Apparatus for enhancing and thresholding scanned microfilm images and methods for use therein

• US 4,982,294 A
• Filed: 07/24/1987
• Issued: 01/01/1991
• Est. Priority Date: 07/24/1987
• Status: Expired due to Fees

• Current Assignee: Eastman Kodak Company
• Original Assignee: Eastman Kodak Company
• Inventors: Lewis, Scott, Morton, Roger R. A., Redden, John E.
• Primary Examiner(s): Groody, James J.
• Assistant Examiner(s): Harvey, David E.
• Application Number:
  US07/077,529
• Time in Patent Office: 1,257 Days
• Field of Search: 358/282, 358/284, 358/167, 358/443, 358/464, 358/465, 358/466, 307/358, 307/359, 328/147, 328/149, 328/151, 382/50-54
• US Class Current: 358/465
• CPC Class Codes: G06T 5/20 using local operators
• CPC Class Codes: G06V 10/28 Quantising the image, e.g. histogram thresholding for discrimination between background and foreground patterns
• CPC Class Codes: H04N 1/403 Discrimination between the two tones in the picture signal of a two-tone original
• CPC Class Codes: H04N 1/409 Edge or detail enhancement; Noise or error suppression

Abstract

Apparatus for enhancing and thresholding scanned microfilm images, specifically for removing single pixel noise therefrom, and accompanying methods for use therein are disclosed. Specifically, this apparatus convolves each one of a pre-defined group of incoming pixel values with a corresponding one of a pre-defined set of coefficients in order to generate an convolved pixel value. The convolved pixel is then compared against a threshold level to produce a thresholded pixel value indicative of the results of the comparison. The inventive apparatus also includes a noise processing circuit that contains a noise tracker and a noise detector and filter circuit. The noise tracker is used to produce a noise value that tracks a noise level present in the threshold pixel value. The noise detector and filter circuit filters pixel noise from the thresholded pixel value and, in response thereto, produces a corresponding output pixel value for a current incoming pixel value being processed and provides a signal to the noise tracker that pixel noise has been detected. A background tracker, which also forms part of the inventive apparatus, produces a background value that tracks a background level present in the incoming pixel values. The background value and the noise value are combined to produce an error signal which, in turn, is used, along with a deaveraged value of the current incoming pixel value, to set the threshold level for the current pixel value being processed. Specific apparatus for the background tracker and the noise tracker, and accompanying methods for use therein, are also disclosed herein.

Claims

1. In an image processing system, apparatus for enhancing and thresholding a stream of incoming multi-bit pixel values in order to produce output pixel values, wherein said incoming pixel values collectively comprise a scanned image, said apparatus comprising:

means, responsive to a plurality of incoming pixel values, for convolving each one of a pre-defined group of said incoming pixel values with a corresponding one of a pre-defined set of coefficients in order to generate a convolved pixel value;

means, responsive to said convolved pixel value and to a threshold level, for comparing said convolved pixel value against said threshold level and for producing a thresholded pixel value indicative of the results of said comparison;

noise processing means, responsive to said thresholded pixel value, for producing a noise value that tracks a noise level present in said thresholded pixel value and for filtering noise from said thresholded pixel value and, in response thereto, producing a corresponding output pixel value;

background tracking means, responsive to said incoming pixel values, for producing a background value that tracks a background level present in said incoming pixel values;

means, responsive to said background value and said noise value for producing an error signal; and

means, responsive to said error signal, for providing said threshold level.

2. The apparatus in claim 1 wherein said noise processing means comprises:

means, responsive to said thresholded pixel value, for producing both said output pixel value and a noise signal indicative of the presence of said noise; and

means, responsive to said noise signal for producing said noise value.

3. The apparatus in claim 2 wherein said output pixel value and noise signal producing means comprises:

means for forming a moving window of a plurality of thresholded pixel values;

means, responsive to said moving window, for comparing said thresholded pixel values that form said moving window against at least one pre-determined noise pixel pattern to determine whether pixel noise exists amongst the thresholded pixel values that form said window and, in response thereto, producing said noise signal and for changing the value of a current one of said thresholded pixel values to remove any noise therein so as to form a filtered thresholded pixel value; and

means for providing said filtered thresholded pixel value as the output pixel value.

