EP0647058A2
Expired / Lapsed
EP
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Digital halftone rendering of a gray scale image with frequency-dependent diagonal correlation
Assignee: HP Inc
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Filing: 1994-09-09
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Granted: —
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Expires: 2014-09-09
Abstract
Abstract A method of halftoning a digital gray scale image is disclosed that utilizes a point by point thresholding comparison to a novel diagonal correlation dither matrix. The new dither matrix forces diagonal correlation of adjacent dots in the output image while maximizing dispersion of dots, thereby producing visually unobtrusive output dot patterns. The matrix is generated according to a spatial domain cost function that determines a cost value for each candidate pixel based on respective radial distances and relative angles between a candidate pixel and the ON pixels in the matrix such that unit diagonals are favored over placement of vertically or horizontally adjacent dots.
Inventors
- 👤 David C. Barton
Bibliographic Details
Application Number
—
Jurisdiction
EP
Data Source
google_patents
Last Synchronized
2026-11-09 13:31
CPC Classifications
H; H04; H04N; H04N1/00; H04N1/40; H04N1/405; H04N1/4051
Claims Analysis
Total Claims: 10
Claim 1
Independent Claim
58 words
A method of halftoning a digital gray scale image ([F(i,j)] by utilizing a point by point thresholding comparison (28) to a predetermined diagonal correlation dither matrix [M(i,j)] in which the dither matrix is substantially comprised of a function that forces diagonal correlation of adjacent dots (C1,P10) while maximizing dispersion of dots, thereby producing visually unobtrusive output dot patterns.
Claim 2
Dependent Claim
159 words
A method according to claim 1 wherein the dither matrix is generated, for each desired tone level, by a process comprising the steps of: providing a null matrix (M) of pixel locations having a predetermined size; initializing the null matrix (52) to a predetermined initial state corresponding to a minimum tone level: identifying as candidate pixels (58) all of the OFF pixels in the matrix; determining a cost value (60) for each candidate pixel based on respective radial distances and relative angles between the candidate pixel and the ON pixels in the matrix; comparing the respective cost values of the candidate pixels (78) to determine a minimum cost value; selecting (78) one of the candidate pixels that has the minimum cost value: turning ON the selected candidate pixel (80) in the matrix; and repeating (84) said identifying, determining, comparing, selecting and turning ON steps until the number of ON pixels in the matrix corresponds to the desired tone level.
Claim 3
Dependent Claim
52 words
A method according to claim 2 wherein said determining a cost value includes: selecting a search area (r) surrounding the candidate pixel; determining an individual cost value (110) based on each ON pixel (106) within the search area (108); and summing the individual cost values (112) to determine the total cost value.
Claim 4
Dependent Claim
29 words
A method according to claim 3 wherein selecting the search area includes sizing the search area so as to include an area inversely related to the desired tone value.
Claim 5
Dependent Claim
34 words
A method according to claim 3 wherein selecting the search area includes sizing the search area so as to encompass at least a predetermined number of ON pixels greater than about 10-15 ON pixels.
Claim 6
Dependent Claim
44 words
A method according to claim 3 wherein selecting the search area includes increasing the size of the search area for low tone values so as to reduce human visual detection of low frequency artifacts among dots in output regions of constant low tone value.
Claim 7
Dependent Claim
45 words
A method according to claim 3 wherein selecting the search area includes increasing the size of the search area for high tone values so as to reduce human visual detection of low frequency artifacts among white spaces in output regions of constant high tone value.
Claim 8
Independent Claim
81 words
A method according to claims 3,4,5,6 or 7 wherein said determining the individual cost value includes adjusting a radial distance between the candidate pixel and each ON pixel by a function of the radial distance and a relative angle between the candidate pixel and each ON pixel, the adjusted radial distance having a peak where the relative angle equals approximately 45 degrees, thereby favoring candidate pixels diagonally adjacent to ON dots over candidate pixels vertically or horizontally adjacent to ON dots.
Claim 9
Independent Claim
84 words
A method according to claims 3,4,5,6 or 7 wherein the cost value is determined according to a cost function (FnC) that comprises exponentiating the adjusted radial distance to at least the third power so that the cost value is strongly influenced by the actual radial distance between the candidate pixel and the ON dots, whereby clumps of dots have an influence on the cost value approximately equal to the cost imposed by a dot in the clump that is closest to the candidate pixel.
Claim 10
Independent Claim
59 words
A method according to claims 3,4,5,6 or 7 further comprising compensating for a target output system in which dots that are printed within a predetermined minimum radial distance touch greater than one unit one another, by adjusting the actual radial distance r, so as to form an adjusted radial distance that varies between r and the minimum radial distance.
References Cited (Backward Citations)
No citation records indexed.
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