act of constituting a string of alphanumeric characters 13 from an image 11 of a license plate 7, the reconstructed string corresponding to the characters visible on said image 11. Each license plate 7 comprises a first lateral end 9a and a second lateral end 9b. The first end 9a forms a first angle relative to the reference axis A of the camera 4. The second end 9b forms a second angle relative to the reference axis A of the camera 4, the first angle being greater than the second angle, as illustrated in [Fig. 3].

[0032] The method comprises a step of identifying each of the license plates 7 by the camera 4 of the device 2 and acquiring an image 11 of each of the license plates 7 by the camera 4. The term “identification” means the fact of identifying the presence of a license plate 7 in the shooting field of the camera 4 of the device 1. Naturally, the image 11 can correspond to both a snapshot and a video.

[0033] The method comprises a step of transmission by the camera 4 of the image 11 of each of the license plates 7 to the processing unit 6 of the device 2. The transmission can for example be carried out by a cable connection in the case where the processing unit 6 is in situ, but can alternatively be carried out by wireless communication in the case where the processing unit 6 is located remotely, in particular by means of sub-units as described above.

[0034] The method comprises a detection step on each image 11 of a chain of alphanumeric characters 13 of the license plate 7. The term "detection" means the act of individually delimiting on an image 11 of the license plate 7 each alphanumeric character 13, without authenticating said character. In other words, the camera 4 transmits information on the presence of a license plate 7 on the image 11, which makes it possible, for example, to indicate, for example by means of a light, the presence of a vehicle 3 on the associated space 5. The detection may in particular comprise a calculation of the Haar characteristics, advantageously using the creation of integral images.

[0035] The method comprises a step of correcting a parallax by the processing unit 6 on each of the transmitted images 11. Naturally, the image 11 may require parallax correction, in particular in the case where the plate 7 is aligned with the reference axis A of the camera 4, as illustrated in [Fig. 3]. The correction applied to each image 11 is a function of an angle of incidence 0 between the first angle and the second angle relative to the reference axis A of the camera 4, as illustrated in [Fig. 4]. The angle of incidence 0 during the correction of a parallax of each character 13 detected on each image 11. Such a method makes it possible to authenticate a greater number of registration plates 7 of vehicles 3 using the same number of cameras. Advantageously, the parallax correction applied comprises an application of the Hough transform followed by a rotation of the image 11 of each plate 7.The rotation of the image is then based on angles obtained by the Hough transform and includes in particular the calculation of a 2D affine rotation matrix making it possible to carry out an affine transformation of an image 11 and the application of the affine transformation to said image 11.

[0036] In particular, the method comprises a step of reconstructing the string of alphanumeric characters identified on each image 11 by optical character recognition. Thanks to the correction step previously carried out, such a method makes it possible to significantly reduce the errors due to the parallax effect induced by the angle of incidence 0 on the characters 13, in particular by applying a correction specific to each character 13, which promotes higher quality optical character recognition, as illustrated in [Fig. 5]. Pre-processing of the image 11 can be carried out before the optical character recognition, the pre-processing comprising in particular median filtering followed by a Canny filter.

[0037] The method notably comprises a step of storing in memory by the processing unit 6 the reconstructed alphanumeric character strings 13.

[0038] Advantageously, the method comprises a step of searching for each alphanumeric character string 13 reconstituted in a database of license plates 7. In such a configuration, each character string 13 re constituted and authenticating a given vehicle 3 is compared to a character string 13 stored in a license plate database, making it possible, for example, to verify that the reconstructed string 13 is indeed a valid and existing license plate number 7.

[0039] The authentication device 2 may advantageously comprise a camera (not shown) located at an entrance 15 and / or a camera (not shown) at an exit 17 of the parking lot 1, connected to processing units 6 and making it possible to reconstruct alphanumeric character strings 13 corresponding to plates 7 of vehicles 3 entering and / or leaving the parking lot 1. In such a configuration, the search is carried out on a reduced sample of registration plates 7, considerably increasing the recognition accuracy. The images 11 acquired by the cameras 4 at the entrance 15 and / or at the exit 17 of the parking lot 1 have satisfactory reliability, the registration plates 7 of the vehicles 3 at these crossing points being aligned with the reference axis A of the cameras 4 during the acquisition of images 11.

[0040] Advantageously, the device 2 and more particularly the processing unit 6 comprises an artificial intelligence algorithm based on deep neural networks operating by learning. Preferably, the artificial intelligence algorithm is configured to implement the parallax correction step, making it possible to strengthen the reliability of the correction over time.