4. The apparatus in claim 2 wherein said noise value producing means comprises:

means, responsive to said noise signal, for generating a value representative of two dimensional noise content occurring in a moving window of thresholded pixel values, wherein said two-dimensional noise content includes a first portion attributable to noise content in a vertical direction.

5. The apparatus in claim 4 wherein said two dimensional noise content value generating means comprises:

means, responsive to said noise signal and a first prior noise value, for producing said first portion as a first pre-defined function of a current thresholded pixel value and said first prior noise value, wherein said first prior noise value equals a value of said first pre-defined function for a corresponding thresholded pixel value associated with a pixel situated on a prior scan line and located immediately above a pixel associated with the current thresholded pixel value;

means responsive to said first portion, for delaying said first portion previously determined for a corresponding thresholded pixel value associated with the pixel situated on a prior scan line and located immediately above the pixel associated with the current thresholded pixel, for an interval of time equivalent to a scan line so as to produce said first prior noise value;

means, responsive to said first portion and a second prior noise value, for producing said two dimensional noise content value as a second pre-defined function of said first portion and said second prior noise value, wherein said second prior noise value equals a value of said second pre-defined function for a most recent prior pixel situated on the present scan line; and

means, responsive to the two dimensional noise content value, for delaying said two dimensional noise content value previously determined for the most recent pixel by an interval of time equivalent to a pixel interval so as to produce said second prior noise value.

6. The apparatus in claim 5 wherein said first and second pre-defined functions are each exponentially decaying functions.

7. The apparatus in claim 2 wherein said background tracking means comprises means for producing said background value in response to a current input pixel and to a background value of at least one pixel located adjacent to said current input pixel.

8. The apparatus in claim 7 wherein said background value producing means comprises:

means for generating said background value for a current input pixel (having a pixel value V) in response to a pre-determined function of the pixel value of the current input pixel, a background value of a prior input pixel (background value b), wherein said prior input pixel is situated on a prior scan line and is located immediately above the current input pixel, and a background value of a most recent input pixel (background value c), wherein said most recent input pixel is situated on a current scan line and is located adjacent to said current input pixel.

9. The apparatus in claim 8 wherein said pre-determined function is given by:

min (b,c)-f(min(b,c)-V), where f is a pre-defined function which responds rapidly when the value of said current input pixel changes towards a background level in the scanned image but responds slower when the value of said current input pixel changes away from the background level.

10. The apparatus in claim 9 wherein said background value generating means further comprises:

means, responsive to the background values b and c, for determining a minimum value therebetween;

means for determining, in response to said minimum value and to the current input pixel value (pixel value V), a resulting value given by the pre-determined function of said minimum value and said current input pixel value;

means for combining said resulting value and said minimum value to yield the background value for the current input pixel value;

means, responsive to the background value for the current input pixel value, for delaying the background value previously determined for the prior input pixel situated on a prior scan line and located immediately above the input pixel, by an interval of time equivalent to one scan line to yield said background value b; and

means, responsive to the background value for the current input pixel value, for delaying the background value previously determined for the most recent input pixel situated on a current scan line and located adjacent to said current input pixel, by an interval of time equivalent to one pixel interval to yield said background value c.

11. The apparatus in claim 10 wherein said resulting value determining means comprises a look up table, operative in response to said minimum value and said current input pixel value, for providing said resulting value.

12. The apparatus in claims 2 or 8 further comprising:

means, responsive to a stream of scanned pixel values that comprise said scanned image and are produced by a scanner, for compensating each pixel value in said stream of scanned pixel values for at least one pre-defined characteristic so as to produce a stream of compensated pixel values; and

means for averaging said stream of compensated pixel values on an odd/even basis to generate said plurality of incoming pixel values.

13. The apparatus in claim 12 wherein the compensating means further comprises:

means for compensating each of said scanned pixel values that comprise said scanned image for shading differences occurring in said scanner in order to produce a stream of shading corrected pixel values; and

means for correcting pixel value in said stream each of said shading corrected pixel values for at least one transfer characteristic of a medium that is used to store said image in order to produce said stream of compensated pixel values.

14. The apparatus in claim 12 further comprising:

means, responsive to said plurality of incoming pixel values, for deaveraging at least one pixel value in said plurality of incoming pixel values in order to produce a deaveraged pixel value.

