Automatic perspective distortion detection and correction for document imaging
First Claim
1. A method of correcting perspective distortion in an image captured by an image reader wherein the captured image has a number of special markers located on the boundary of the image having a predetermined shape, comprising the steps of:
- a. calculating the smallest predetermined shape that encloses all of the special boundary markers;
b. building a geometric transform to map the location of the special markers in the captured image to corresponding locations on the predetermined shape; and
c. applying the geometric transform to the captured image.
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Accused Products
Abstract
A method and apparatus for detecting and correcting perspective distortion for document imaging is described. The document template of the present invention contains special markers that define the corners of the document. One of these markers, different from the others, uniquely identifies a particular corner. When an image of a document is captured and it is found to contain perspective distortion, the smallest rectangle that encloses the special markers in the captured image is calculated and geometric transforms are used to map the special markers in the captured image to the corners of the smallest rectangle. To correct for orientation errors during image capture, the captured image is rotated based on the location of the unique marker. The present invention can also provide feedback to the operator as the image is being captured. This feedback guides the operator to properly align the image reader for substantially perspective distortion-free imaging.
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Citations
39 Claims
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1. A method of correcting perspective distortion in an image captured by an image reader wherein the captured image has a number of special markers located on the boundary of the image having a predetermined shape, comprising the steps of:
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a. calculating the smallest predetermined shape that encloses all of the special boundary markers;
b. building a geometric transform to map the location of the special markers in the captured image to corresponding locations on the predetermined shape; and
c. applying the geometric transform to the captured image. - View Dependent Claims (2, 3, 4, 5, 6, 7)
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8. A method of positioning an image reader having a rectangular field of view to avoid perspective distortion in a captured image wherein the captured image has special boundary markers located at the corners of the image having a rectangular shape comprising the steps of:
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a. capturing an image;
b. calculating the distance between the special boundary markers and the field of view corners;
c. determining if the distances are all the same within a predetermined tolerance; and
d. repositioning the image reader and recapturing the image if the distances are not the same within the predetermined tolerance;
e. repeating steps b., c. and d. until the distances are all the same within the predetermined tolerance. - View Dependent Claims (9, 10, 11)
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12. A method of producing an image of a substantially rectangular target having special boundary markers at the corners with one of the markers being a unique corner marker using an image reader having a rectangular field of view comprising the steps of:
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a. capturing an image;
b. positioning the reader as a function of the distances from the markers to corners of the field of view; and
c. correcting orientation errors of the image using the unique corner marker. - View Dependent Claims (13, 14, 15)
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16. A method of producing an image of a substantially rectangular target having special boundary markers at the corners with one of the markers being a unique corner marker using an image reader comprising the steps of:
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a. capturing an image;
b. correcting perspective distortion on the captured image using the special boundary markers; and
c. correcting orientation errors of the image using the unique corner marker. - View Dependent Claims (17, 18, 19, 20)
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21. An apparatus for correcting perspective distortion in an image captured by an image reader wherein the captured image has a number of special markers located on the boundary of the image having a predetermined shape comprising:
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means for calculating the smallest predetermined shape that encloses all of the special boundary markers;
means for building a geometric transform to map the location of the special markers in the captured image to corresponding locations of the predetermined shape; and
means for applying the geometric transform to the captured image. - View Dependent Claims (22, 23, 24, 25, 26)
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27. An apparatus for positioning an image reader having a rectangular field of view to avoid perspective distortion in a captured image wherein the captured image has special boundary markers located at the corners of the image having a rectangular shape comprising:
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means for capturing an image;
means for calculating the distance between the special boundary markers and the field of view corners;
means for determining if the distances are all the same within a predetermined tolerance;
means for recapturing the image until the distances are all the same within a predetermined tolerance; and
means for indicating when the distances are all the same within a predetermined tolerance. - View Dependent Claims (28, 29, 30)
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31. An apparatus for producing an image of a substantially rectangular target having special boundary markers at the corners with one of the markers being a unique corner marker using an image reader having a rectangular field of view comprising:
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means for capturing an image;
means for providing an indication to position the reader as a function of the distances from the markers to corners of the field of view; and
means for correcting orientation errors of the image using the unique corner marker. - View Dependent Claims (32, 33, 34)
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35. An apparatus for producing an image of a substantially rectangular target having special boundary markers at the corners with one of the markers being a unique corner marker using an image reader comprising the steps of:
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means for capturing an image;
means for correcting perspective distortion on the captured image using the special boundary markers; and
means for correcting orientation errors of the captured image using the unique corner marker. - View Dependent Claims (36, 39)
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37. The apparatus as claimed in 35 wherein the means for correcting perspective distortion comprises:
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means for calculating the smallest predetermined shape that encloses all of the special boundary markers;
means for building a geometric transform to map the location of the special markers in the captured image to corresponding locations of the predetermined shape; and
means for applying the geometric transform to the captured image. - View Dependent Claims (38)
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Specification