EP0544511A2
Expired / Lapsed
EP
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Halftoning with enhanced dynamic range and edge enhanced error diffusion
Assignee: Xerox Corp
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Filing: 1992-11-25
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Granted: —
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Expires: 2012-11-25
Abstract
Abstract Gray pixel values, where the value of the pixel has one of a relatively large number of input levels, are directed through an error diffusion quantization process, for the conversion to one of a relatively small number of values. Each pixel of the gray level data (18) is modified (10) by a correspondingly weighted error correction term (20,22,24,28) or terms from previously processed pixels, generating a modified pixel value (12). This modified pixel value is compared (14) to a threshold value, the threshold value determined adaptively (15), based on 1) a dither pattern, having a dynamic range greater than 1, and 2) the value of the pixel to be thresholded, and optionally on its predetermined neighborhood, multiplied by a value representing an edge enhancement factor. The difference value between the modified pixel value (12) and the output pixel value (16) is distributed in accordance with a weighted distribution scheme to a set of neighboring unprocessed pixels, increasing or decreasing the gray values of such pixels.
Inventors
- 👤 Reiner Eschbach
Bibliographic Details
Application Number
—
Jurisdiction
EP
Data Source
google_patents
Last Synchronized
2026-11-09 12:59
CPC Classifications
H; H04; H04N; H04N1/00; H04N1/40; H04N1/405; H04N1/4051; H04N1/4052; H04N1/4053
Claims Analysis
Total Claims: 10
Claim 1
Independent Claim
249 words
A method of quantizing pixel values in an original image formed by a plurality of pixels, each pixel representing an optical density of the image at a location within the image, and having an original optical density value associated therewith selected from one of a set of c original optical density values that has a number of members larger than a desired output set of d optical density values, and having a dynamic range associated therewith, the steps comprising: for each pixel, adding a previously determined error term to the original optical density value thereof, to derive a modified pixel value; determining for the pixel to be quantized, a threshold level that is a function of the original optical density of the pixel, and optionally its predetermined neighborhood, to be quantized, and a threshold value selected from a set of threshold values having a dynamic range that is greater than that of the original image; applying the determined threshold level to each optical density value of each pixel in the image, to produce a output optical density value that is a member of the desired output set, each member of the output set of d optical density values a legal output value; determining an error term that is a difference between the output optical density value and the modified optical density value of the pixel; applying a proportional amount of the determined error term to the original optical density value of each of a predetermined set of neighboring pixels.
Claim 2
Dependent Claim
18 words
The method as defined in claim 1, wherein the the set of threshold values describes a halftone cell.
Claim 3
Dependent Claim
63 words
The method as defined in claim 1, wherein the function determining the threshold level is the difference between a product of the original optical density value of the pixel to be quantized and a selected enhancement factor K -1, and a threshold value selected from the set of threshold values having a dynamic range that is greater than that of the original image.
Claim 4
Dependent Claim
30 words
The method as defined in claim 3, wherein the enhancement factor K is selected to be greater than or equal to the dynamic range of the set of threshold values.
Claim 5
Dependent Claim
30 words
The method as defined in claim 1, where the dynamic range of the set of threshold values is between 4 to 8 times the dynamic range of the original image.
Claim 6
Dependent Claim
305 words
An arrangement for quantizing pixel values in an image formed by a plurality of pixels, each pixel representing an optical density of the image at a location within the image, and having an original optical density value associated therewith selected from one of a set of c original optical density values that has a number of members larger than a desired output set of d optical density values, and having a dynamic range associated therewith defined as 1, the quantizing arrangement comprising: means for inputting an image pixel to be quantized having an original optical density value; means for adding an error term derived from the quantization of at least one previous pixel to the original optical density value to produce a modified image pixel; means for determining for the pixel to be quantized, a threshold level, that is a function of the original optical density of the pixel, and optionally its predetermined neighborhood, to be quantized and a threshold value that is selected from a set of threshold values having a dynamic range that is greater than that of the original image; means for thresholding the optical density value of each modified pixel in the image with the threshold level determined at said determining means, to produce a new optical density value that is a member of the desired output value set, each member of the output set of d new optical density values a legal output value which may be represented by an output device; means for determining an error term that is a difference between the new optical density value and the modified optical density value; means for storing a set of weighted error terms to be applied to each original optical density value in a predetermined set of neighboring pixels, each weighted error term a preselected portion of the error term.
Claim 7
Dependent Claim
18 words
The arrangement as defined in claim 6, wherein the the set of threshold values describes a halftone cell.
Claim 8
Dependent Claim
62 words
The arrangement as defined in claim 6, wherein the function determining the threshold level is a difference between a product of the original optical density value of the pixel to be quantized and a selected enhancement factor K-1, and the threshold value selected from a set of threshold values having a dynamic range that is greater than that of the original image.
Claim 9
Dependent Claim
30 words
The arrangement as defined in claim 8, wherein the enhancement factor K is selected to be greater than or equal to the dynamic range of the set of threshold values.
Claim 10
Dependent Claim
30 words
The arrangement as defined in claim 6, where the dynamic range of the set of threshold values is between 4 to 8 times the dynamic range of the original image.
References Cited (Backward Citations)
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