15. The apparatus in claim 14 wherein said threshold level providing means comprises:

means, operative in response to said error signal and to said deaveraged pixel value, for providing a pre-defined pixel value as the threshold level for use by said comparing means.

16. The apparatus in claim 15 wherein said pre-defined pixel value providing means comprises a look up table, operative in response to said error signal and to said deaveraged pixel value, for producing said pre-defined pixel value.

17. The apparatus in claim 14 wherein said error signal producing means comprises means for generating said error signal as an algebraic combination of said background value provided by said background tracking means and said noise value provided by said noise processing means.

18. The apparatus in claim 17 wherein the comparing means comprises means for providing a single bit pixel value for said output pixel value.

19. In an image processing system suitable for use in conjunction with an image management system, apparatus for enhancing and thresholding a stream of incoming multi-bit gray scale pixel values in order to produce single bit output pixel values, wherein said incoming pixel values collectively comprise a scanned microfilm image, said apparatus comprising:

means, responsive to a plurality of incoming pixel values, for convolving each one of a pre-defined group of said incoming pixel values with a corresponding one of a pre-defined set of coefficients in order to generate a convolved pixel value;

means, responsive to said convolved pixel value and to a threshold level, for comparing said convolved pixel value against said threshold level and for producing a thresholded pixel value indicative of the results of said comparison;

noise processing means, responsive to said thresholded pixel value, for producing a noise value that tracks a noise level present in said thresholded pixel value and for filtering noise from said thresholded pixel value and, in response thereto, producing a corresponding output pixel value;

wherein said noise processing means comprises;

means, responsive to said thresholded pixel value, for producing both said output pixel value and a noise signal indicative of the presence of said noise; and

means, responsive to said noise signal for producing said noise value;

background tracking means, responsive to said incoming pixel values, for producing a background value that tracks a background level present in said incoming pixel values;

wherein said background tracking means comprises;

means for producing said background value for a current input pixel in response to a current one of said incoming pixel values and to a background value of at least one pixel located adjacent to said current input pixel;

means, responsive to said background value and said noise value for producing an error signal; and

means, responsive to said error signal, for providing said threshold level.

20. The apparatus in claim 19 wherein said output pixel value and noise signal producing means comprises:

means for forming a moving window of a plurality of thresholded pixel values;

means, responsive to said moving window, for comparing said thresholded pixel values that form said moving window against at least one pre-determined noise pixel pattern to determine whether pixel noise exists amongst the thresholded pixel values that form said window and, in response thereto, producing said noise signal and for changing the value of a current one of said thresholded pixel values to remove any noise therein so as to form a filtered thresholded pixel value; and

means for providing said filtered thresholded pixel value as the output pixel value.

21. The apparatus in claim 20 wherein said noise value producing means comprises:

means, responsive to said noise signal, for generating a value representative of two dimensional noise content occurring in the moving window of thresholded pixel values, wherein said two-dimensional noise content includes a first portion attributable to noise content in a vertical direction.

22. The apparatus in claim 21 wherein said two dimensional noise content value generating means comprises:

means, responsive to said noise signal and a first prior noise value, for producing said first portion as a first pre-defined function of a current thresholded pixel value and said first prior noise value, wherein said first prior noise value equals a value of said first pre-defined function for a corresponding thresholded pixel value associated with a pixel situated on a prior scan line and located immediately above a pixel associated with the current thresholded pixel value;

means, responsive to said first portion, for delaying said first portion previously determined for a corresponding thresholded pixel value associated with the pixel situated on a prior scan line and located immediately above the pixel associated with the current thresholded pixel value, for an interval of time equivalent to a scan line so as to produce said first prior noise value;

means, responsive to said first portion and a second prior noise value, for producing said two dimensional noise content value as a second pre-defined function of said first portion and said second prior noise value, wherein said second prior noise value equals a value of said second pre-defined function for a most recent prior pixel situated on the present scan line; and

means, responsive to the two dimensional noise content value, for delaying said two dimensional noise content value previously determined for the most recent pixel by an interval of time equivalent to a pixel interval so as to produce said second prior noise value.

23. The apparatus in claim 22 wherein said first and second pre-defined functions are each exponentially decaying functions.

