Geometric pre-correction for automatic license plate recognition

• US 20130329961A1
• Filed: 06/12/2012
• Published: 12/12/2013
• Est. Priority Date: 06/12/2012
• Status: Active Grant

• Current Assignee: Conduent Business Services, LLC (Conduent, Inc.)
• Original Assignee: Xerox Corporation (Xerox Holdings Corp.)
• Inventors: Fan, Zhigang, Zhao, Yonghui, Wang, Shen-ge
• Granted Patent: US 9,020,200 B2
• Time in Patent Office: Days
• US Class Current: 382/105
• CPC Class Codes: G06T 5/006 Geometric correction correction of chromatic aberrations adapted to be used in an image pickup device containing an electronic image sensor H04N23/10; detecting, correcting, reducing or removing artefacts resulting only from the lens unit, e.g. flare, shading, vignetting or "cos4" H04N25/61
• CPC Class Codes: G06T 5/80 Geometric correction
• CPC Class Codes: G06V 10/243 by compensating for image skew or non-uniform image deformations
• CPC Class Codes: G06V 20/63 Scene text, e.g. street names

Abstract

A method for adjusting a license plate that is detected in a captured image includes automatically determining at least one set of correction parameters corresponding to a slant-oriented license plate. The method further includes receiving an input image representing a detected license plate. In response to receiving the input image, the method includes automatically adjusting the input image to obtain a corrected image using the at least one set of correction parameters.

Claims

1. A method for adjusting a license plate detected in a captured image, the method comprising:

automatically determining at least one set of correction parameters corresponding to a slant-oriented plate;

receiving an input image representing a detected license plate;

in response to receiving the input image, automatically adjusting the input image to obtain a corrected image using the at least one set of correction parameters.

2. The method of claim 1, wherein the adjusting further includes:

determining if the slant-oriented plate was detected after applying the first set of correction parameters;

in response to the slant-oriented plate not being determined, applying at least a second set of correction parameters corresponding to an upright-oriented plate.

3. The method of claim 1, wherein the adjusting includes:

applying one matrix if the input image includes a front plate and applying two matrices if the input image includes a rear plate.

4. The method of claim 1, wherein the adjusting includes:

scaling the detected plate in the input image to a predetermined size; and

,

adjusting the detected plate in the input image to compensate for distortion.

5. The method of claim 1, wherein the providing the at least one correction matrix includes:

providing an image capture device;

receiving images of N number of license plates captured by the image capture device;

determining x,y coordinates for each of four corners of each license plate captured in the images;

determining correction matrices using optimization.

6. The method of claim 5, wherein the determining the correction matrices includes:

determining a feature set by calculating differences between the x,y coordinates of the four corners;

determining an original plate geometry matrix V;

of the feature set;

determining an ideal plate geometry D, wherein vectors of the D matrix are defined by a height and width of an ideal rectangular plate;

determining a corrected plate geometry CV_i;

determining a distance between the corrected plate geometry and the ideal plate geometry by subtracting the ideal plate geometry from the corrected plate geometry (CV_i−

D); and

,

determining the correction matrix C by squaring a sum of all matrix entries for the license plates.

7. The method according to claim 5, wherein the providing the at least one correction matrix further includes:

determining a matrix V′

containing the plate information for the N number of plates;

determining an ideal plate geometry D′

; and

,

calculating vector c′

using the matrix V′ and

matrix D′

.

8. The method according to claim 7, wherein the N number of plates all include one of upright-oriented and slant-oriented plates.

9. The method according to claim 7, wherein the determining the ideal plate geometry D′

includes;

specifying an ideal height and width of the number N of license plates.

10. A computer program product comprising tangible media which encodes instructions for performing the method of claim 1.

11. A system for adjusting an input image, the system comprising:

a correction device comprising memory which stores instructions for performing the method of claim 1 and a processor, in communication with the memory for executing the instructions.

12. A system for adjusting an input image, the system comprising:

an input device adapted to capture an image of a license plate;

a correction device in communication with the input device, the correction device adapted to;

receive the input image transmitted from the input device, and

in response to receiving the input image, automatically adjusting the input image v[(x′

,y′

)] to obtain a corrected image v′

(x,y) by applying at least one correction matrix;

a storage device in communication with the correction device, the storage device adapted to store the at least one correction matrix;

an interface in communication with the correction device, the interface adapted to display the corrected image.

13. The system according to claim 12, wherein the correction device is further adapted to:

determine x,y coordinates for each of four corners of the license plate in the input image;

determine a feature set by calculating differences between the x,y coordinates of the four corners;

determine an original plate geometry matrix V_i of the feature set;

determine an ideal plate geometry D, wherein vectors of the D matrix are defined by a height and width of an ideal rectangular plate;

determine a corrected plate geometry CV_i;

determine a distance between the corrected plate geometry and the ideal plate geometry by subtracting the ideal plate geometry from the corrected plate geometry (CV_i−

D); and

,

obtaining an optimization using a least square root method.

14. The system according to claim 12, further comprising:

a training device in communication with the storage device, the training device adapted to;

determine a matrix V′

containing the plate information for the N number of plates;

determine an ideal plate geometry D′

; and

,

calculate vector c′

using the matrix V′ and

matrix D′

.

15. The system according to claim 14, wherein the N number of plates includes one of front plates, rear slant-oriented plates, and rear upright-oriented plates.

16. The system according to claim 12, wherein the at least one correction matrix scales the image and corrects for distortion caused by the input device.

17. The system according to claim 12, wherein the correction device determines the four corners by detecting boundaries of the license plate and calculating intersections of the boundaries.

18. The system according to claim 12, wherein the at least one correction matrix corresponds to a slant-oriented license plate.

19. The system according to claim 18, wherein the storage device is adapted to store at least a second correction matrix corresponding to an upright-oriented license plate and the correction device is adapted to automatically adjust the input image by applying the at least second correction matrix after applying the at least first correction matrix.

20. A system for adjusting an input image, the system comprising:

a training device adapted to;

receive images of N number of plates,

determine a matrix V′

containing feature sets for each of the N number of plates,

determine an ideal geometry matrix D′

by specifying height and width feature sets of N ideal plates, and

calculate a correction matrix using the matrix V′ and

ideal geometry D′

;

a storage device in communication with the training device, the storage device adapted to store the at least one correction matrix;

an input device adapted to capture an image of a detected license plate;

a correction device in communication with the input device and the storage device, the correction device adapted to;

receive the captured image transmitted from the input device, and

in response to receiving the input image, automatically adjusting the input image v[(x′

,y′

)] to obtain a corrected image v′

(x,y) by applying the at least one correction matrix;

an interface in communication with the correction device, the interface adapted to display the corrected image.