Method and apparatus for decoding two-dimensional bar code using CCD/CMD camera

• US 5,319,181 A
• Filed: 03/16/1992
• Issued: 06/07/1994
• Est. Priority Date: 03/16/1992
• Status: Expired due to Term

• Current Assignee: Symbol Technologies, Inc. (Zebra Technologies Corporation)
• Original Assignee: Symbol Technologies, Inc. (Zebra Technologies Corporation)
• Inventors: Chen, Ming-Hua, Shellhammer, Stephen J., Nikzad, Arman
• Primary Examiner(s): Dolinar, Andrew M.
• Application Number:
  US07/851,493
• Time in Patent Office: 813 Days
• Field of Search: 235/462, 235/470, 235/471
• US Class Current: 235/462.09
• CPC Class Codes: G06K 19/06037 multi-dimensional coding
• CPC Class Codes: G06K 7/0166 by means of clock-signals derived from the code marks, e.g. self-clocking code
• CPC Class Codes: G06K 7/1093 sensing, after transfer of the image of the data-field to an intermediate store, e.g. storage with cathode ray tube
• CPC Class Codes: G06K 7/1417 2D bar codes
• CPC Class Codes: G06K 7/1443 locating of the code in an image
• CPC Class Codes: G06K 7/1456 determining the orientation of the optical code with respect to the reader and correcting therefore

Abstract

A method and apparatus for decoding a two-dimensional bar code symbol using a charge-coupled device (CCD) camera or a charge-modulation device (CMD) camera. The CCD/CMD camera takes pictures of the symbol and the picture is converted into digital data. The location and orientation of the two-dimensional bar code symbol is determined and verified. Defects and damages on the symbol are detected and corrected. The symbol is scanned to read the codewords of the two-dimensional bar code symbol.

Claims

1. A method for decoding a two-dimensional bar code symbol having rows of bar coded information in which data is represented in each row by a plurality of codewords, each row including a predetermined control codeword which is the same for each row, the codewords being formed of a plurality of modules, the method comprising the steps of:

optically imaging the two-dimensional bar code symbol, converting the image to lines of image data corresponding to a field of view including the bar code symbol, and storing the image data in a memory;

determining an orientation of the two-dimensional bar code symbol in the field of view by locating the positions of at least two of the control codewords in the image data;

determining a sequence of lines passing through the rows of the bar code symbol in the image data from the orientation derived from the positions of the control codewords; and

scanning the two-dimensional bar code symbol in the image data along the sequence of lines to read the codewords.

2. The method of decoding a two-dimensional bar code symbol according to claim 1, further comprising the steps of

detecting defects on the two-dimensional bar code symbol;

and

scanning the two-dimensional bar code symbol properly despite the presence of defects.

3. The method of decoding a two-dimensional bar code symbol according to claim 1, wherein the orientation determining step comprises the steps of:

scanning the lines of image data to determine edge locations of the symbol;

searching for control codewords along the edge locations of the symbol in the image data; and

determining a plurality of control points adjacent the symbol in the image data relative to the positions of the control codewords.

4. The method for decoding a two-dimensional bar code symbol according to claim 3, further comprising the step of verifying the orientation of the two-dimensional bar code symbol.

5. The method for decoding a two-dimensional bar code symbol according to claim 3, wherein the image data scanning step includes a horizontal scan.

6. The method for decoding a two-dimensional bar code symbol according to claim 3, wherein the image data scanning step includes a vertical scan.

7. The method for decoding a two-dimensional bar code symbol according to claim 3, wherein the image data scanning step includes skipping a predetermined number of lines while searching for the control codewords.

8. The method for decoding a two-dimensional bar code symbol according to claim 3, wherein each row of the symbol includes two predetermined control codewords, one control codeword being a start codeword at the beginning of each row and the other control codeword being a stop control codeword at the end of each row, and wherein the searching step is repeated until a predetermined number of start and stop codewords have been found.