[0041] It will also be noted that the invention is not limited to the embodiments described above. It will indeed appear to those skilled in the art that various modifications can be made to the embodiment described above, in light of the teaching which has just been disclosed to them.

[0042] In the detailed presentation of the invention which is made above, the terms used must not be interpreted as limiting the invention to the embodiment set out in the present description, but must be interpreted to include all equivalents whose prediction is within the reach of a person skilled in the art by applying his general knowledge to the implementation of the teaching which has just been disclosed to him.

Claims

Claims

1. Method for authenticating license plates (7) implemented by a device (2) for authenticating license plates (7), said device (2) comprising at least one camera (4) arranged on a traffic lane of a parking lot (2), opposite a plurality of parking spaces (5), so that license plates (7) of at least two vehicles (3) parked in adjacent parking spaces (5) are visible by the camera (4), each license plate (7) comprising a first lateral end (9a) and a second lateral end (9b), the first end (9a) of each plate (7) forming a first angle relative to a reference axis (A) of the camera (4) and the second end (9b) of each plate (7) forming a second angle relative to the reference axis (A) of the camera (4), the first angle being greater than the second angle, the method comprising the following steps: • the identification of each of the license plates (7) by the camera (4) of the device (2) and the acquisition of an image (11) of each of the license plates (7) by the camera (4); • the transmission by the camera (4) of said image (11) of each of the license plates (7) to a processing unit (6) of the device (2) for authenticating the license plate (7); • the detection on each image (11) of the registration plate (7) of a string of alphanumeric characters (13) of the registration plate (7); • correction of a parallax by the processing unit (6) of the device (2) on each of the images (11) of license plates (7) transmitted; • the reconstruction of the alphanumeric character string (13) identified on each image (11) by optical character recognition; • storage in memory by the processing unit (6) of the device (2) of the reconstructed alphanumeric character strings (13); the method being characterized in that the parallax correction applied to each of the images (11) of license plates (7) is a function of an angle of incidence (0), the angle of incidence (0) being for each image (11) of license plate (7) between the first angle and the second angle relative to the reference axis (A) of the camera (4), the angle of incidence (0) varying for the correction of a parallax of each character (13) detected on each image (11) of license plate (7).

2. Method for authenticating plates (7) according to claim 1, characterized in that it comprises a step of searching for each alphanumeric character string (13) reconstituted in a database of registration plates (7) acquired subsequently or previously.

3. Method for authenticating plates (7) according to claim 2, characterized in that the database consists of alphanumeric character strings (13) each associated with an image (11) of a license plate (7) acquired by a camera (4) of the device (2) at an entrance (15) of the parking lot (1) and / or images (11) of license plates (7) acquired by a camera (4) of the device (2) at an exit (17) of the parking lot (1).

4. Method for authenticating plates (7) according to claim 3, characterized in the case where a string of alphanumeric characters (13) stored in memory corresponds to an image (11) of license plates (7) from the database acquired by a camera (4) of the device (2) at the exit (17) of the parking lot (1), said string of characters (13) is erased from the memory storage.

5. Method for authenticating plates (7) according to any one of the preceding claims, characterized in that the parallax correction applied comprises an application of the Hough transform followed by a rotation of the image (11) of each plate (7).

6. Method for authenticating plates (7) according to any one of the preceding claims, characterized in that the parallax correction step is carried out by an artificial intelligence of the processing unit (6), based on deep neural networks operating by learning.

7. Authentication device (2) for license plates (7) of a parking lot (1) configured to implement the method according to any one of claims 1 to 6, comprising: • at least one camera (4) arranged on a traffic lane of the parking lot (1), opposite a plurality of parking spaces (5), so that license plates (7) of at least two vehicles (3) parked in adjacent parking spaces (5) are visible by the camera (4); • a processing unit (6) connected to the camera (4) and comprising a storage memory, the processing unit (6) being configured to process images (11) acquired by the camera (4); the authentication device (2) being configured to correct a parallax induced by an angle of incidence (0) between a license plate (7) and a reference axis (A) of the camera (4).

8. Authentication device (2) for license plates (7) according to claim 7, characterized in that it comprises at least two cameras (4) arranged on a traffic lane of the parking lot (1), each camera facing a plurality of parking spaces (5) on a given side of the traffic lane.

9. Authentication device (2) for license plates (7) according to claim 7 or 8, characterized in that it comprises a camera positioned at an entrance (15) of the parking lot (1) and / or a camera positioned at an exit (17) of the parking lot (1).

10. Authentication device (2) for license plates (7) according to any one of claims 7 to 9, characterized in that the processing unit (6) comprises artificial intelligence based on deep neural networks operating by learning.

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