Automated license plate locator and reader including perspective distortion correction

• US 5,651,075 A
• Filed: 08/30/1995
• Issued: 07/22/1997
• Est. Priority Date: 12/01/1993
• Status: Expired due to Term

• Current Assignee: Hughes Missile Systems Company
• Original Assignee: Hughes Missile Systems Company
• Inventors: Reinhart, Craig C., Frazier, James F., Alves, James F.
• Primary Examiner(s): JOHNS, ANDREW W
• Application Number:
  US08/521,197
• Time in Patent Office: 692 Days
• Field of Search: 348/148, 348/149, 340/933, 340/937, 382/105, 382/177, 382/199, 382/209, 382/278, 382/291, 382/293
• US Class Current: 382/105
• CPC Class Codes: G06V 10/32 Normalisation of the pattern dimensions
• CPC Class Codes: G06V 10/751 Comparing pixel values or logical combinations thereof, or feature values having positional relevance, e.g. template matching
• CPC Class Codes: G06V 20/625 License plates
• CPC Class Codes: G06V 20/63 Scene text, e.g. street names

Abstract

An automated license plate locator and reader (10) which provides novel methods for 1) correcting for perspective distortion (16), 2) locating the license plate (20), 3) reading the license plate (22) and 4) improving the confidence rating of the output signal (228). The novel method for accommodating a wide range of viewing geometries involves warping the image to eliminate perspective distortion of the plate resulting from roadside camera placement. The novel method for locating plates involves correlating an "idealized" edge template of a license plate with an edge-enhanced version of the image. Edge enhancement and shadow reduction is achieved by applying a Laplacian operator over the input image. Performance is further improved by filtering out distracting edges within the resulting correlation surface and then locating the highest peaks within this filtered correlation surface. The location of the highest peaks provides the location of the plate. The novel method for reading the license plates involves performing a modified binary correlation of an image of the plate with reference characters to provide output signals indicative of the symbols on the plate. This output is correlated with valid license plate signals to improve the confidence rating thereof.

Claims

1. A system for reading symbols including characters and numerals on a license plate mounted on a vehicle comprising:

camera means for providing an input image of said license plate, the camera means including a camera positioned to a side of the license plate so that said input image contains a lateral perspective distortion wherein a first side of the license plate is closer to the camera than a second side of the license plate;

means for warping said image to mitigate said lateral perspective distortion in said input image to provide a corrected image; and

means for processing said corrected image to provide an output signal representative of the symbols on said license plate.

2. The system of claim 1, wherein the means for warping comprises a digital image warping circuit.

3. In a system for reading symbols including characters and numerals on a license plate mounted on a vehicle having a camera for providing an input image of the license plate, a system for locating the license plate on the vehicle comprising:

first means for providing an idealized edge template of a license plate by averaging a plurality of images of a license plate;

second means for providing an edge-enhanced version of the input image; and

third means for correlating the idealized edge template of the license plate with the edge-enhanced version of the input image and locating the license plate on the vehicle based on the correlation.

4. The invention of claim 3 further including means for filtering edges within a correlation surface provided by said third means.

5. The invention of claim 4 further including means for locating peaks within the filtered correlation surface to provide an output representative of the location of the license plate on the vehicle.

6. A system for reading symbols including characters and numerals on a license plate mounted on a vehicle comprising:

camera means for providing an input image of said license plate, the camera means including a camera positioned to a side of the license plate so that said input image contains a lateral perspective distortion wherein a first side of the license plate is closer to the camera than a second side of the license plate;

means for warping said lateral perspective distortion in said input image to provide a corrected image;

means for applying a Laplacian operator to said corrected image to enhance edges and reduce the effects of shadows therein;

means for locating the license plate on the vehicle comprising;

first means for providing an idealized edge template of the license plate;

second means for providing an edge-enhanced version of the input image; and

third means for correlating the idealized edge template of the license plate with the edge-enhanced version of the input image; and

means for reading the license plate comprising;

means for storing said input image in memory;

means for storing images of reference symbols in memory; and

means for correlating said input image with said images of symbols to provide output signals indicative of the symbols on said license plate.

7. The invention of claim 6 further including means for correlating said output signals indicative of the symbols on said license plate with valid license plate symbols to improve a confidence rating of said output signals.

8. A method for reading symbols including characters and numerals on a license plate mounted on a vehicle comprising the steps of;

providing an input image of said license plate using a camera positioned to a side of the license plate, said input image containing lateral perspective distortion resulting from a first side of the license plate being closer to the camera than a second side of the license plate;

warping said input image to mitigate said lateral perspective distortion to provide a corrected image; and

processing said corrected image to provide an output signal representative of the symbols on said license plate.

9. In a system for reading symbols including characters and numerals on a license plate mounted on a vehicle having a camera for providing an input image of the license plate, a method for locating the license plate on the vehicle comprising the steps of:

providing an idealized edge template of the license plate by averaging a plurality of images of a license plate;

providing an edge-enhanced version of the input image;

correlating the idealized edge template of the license plate with the edge-enhanced version of the input image to provide a correlation surface; and

locating the license plate on the vehicle based on the correlation surface.

10. The invention of claim 9 further including the step of filtering edges within said correlation surface.

11. The invention of claim 10 further including the step of locating peaks within the filtered correlation surface to provide an output representative of the location of the license plate on the vehicle.

12. A method for reading symbols including characters and numerals on a license plate mounted on a vehicle comprising the steps of:

providing an input image of said license plate using a camera positioned to a side of the license plate, said input image containing lateral perspective distortion resulting from a first side of the license plate being closer to the camera than a second side of the license plate;

mitigating said lateral perspective distortion in said input image to provide a corrected image by warping same;

applying a Laplacian operator to said corrected image to enhance edges and reduce the effects of shadows therein;

locating the license plate on the vehicle further including the steps of;

providing an idealized edge template of the license plate;

providing an edge-enhanced version of the input image; and

correlating the idealized edge template of the license plate with the edge-enhanced version of the input image and locating the license plate on the vehicle; and

reading the license plate, further including the steps of;

storing said input image in memory;

storing images of reference symbols in memory; and

correlating said input image with said images of symbols to provide output signals indicative of the symbols on said license plate.

13. The invention of claim 12 further including the step of correlating said output signals indicative of the symbols on said license plate with valid license plate symbols to improve a confidence rating of said output signals.

14. A system for reading symbols including characters and numerals on a license plate mounted on a vehicle comprising

camera means for providing an input image of said license plate, the camera means including a camera positioned to one side of the license plate so that said input image contains a lateral perspective distortion wherein a first side of the license plate is closer to the camera than a second side of the license plate;

means for warping said image to mitigate said lateral perspective distortion in said input image to provide a corrected image;

means for processing said corrected image to provide an output signal representative of the symbols on said license plate;

means for providing an idealized edge template of the license plate;

means for providing an edge-enhanced version of the input image; and

means for correlating the idealized edge template of the license plate with the edge-enhanced version of the input image to provide at least one signal indicative of the location of the license plate.