9. The method for decoding a two-dimensional bar code symbol according to claim 8, further comprising the step of selecting the best two start and best two stop codewords from the predetermined number of located start and stop codewords.

10. The method for decoding a two-dimensional bar code symbol according to claim 3, wherein the image data scanning step for determining the edge locations includes the steps of:

selecting two end points to find edge locations therebetween;

traversing a line between the two end points;

calculating a slope of the line;

comparing a gray level of a first pixel position to a second pixel position on said line;

determining the first pixel position to be an edge location when the comparison is greater than or equal to a first predetermined threshold value;

incrementing said first and second pixel positions along said line according to said slope; and

repeating said comparison step until the last end point is reached.

11. The method for decoding a two-dimensional bar code symbol according to claim 3, wherein the control point determining step comprises the steps of:

determining a line passing through a first bar of the control codewords; and

locating points both on said line and at an edge of the two-dimensional bar code symbol.

12. The method for decoding a two-dimensional bar code symbol according to claim 4, wherein the verifying step includes checking whether the two-dimensional bar code symbol forms a rectangle having a shape meeting requirements of a first predefined acceptable condition.

13. The method for decoding a two-dimensional bar code symbol according to claim 12, wherein the checking step comprises the steps of:

connecting the control points to form a rectangle;

calculating a slope for each line of said rectangle; and

comparing the slopes of intersecting lines at each corner of said rectangle, wherein the symbol has the acceptable rectangular shape when each of said comparison at said each corner of said rectangle is less than a second predetermined threshold value.

14. The method for decoding a two-dimensional bar code symbol according to claim 2, wherein each row of the symbol includes two predetermined control codewords, one control codeword being a start codeword at the beginning of each row and the other control codeword being a stop control codeword at the end of each row, and wherein the detecting step comprises the steps of:

determining a control line at top and bottom edges of the symbol;

forming first and second neighboring lines adjacent and at opposite sides of each said control lines;

searching for the start and stop codewords on each side neighboring lines; and

determining the symbol to have a defect when each said control line fails to meet predefined conditions.

15. The method for decoding a two-dimensional bar code symbol according to claim 14, wherein the second predefined condition includes said first neighboring line having both start and stop codewords and said second neighboring line having neither of the start and stop codewords.

16. The method for decoding a two-dimensional bar code symbol according to claim 14, wherein the correcting step corrects for one defective control line with the other control line being a valid control line, the correcting step comprising the step of adjusting the defective control line to be parallel with the valid control line and to include one of the control points.

17. The method for decoding a two-dimensional bar code symbol according to claim 16, further comprising the step of validating the adjusted control line, said validating step including the step of:

forming two neighboring lines adjacent and at opposite sides of said adjusted control lines;

searching for one of said start and stop codewords on each said neighboring lines; and

determining the valid correction of the defective control line by testing whether said adjusted control line meets a condition of having one neighboring line having one of start and stop codewords and the other neighboring line having neither of the start and stop codewords.

18. The method for decoding a two-dimensional bar code symbol according to claim 1, wherein the line determining step comprises the steps of:

forming rescan start and rescan stop lines;

selecting virtual scan head and corresponding virtual scan tail points on said rescan start and scan stop lines, respectively; and

retrieving a pair of head and corresponding tail points and forming a line connecting the two points.

19. The method for decoding a two-dimensional bar code symbol according to claim 18, wherein the scanning step comprises the steps of:

computing gray-level differences between successive pixels on said line connecting the head and tail points;

finding extreme of the computed differences;

comparing each extremum to a third predetermined threshold value;

determining the extremum to be an edge when the comparison is greater than said third predetermined threshold value; and

calculating distances between adjacent edges.

20. The method for decoding a two-dimensional bar code symbol according to claim 19, wherein the scanning step further comprises the step of optimizing the scanning step to reduce the scanning time and overhead by reducing the number of head and tail pairs.