24. The apparatus in claim 19 wherein said background value producing means comprises:

means for generating said background value for a current input pixel (having a pixel value V) in response to a pre-determined function of the value of the current input pixel, a background value of a prior input pixel (background value b), wherein said prior input pixel is situated on a prior scan line and is located immediately above the current input pixel, and a background value of a most recent input pixel (background value c), wherein said most recent input pixel is situated on a current scan line and is located adjacent to said current input pixel; and

wherein said pre-determined function is given by;

min (b,c)-f(min (b,c)-V), wherein f is a pre-defined function which responds rapidly when the value of said current input pixel changes towards a background level in the scanned microfilm image but responds slower when the value of said current input pixel changes away from the background level.

25. The apparatus in claim 24 wherein said background value generating means further comprises:

means, responsive to the background values b and c, for determining a minimum value therebetween;

means for determining, in response to said minimum value and to the current input pixel value (pixel value V), a resulting value given by the pre-determined function of said minimum value and said current input pixel value;

means for combining said resulting value and said minimum value to yield the background value for the current pixel value;

means, responsive to the background value for the current input pixel value, for delaying the background value previously determined for the prior input pixel situated on a prior scan line and located immediately above the input pixel, by an interval of time equivalent to one scan line to yield said background value b; and

means, responsive to the background value for the current input pixel value, for delaying the background value previously determined for the most recent input pixel situated on a current scan line and located adjacent to said current input pixel, by an interval of time equivalent to one pixel interval to yield said background value c.

26. The apparatus in claim 25 wherein said resulting value determining means comprises a look up table, operative in response to said minimum value and said current input pixel value, for providing said resulting value.

27. The apparatus in claim 19 further comprising:

means, responsive to a stream of scanned pixel values that comprise said scanned microfilm image and are produced by a microfilm scanner, for compensating each pixel values in said stream of scanned pixel values for at least one pre-defined characteristic so as to produce a stream of compensated pixel values; and

means for averaging said stream of compensated pixel values on an odd/even basis to generate said plurality of incoming pixel values.

28. The apparatus in claim 27 wherein the compensating means further comprises:

means for compensating each of said scanned pixel values that comprise said scanned microfilm image for shading differences occurring in said microfilm scanner in order to produce a stream of shading corrected pixel values; and

means for correcting each pixel value in said stream of said shading corrected pixel values for at least one transfer characteristic of a film that is used to store said microfilm image in order to produce said stream of compensated pixel values.

29. The apparatus in claim 28 further comprising:

means, responsive to said plurality of pixel values, for deaveraging at least one pixel value in said plurality of incoming pixel values in order to produce a deaveraged pixel value.

30. The apparatus in claim 29 wherein said threshold level providing means comprises:

means, operative in response to said error signal and to said deaveraged pixel value, for providing a pre-defined pixel value as the threshold level for use by said comparing means; and

wherein said pre-defined pixel value providing means comprises a look up table, operative in response to said error signal and to said deaveraged pixel value, for producing said pre-defined pixel value.

31. The apparatus in claim 30 wherein said error signal producing means comprises means for generating said error signal as an algebraic combination of said background value provided by said background tracking means and said noise value provided by said noise processing means.

32. In an image processing system, apparatus for tracking a noise level appearing in a stream of pixel values and for filtering noise therefrom, wherein said pixel values collectively comprise a scanned image, said apparatus comprising:

means, responsive to a plurality of incoming pixel values, for convolving each one of a pre-defined group of said incoming pixel values with a corresponding one of a pre-defined set of coefficients in order to generate a convolved pixel value;

means, responsive to said convolved pixel value and to a threshold level, for comparing said convolved pixel value against said threshold level and for producing a thresholded pixel value indicative of the results of said comparison;

means, responsive to said thresholded pixel value, for producing a noise signal indicative of the presence of noise in said thresholded pixel value and for filtering the noise from said thresholded pixel value and producing a corresponding output pixel value;

means for producing a noise value in response to said noise signal;

means for producing an error signal in response to said noise value; and

means for providing said threshold level in response to said error signal.

33. The apparatus in claim 32 wherein said producing and filtering means comprises:

means for forming a moving window of a plurality of thresholded pixel values;

means, responsive to said moving window, for comparing said thresholded pixel values that form said moving window against at least one pre-determined noise pixel pattern to determine whether pixel noise exists amongst the thresholded pixel values that form said window and, in response thereto, producing said noise signal and for changing the value of a current one of said thresholded pixel values to remove any noise therein so as to form a filtered thresholded pixel value; and

means for providing said filtered thresholded pixel value as the output pixel value.

