Reconstruction of virtual raster

• US 7,543,753 B2
• Filed: 04/27/2005
• Issued: 06/09/2009
• Est. Priority Date: 12/06/2001
• Status: Active Grant

• Current Assignee: Anoto Group AB
• Original Assignee: Anoto Group AB
• Inventors: Pettersson, Mats Petter
• Primary Examiner(s): LE, THIEN MINH
• Application Number:
  US11/115,240
• Publication Number:
  US 20050199729A1
• Time in Patent Office: 1,504 Days
• Field of Search: 235/472.01, 235/472.02, 235/472.03, 23546201-46225, 348/E3.045, 348/E9.021, 348/807, 348/190
• US Class Current: 235/462.25
• CPC Class Codes: G06K 7/14 using light without selection of wavelength, e.g. sensing reflected white light G06K7/10831 - G06K7/1097 take precedence
• CPC Class Codes: G06T 3/00 Geometric image transformations in the plane of the image
• CPC Class Codes: G06V 10/19 by sensing codes defining pattern positions
• CPC Class Codes: G06V 10/20 Image preprocessing
• CPC Class Codes: G06V 10/247 by affine transforms, e.g. correction due to perspective effects; Quadrilaterals, e.g. trapezoids
• CPC Class Codes: G06V 30/142 using hand-held instruments; Constructional details of the instruments
• CPC Class Codes: G06V 30/1423 the instrument generating sequences of position coordinates corresponding to handwriting preprocessing or recognising digital ink G06V30/32

Abstract

A method aims at identifying, starting from objects in a digital image, a virtual raster included in a coding pattern. The objects in the image at least partially reproduce marks on a base, each mark being associated with a respective point of intersection of raster lines belonging to the virtual raster. The method comprises the steps of matching sets of objects against a cell unit, which corresponds to a recurring known basic element of said raster; when a set corresponds with the cell unit, identifying the objects in the set as approved; and reconstructing the virtual raster on the basis of the mutual arrangement of the approved objects. A computer program, a storage medium and a device for position determination are also described.

Claims

1. A method of correcting for geometrical perspective distortion in a sequence of digital representations of a coding pattern, said method comprising:

deriving current correction data for a digital representation in said sequence;

using the current correction data to identify a correspondence between objects in the digital representation and elements of the coding pattern;

generating, from the digital representation and based upon said correspondence, a distortion-corrected digital representation; and

deriving subsequent correction data for a subsequent digital representation in said sequence based upon the current correction data.

2. The method of claim 1, wherein said generating comprises:

computing homogeneous transformation data based upon said correspondence.

3. The method of claim 2, wherein said generating further comprises:

operating the homogeneous transformation data on the digital representation to generate the distortion-corrected digital representation.

4. The method of claim 2, wherein the subsequent correction data is derived from the homogeneous transformation data.

5. The method of claim 4, wherein the subsequent correction data comprises linear transformation data extracted from the homogeneous transformation data.

6. The method of claim 1, wherein the current correction data comprises linear transformation data, and wherein said using comprises:

operating the linear transformation data on said objects.

7. The method of claim 1, wherein said elements comprise at least one of:

code marks of the coding pattern and points of intersection of raster lines included in the coding pattern.

8. The method of claim 1, wherein said correspondence relates a number of object locations in the digital representation to a corresponding number of intersections of raster lines included in the coding pattern.

9. The method of claim 8, wherein said number is at least four.

10. The method of claim 2, wherein said computing comprises computing eight homogeneous transformation parameters.

11. The method of claim 1, wherein the current correction data comprises linear transformation data.

12. The method of claim 11, wherein said deriving current correction data comprises:

identifying at least two main vectors of the current digital representation; and

deriving the linear transformation data that transforms the main vectors to essentially coincide with corresponding reference vectors of the coding pattern.

13. The method of claim 12, wherein the linear transformation data is derived to adapt the magnitude of the main vectors to the magnitude of the corresponding reference vectors.

14. The method of claim 12, wherein the reference vectors are mutually orthogonal.

15. The method of claim 1, wherein each digital representation corresponds to an image in a sequence of two-dimensional digital images.

16. The method of claim 15, wherein each digital representation is binarized version of said image.

17. The method of claim 15, wherein each digital representation comprises a data structure containing location indications for objects in said image.

18. The method of claim 1, wherein the coding pattern comprises a raster formation comprising a first plurality of raster line intersections, and a second plurality of code marks, each raster line intersection being assigned at least one code mark.

19. The method of claim 18, wherein each raster line intersection represents a value by way of a relative location of said at least one code mark.

20. The method of claim 19, wherein said value is given by a displacement of the center of gravity of the code mark relative to the raster line intersection.

21. A computer-readable medium comprising program code which, when executed in a computer, causes the computer to implement a method of correcting for geometrical perspective distortion in a sequence of digital representations of a coding pattern, said method comprising:

deriving current correction data for a digital representation in said sequence;

using the current correction data to identify a correspondence between objects in the digital representation and elements of the coding pattern;

generating, from the digital representation and based upon the current correction datasaid correspondence, a distortion-corrected digital representation; and

deriving subsequent correction data for a subsequent digital representation in said sequence based upon the current correction data.

22. A device for correcting for geometrical perspective distortion in a sequence of digital representations of a coding pattern, said device comprising:

a first calculator which derives current correction data for a digital representation in said sequence;

a corrector which uses the current correction data to identify a correspondence between objects in the digital representation and elements of the coding pattern and which generates, from the digital representation and based upon said correspondence, a distortion-corrected digital representation; and

a second calculator which derives subsequent correction data for a subsequent digital representation in said sequence based upon the current correction data.

23. A handheld apparatus, comprising:

a reader for generating a sequence of digital representations of a position-coding pattern,

a device for generating a sequence of distortion-corrected digital representations, said device comprising a first calculator which derives current correction data for a digital representation in said sequence of digital representations, a corrector which generates, from the digital representation and based upon the current correction data, a distortion-corrected digital representation, and a second calculator which derives subsequent correction data for a subsequent digital representation in said sequence based upon the current correction data; and

a position decoder for generating position data based upon said sequence of distortion-corrected digital representations.