21. The method for decoding a two-dimensional bar code symbol according to claim 1, wherein the line determining step comprises the steps of:

defining an interval having a beginning point and an end point at a center of each row of said two-dimensional bar code symbol and forming a line connecting the beginning and end points;

partitioning the defined interval into a predetermined unit sub-intervals; and

approximating width of the modules to correspond to a nearest unit sub-interval.

22. The method for decoding a two-dimensional bar code symbol according to claim 21, wherein the scanning step comprises the steps of:

summing gray-level values for each module interval;

comparing the sum to a fourth predetermined threshold value; and

determining the module to be one of a plurality of predefined values corresponding to the comparison.

23. The method for decoding a two-dimensional bar code symbol according to claim 1, further comprising the step of detecting and correcting damages due to destruction of one of the start and stop codewords of the symbol.

24. An apparatus for decoding a two-dimensional bar code symbol having rows of bar coded information in which data is represented in each row by a plurality of codewords, each row including a predetermined control codeword, the codewords being formed of a plurality of modules, comprising:

means for optically imaging the two-dimensional bar code symbol, converting the image to lines of image data corresponding to a field of view including the bar code symbol, and storing the image data in a memory;

means for determining an orientation of the two-dimensional bar code symbol in the field of view by locating the positions of at least two of the control codewords in the image data; and

means for determining a sequence of lines passing through the rows of the bar code symbol in the image data from the orientation derived from the positions of the control codewords; and

means for scanning the two-dimensional bar code symbol in the image data along the sequence of lines line to read the codewords.

25. The apparatus for decoding a two-dimensional bar code symbol according to claim 24, further comprising means for detecting defects on the two-dimensional bar code symbol and correcting the detected defects on the two-dimensional bar code symbol.

26. The apparatus for decoding a two-dimensional bar code symbol according to claim 24, wherein the orientation determining means comprises:

means for scanning the lines of image data to determine edge locations of the symbol;

means for searching for control codewords along the edge locations of the symbol in the image data; and

means for determining a plurality of control points adjacent the symbol in the image data relative to the positions of the control codewords.

27. The apparatus for decoding a two-dimensional bar code symbols according to claim 26, further comprising means for verifying the orientation of the two-dimensional bar code symbol.

28. The apparatus for decoding a two-dimensional bar code symbol according to claim 26, wherein the image data scanning means includes means for scanning the image data horizontally.

29. The apparatus for decoding a two-dimensional bar code symbol according to claim 26, wherein the image data scanning means includes means for scanning the image data vertically.

30. The apparatus for decoding a two-dimensional bar code symbol according to claim 26, wherein the image data scanning means includes means for skipping a predetermined number of lines while searching for the control codewords.

31. The apparatus for decoding a two-dimensional bar code symbol according to claim 26, wherein each row of the symbol includes two predetermined control codewords, one control codeword being a start codeword at the beginning of each row and the other control codeword being a stop control codeword at the end of each row, and wherein the search for the control codewords in the searching means is repeated until a predetermined number of start and stop codewords have been found.

32. The apparatus for decoding a two-dimensional bar code symbol according to claim 31, further comprising means for selecting the best two start and best two stop codewords from the predetermined number of located start and stop codewords.

33. The apparatus for decoding a two-dimensional bar code symbol according to claim 26, wherein the image data scanning means for determining the edge locations includes:

means for selecting two end points to find edge locations therebetween;

means for traversing a line between the two end points;

means for calculating a slope of the line;

means for comparing a gray level of a first pixel position to a second pixel position on said line;

means for determining the first pixel position to be an edge location when the comparison is greater than or equal to a first predetermined threshold value;

means for incrementing said first and second pixel positions along said line according to said slope; and

means for repeating the comparison in said comparing means until the last end point is reached.

34. The apparatus for decoding a two-dimensional bar code symbol according to claim 26, wherein the control point determining means comprises:

means for determining a line passing through a first bar of the control codewords; and

means for locating points both on said line and at an edge of the two-dimensional bar code symbol.

