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EP0187911A2 Expired / Lapsed EP ⏱️ 0.0 years remaining

Method and apparatus for thresholding image data

Assignee: International Business Machines Corp
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Filing: 1985-11-12
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Granted: —
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Expires: 2005-11-12
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Abstract

Abstract A system is described for thresholding graphics images to reduce the information content for transmission and reproduction and which uses a particular image data thresholding technique that may be used to obtain display images with a number of intensity levels. Firstly, an histogram of the image to be encoded is generated and analyzed to determine essential parameters. In particular, the background colour of the image is determined by summing the histogram elements to find the median (B) of the distribution, that is, the point at which the half sum of total pixels is reached. The extrema (A. G) of the histogram are calculated and the skew is determined from the position of the median (B) with respect thereto. If the difference between the median (B) and the black extreme (G) is less than half the difference between the median (B) and the white extreme (A) then the background of the image is deemed to be black, otherwise, the image will be defined as having a white background. After the background colour is determined, the width of the background is determined from the difference between the median (B) and the background colour extreme (A), and the edge between the background and foreground regions, is defined as being at a distance one-half the background width from the median (B) in the direction of the foreground colour extreme (G). A sequence of threshold levels (D, E, F) may then be determined using this edge (C) as a base, by setting them at points disposed at fractionally different distances between the edge (C) and the foreground colour extreme (G). Preferably the levels are set at points 1/8, 1/4 and 1/2 the defined distance or at 3/16, 3/8 and 3/4. The selected points will respectively define the threshold levels such as between white/gray-white, gray-white/gray-black, and gray-black/black. Thus, the resulting image may then, after encosing and transmission, be decoded and displayed on a suitable teleconferencing system or fascimile machine with four intensity levels.

Inventors

  • 👤 William Boone Pennebaker

Bibliographic Details

Application Number —
Jurisdiction EP
Data Source google_patents
Last Synchronized 2026-11-03 15:28
CPC Classifications
H; H04; H04N; H04N1/00; H04N1/40; H04N1/407; H04N1/4072; H04N1/4074; G; G06; G06T; G06T7/00; G06T7/10; G06T7/11; G06T7/136; G06V; G06V10/00; G06V10/20; G06V10/28

Claims Analysis

Total Claims: 12
Claim 1
Independent Claim 176 words
A method for thresholding image data comprising the steps of: generating a histogram of an image; computing a median (B) of the distribution of the image by summing the histogram elements until the half sum of total pixels is reached; computing the extremes (A, G) of said distribution; determining the skew of said distribution from the relative position of said median (B) with respect to said extremes - (A, G) to determine the colour of the image background; defining the edge between background and foreground regions of the image by determining a point (C) located between the median (B) and the foreground colour extreme (G) a distance equal to half the distance between the median (B) and the background colour extreme (A), which edge point is used as a base for calculating the sequence of threshold levels; and determining a sequence of threshold levels (d, E, F) in a foreground region of said histogram by establishing a plurality of points differentially spaced along the distance between said edge point (C) and said foreground colour extreme (G).
Claim 2
Independent Claim 176 words
A method for thresholding image data comprising the steps of: generating a histogram of an image; computing a median (B) of the distribution of the image by summing the histogram elements until the half sum of total pixels is reached; computing the extremes (A, G) of said distribution; determining the skew of said distribution from the relative position of said median (B) with respect to said extremes - (A, G) to determine the colour of the image background; defining the edge between background and foreground regions of the image by determining a point (C) located between the median (B) and the foreground colour extreme (G) a distance equal to half the distance between the median (B) and the background colour extreme (A), which edge point is used as a base for calculating the sequence of threshold levels; and determining a sequence of threshold levels (d, E, F) in a foreground region of said histogram by establishing a plurality of points differentially spaced along the distance between said edge point (C) and said foreground colour extreme (G).
Claim 3
Dependent Claim 59 words
Method as in claim 1 wherein the step of computing the extremes (A, G) of said distribution is carried out by summing the histogram elements from each end of the intensity range until a small but significant number is found and then scanning back toward the respective intensity range ends until respective zero elements are found in the histogram.
Claim 4
Dependent Claim 24 words
Method as in claim 1 wherein said C plurality of points - (D, E, F) are spaced 1/8, 1/4, and 1/2 of said distance.
Claim 5
Dependent Claim 24 words
Method as in claim 1 wherein said C plurality of points - (D, E, F) are spaced 3/16, 3/8, and 3/4 of said distance.
Claim 6
Dependent Claim 52 words
Method as in claim 1 wherein the step of determining the skew of said distribution comprises measuring the lengths of the respective distances between said median (B) and said extremes (A, G) and then determining whether the length of one distance is less than one half the length of the other distance.
Claim 7
Dependent Claim 186 words
Apparatus for thresholding image data comprising: means for generating a histogram of an image; means for computing a median (B) of the distribution of the image by summing the histogram elements until the half sum of total pixels is reached; means for computing the extremes (A, G) of said distribution means for determining the skew of said distribution from the relative position of said median - (B) with respect to said extremes (A, G) to determine the colour of the image background; means for defining the edge between background and foreground regions of the image by determining a point (C) located between the median (b) and the foreground colour extreme (G) a distance equal to half the distance between the median (B) and the background colour extreme (A), which edge point (C) is used as a base for calculating the sequence of threshold levels (D, E, F); and means for determining a sequence of threshold levels (D, E, F) in a foreground region of said histogram by establishing a plurality of points differentially spaced along the distance between said edge point and said background colour extreme.
Claim 8
Dependent Claim 186 words
Apparatus for thresholding image data comprising: means for generating a histogram of an image; means for computing a median (B) of the distribution of the image by summing the histogram elements until the half sum of total pixels is reached; means for computing the extremes (A, G) of said distribution means for determining the skew of said distribution from the relative position of said median - (B) with respect to said extremes (A, G) to determine the colour of the image background; means for defining the edge between background and foreground regions of the image by determining a point (C) located between the median (b) and the foreground colour extreme (G) a distance equal to half the distance between the median (B) and the background colour extreme (A), which edge point (C) is used as a base for calculating the sequence of threshold levels (D, E, F); and means for determining a sequence of threshold levels (D, E, F) in a foreground region of said histogram by establishing a plurality of points differentially spaced along the distance between said edge point and said background colour extreme.
Claim 9
Dependent Claim 62 words
Apparatus as in claim 6 wherein said means for computing the extremes (A, G) of said distribution comprises means for summing the histogram elements (A, G) from each end of the intensity range until a small but significant number is found and then scanning back toward the respective ends of said intensity range until respective zero elements are found in the histogram.
Claim 10
Dependent Claim 23 words
Apparatus as in claim 6 wherein said C plurality of points (D, E, F) are spaced 1/8, 1/4, and 1/2 of said distance.
Claim 11
Dependent Claim 23 words
Apparatus as in claim 6 wherein said plurality of points - (D, E, F) are spaced 3/16, 3/8, and 3/4 of said distance.
Claim 12
Dependent Claim 55 words
Apparatus as In claim 6 wherein said means for determining the skew of said distribution comprises means for measuring the lengths of the respective distances between said median (B) and said extremes (A, G) and means for determining whether the length of one distance is less than one half the length of the other distance.

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