Automated license plate locator and reader including perspective distortion correction

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.

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.

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.

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.

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.

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.