34. The apparatus in claim 33 wherein said noise value producing means comprises:

means, responsive to said noise signal, for generating a value representative of two dimensional noise content occurring in the moving window of thresholded pixel values wherein said two-dimensional noise content includes a first portion attributable to noise content in a vertical direction.

35. The apparatus in claim 34 wherein said two dimensional noise content value generating means comprises:

means, responsive to said noise signal and a first prior noise value, for producing said first portion as a first pre-defined function of a current thresholded pixel value and said first prior noise value, wherein said first prior noise value equals a value of said first pre-defined function for a corresponding thresholded pixel value associated with a pixel situated on a prior scan line and located immediately above a pixel associated with the current thresholded pixel value;

means, responsive to said first portion, for delaying said first portion previously determined for a corresponding thresholded pixel value associated with the pixel situated on a prior scan line and located immediately above the pixel associated with the current thresholded pixel, for an interval of time equivalent to a scan line so as to produce said first prior noise value;

means, responsive to said first portion and a second prior noise value, for producing said two dimensional noise content value as a second pre-defined function of said first portion and said second prior noise value, wherein said second prior noise value equals a value of said second pre-defined function for a most recent prior pixel situated on the present scan line; and

means, responsive to the two dimensional noise content value, for delaying said two dimensional noise content value previously determined for the most recent pixel by an interval of time equivalent to a pixel interval so as to produce said second prior noise value.

36. The apparatus in claim 35 wherein said first and second pre-defined functions are each exponentially decaying functions.

37. Apparatus for tracking a background level in a stream of incoming multi-bit pixel values, wherein said pixel values collectively form a scanned image, said apparatus comprising:

means, responsive to background values b and c, for determining a minimum value therebetween;

wherein background value b is the background value for a first incoming pixel value in said stream, wherein said first incoming pixel value is associated with a pixel situated on a prior scan line and located immediately above a current one of a plurality of incoming pixels, said current one incoming pixel having a pixel value V, and value c is the background value for a second incoming pixel value in said stream, wherein said second incoming pixel value is associated with a pixel situated on a current scan line and located adjacent to said current one of the incoming pixels;

means for determining, in response to said minimum value and to the current one incoming pixel value, a resulting value given by the pre-determined function of said minimum value and said current one incoming pixel value;

means for combining said resulting value and said minimum value to yield a background value for the current one incoming pixel value;

means, responsive to the background value for the current one incoming pixel value, for delaying the background value previously determined for the first incoming pixel value, by an interval of time equivalent to one scan line to yield said background value b; and

means, responsive to the background value for the current one incoming pixel value, for delaying the background value previously determined for the second incoming pixel value, by an interval of time equivalent to one pixel interval to yield said background value c.

38. The apparatus in claim 37 wherein said pre-determined function is given by:

min (b,c)-f(min (b,c)-V), where f is a pre-defined function which responds rapidly when the value of said current one incoming pixel changes towards a background level in the scanned image but responds slower when the value of said current one incoming pixel changes away from the background level.

39. The apparatus in claim 38 wherein said resulting value determining means comprises a look up table, operative in response to said minimum value and said current one incoming pixel value, for providing said resulting value.

40. In an image processing system, a method for enhancing and thresholding a stream of incoming multi-bit pixel values in order to produce output pixel values, wherein said incoming pixel values collectively comprise a scanned image, said method comprising the steps of:

convolving, in response to a plurality of incoming pixel values, each one of a pre-defined group of said incoming pixel values with a corresponding one of a pre-defined set of coefficients in order to generate a convolved pixel value;

comparing said convolved pixel value against a threshold level and for producing a thresholded pixel value indicative of the results of said comparison;

a noise processing step comprised of the steps of;

producing a noise value that tracks a noise level present in said thresholded pixel value and for filtering noise from said thresholded pixel value to form a filtered thresholded pixel value and,

producing, in response to said filtered thresholded pixel value, a corresponding output pixel value;

producing a background value that tracks a background level present in said incoming pixel values;

producing an error signal in response to said background value and said noise value; and

providing said threshold level in response to said error signal.