35. The apparatus for decoding a two-dimensional bar code symbol according to claim 27, wherein the verifying means includes means for checking whether the two-dimensional bar code symbol forms a rectangle having a shape meeting requirements of a first predefined acceptable condition.

36. The apparatus for decoding a two-dimensional bar code symbol according to claim 35, wherein the checking means comprises:

means for connecting the control points to form a rectangle;

means for calculating a slope for each line of said rectangle; and

means for comparing the slopes of intersecting lines at each corner of said rectangle, wherein the symbol has the acceptable rectangular shape when each of said comparison at said each corner of said rectangle is less than a second predetermined threshold value.

37. The apparatus for decoding a two-dimensional bar code symbol according to claim 25, wherein each row of the symbol includes two predetermined control codewords, one control codeword being a start codeword at the beginning of each row and the other control codeword being a stop control codeword at the end of each row, and wherein the detecting means comprises:

means for determining a control line at top and bottom edges of the symbol;

means for forming first and second neighboring lines adjacent and at opposite sides of each said control lines;

means for searching for the start and stop codewords on each said neighboring lines; and

means for determining the symbol to have a defect when each said control line fails to meet second predefined conditions.

38. The apparatus for decoding a two-dimensional bar code symbol according to claim 37, wherein the second predefined condition includes said first neighboring line having both start and stop codewords and said second neighboring line having neither of the start and stop codewords.

39. The apparatus for decoding a two-dimensional bar code symbol according to claim 37, wherein the correcting means corrects for one defective control line with the other control line being a valid control line, the correcting means including means for adjusting the defective control line to be parallel with the valid control line and to include one of the control points.

40. The apparatus for decoding a two-dimensional bar code symbol according to claim 39, further comprising means for validating the adjusted control line, said validating means including:

means for forming two neighboring lines adjacent and at opposite sides of said adjusted control line;

means for searching for one of said start and stop codewords on each said neighboring lines; and

means for determining the valid correction of the defective control line by testing whether said adjusted control line meets a condition of having one neighboring line having one of start and stop codewords and the other neighboring line having neither of the start and stop codewords.

41. The apparatus for decoding a two-dimensional bar code symbol according to claim 24, wherein the line determining means comprises:

means for forming rescan start and rescan stop lines;

means for selecting rescan head and corresponding rescan tail points on said rescan start and rescan stop lines, respectively; and

means for retrieving a pair of head and corresponding tail points and forming a line connecting the two points.

42. The apparatus for decoding a two-dimensional bar code symbol according to claim 41, wherein the scanning means comprises:

means for computing gray-level differences between successive pixels on said line connecting the head and tail points;

means for finding extrema of the computed differences;

means for comparing each extremum to a third predetermined threshold value;

means for determining the extremum to be an edge when the comparison is greater than said third predetermined threshold value; and

means for calculating distances between adjacent edges.

43. The apparatus for decoding a two-dimensional bar code symbol according to claim 41, wherein the scanning means further comprises means for optimizing the scanning of the image data in the scanning means to reduce the scanning time and overhead by reducing the number of head and tail pairs.

44. The apparatus for decoding a two-dimensional bar code symbol according to claim 24, wherein the line determining means comprises:

means for defining an interval having a beginning point and an end point at a center of each row of said two-dimensional bar code symbol and forming a line connecting the beginning and end points;

means for partitioning the defined interval into a predetermined unit sub-intervals; and

means for approximating width of the modules to correspond to a nearest unit sub-interval.

45. The apparatus for decoding a two-dimensional bar code symbol according to claim 44, wherein the scanning means comprises:

means for summing gray-level values for each module interval;

means for comparing the sum to a fourth predetermined threshold value; and

means for determining the module to be one of a plurality of predefined values corresponding to the comparison.

46. The apparatus for decoding a two-dimensional bar code symbol according to claim 24, further comprising means for detecting and correcting damages due to destruction of one of the start and stop codewords of the symbol.