41. The method in claim 40 wherein said noise processing step comprises the steps of:

producing, in response to said thresholded pixel value, both said output pixel value and a noise signal indicative of the presence of said noise; and

producing said noise value in response to said noise signal.

42. The method in claim 41 wherein said output pixel value and noise signal producing step comprises the steps of:

forming a moving window of a plurality of thresholded pixel values;

comparing said thresholded pixel values that form said moving window against at least one pre-determined noise pixel pattern to determine whether pixel noise exists amongst the thresholded pixel values that form said window and, in response thereto, producing said noise signal and changing the value of a current one of said thresholded pixel values to remove any noise therein so as to form the filtered thresholded pixel value; and

providing said filtered thresholded pixel value as the output pixel value.

43. The method in claim 42 wherein said noise value producing step comprises the step of:

generating, in response to said noise signal, a value representative of two dimensional noise content occurring in the moving window of thresholded pixel values, wherein said two-dimensional noise content includes a first portion attributable to noise content in a vertical direction.

44. The method in claim 43 wherein said two dimensional noise content value generating step comprises the steps of:

producing, in response to said noise signal and a first prior noise value, said first portion as a first pre-defined function of a current thresholded pixel value and said first prior noise value, wherein said first prior noise value equals a value of said first pre-defined function for a corresponding thresholded pixel value associated with a pixel situated on a prior scan line and located immediately above a pixel associated with the current thresholded pixel value;

delaying said first portion previously determined for a corresponding thresholded pixel value associated with the pixel situated on a prior scan line and located immediately above the pixel associated with the current thresholded pixel value, for an interval of time equivalent to a scan line so as to produce said first prior noise value;

producing, in response to said first portion and a second prior noise value, said two dimensional noise content value as a second pre-defined function of said first portion and said second prior noise value, wherein said second prior noise value equals a value of said second pre-defined function for a most recent prior pixel situated on the present scan line; and

delaying said two dimensional noise content value previously determined for the most recent pixel by an interval of time equivalent to a pixel interval so as to produce said second prior noise value.

45. The method in claim 41 wherein said background value producing step comprises the step of:

generating said background value for a current input pixel (having a pixel value V) in response to a pre-determined function of the value of the current input pixel, a background value of a prior input pixel (background value b), wherein said prior input pixel is situated on a prior scan line and is located immediately above the current input pixel, and a background value of a most recent input pixel (background value c), wherein said most recent input pixel is situated on a current scan line and is located adjacent to said current input pixel; and

wherein said pre-determined function is given by;

min b,c)-f(min(b,c)-V), where f is a pre-defined function which responds rapidly when the value of said current input pixel changes towards a background level in the scanned image but responds slower when the value of said current input pixel changes away from the background level.

46. The method in claim 45 wherein said background value generating step further comprises the steps of:

determining, in response to the background values b and c, a minimum value therebetween;

determining, in response to said minimum value and to the current input pixel value (pixel value V), a resulting value given by the pre-determined function of said minimum value and said current input pixel value;

combining said resulting value and said minimum value to yield the background value for the current input pixel value;

delaying a background value previously determined for the prior input pixel situated on a prior scan line and located immediately above the current input pixel, by an interval of time equivalent to one scan line, to yield said background value b; and

delaying a background value previously determined for the most recent input pixel situated on a current scan line and located adjacent to said current input pixel, by an interval of time equivalent to one pixel interval, to yield said background interval c.

47. The method in claims 41 or 45 further comprising the steps of:

compensating each pixel value in a stream of scanned pixel values in said scanned image, for at least one pre-defined characteristic so as to produce a stream of compensated pixel values; and

averaging said stream of compensated pixel values on an odd/even basis to generate said plurality of incoming pixel values.

48. The method in claim 47 wherein the compensating step further comprises the steps of:

compensating each of said scanned pixel values that comprise said scanned image for shading differences occurring in said scanner in order to produce a stream of shading corrected pixel values; and

correcting each pixel value in said stream of shading corrected pixel values for at least one transfer characteristic of a medium that is used to store said image in order to produce said stream of compensated pixel values.

49. The method in claim 48 further comprising the step of:

deaveraging, in response to said plurality of incoming pixel values, at least one pixel value in said plurality of incoming pixel values in order to produce a deaveraged pixel value.

50. The method in claim 49 wherein said threshold level providing step comprises the step of:

providing, in response to said error signal and to said deaveraged pixel value, a pre-defined pixel value as the threshold level for use by said comparing step.

51. The method in claim 50 wherein said error signal producing step comprises the step of generating said error signal as an algebraic combination of said background value and said noise value.

52. In an image processing system, a method for tracking a noise level appearing in a stream of pixel values and for filtering noise therefrom, wherein said pixel values collectively comprise a scanned image, said method comprising the steps of:

convolving, in response to a plurality of incoming pixel values, each one of a pre-defined group of said incoming pixel values with a corresponding one of a pre-defined set of coefficients in order to generate a convolved pixel value;

comparing said convolved pixel value against a threshold level;

producing, in response to said comparing step, a thresholded pixel value indicative of the results of said comparison;

producing, in response to said thresholded pixel value, a noise signal indicative of the presence of noise in said thresholded pixel value;

filtering the noise from said thresholded pixel value to produce a filtered thresholded pixel value;

producing, in response to said filtered thresholded pixel value, a corresponding output pixel value;

producing a noise value in response to said noise signal;

producing an error signal in response to said noise value; and

providing said threshold level in response to said error signal.

53. The method in claim 52 wherein said producing and filtering step comprises the steps of:

forming a moving window of a plurality of thresholded pixel values;

comparing said thresholded pixel values that form said moving window against at least one pre-determined noise pixel pattern to determine whether pixel noise exists amongst the thresholded pixel values that form said window and, in response thereto, producing said noise signal and for changing the value of a current one of said thresholded pixel values to remove any noise therein so as to form the filtered thresholded pixel value; and

providing said filtered thresholded pixel value as the output pixel value.

54. The method in claim 53 wherein said noise signal producing step means further comprises the step of:

generating, in response to said noise signal, a value representative of two dimensional noise content occurring in said moving window of thresholded pixel values, wherein said two-dimensional noise content includes a first portion attributable to noise content in a vertical direction.

55. The method in claim 54 wherein said two dimensional noise content value generating step comprises the steps of:

producing, in response to said noise signal and a first prior noise value, said first portion as a first pre-defined function of a current thresholded pixel value and said first prior noise value, wherein said first prior noise value equals a value of said first pre-defined function for a corresponding thresholded pixel value associated with a pixel situated on a prior scan line and located immediately above a pixel associated with the current thresholded pixel value;

delaying said first portion previously determined for a corresponding thresholded pixel value associated with the pixel situated on a prior scan line and located immediately above the pixel associated with the current thresholded pixel value, for an interval of time equivalent to a scan line so as to produce said first prior noise value;

producing, in response to said first portion and a second prior noise value, said two dimensional noise content value as a second pre-defined function of said first portion and said second prior noise value, wherein said second prior noise value equals a value of said second pre-defined function for a most recent prior pixel situated on the present scan line; and

delaying said two dimensional noise content value previously determined for the most recent pixel by an interval of time equivalent to a pixel interval so as to produce said second prior noise value.

56. A method for tracking a background level in a stream of incoming multi-bit pixel values, wherein said pixel values collectively form a scanned image, said method comprising the steps of:

determining a minimum value between background values b and c;

wherein background value b is the background value for a first incoming pixel value in said stream, wherein said first incoming pixel value is associated with a pixel situated on a prior scan line and located immediately above a current one of a plurality of incoming pixels, said current one incoming pixel having a pixel value V, and value c is the background value for a second incoming pixel value in said stream, wherein said second incoming pixel value is associated with a pixel situated on a current scan line and located adjacent to said current one of the incoming pixels;

determining, in response to said minimum value and to the current one incoming pixel value, a resulting value given by the pre-determined function of said minimum value and said current one incoming pixel value;

combining said resulting value and said minimum value to yield a background value for the current one incoming pixel value;

delaying the background value previously determined for the first incoming pixel value, by an interval of time equivalent to one scan line to yield said background value b; and

delaying the background value previously determined for the second incoming pixel value, by an interval of time equivalent to one pixel interval to yield said background value c.

57. The method in claim 56 wherein said pre-determined function is given by:

min(b,c)-f(min(b,c)-V), where f is a pre-defined function which responds rapidly when the value of said current one incoming pixel changes towards a background level in the scanned image but responds slower when the value of said current one incoming pixel changes away from the background level.