Method of and apparatus for correcting halftone-dot image data, and method of correcting halftone-dot threshold data
First Claim
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1. A method of correcting halftone-dot image data, comprising the steps of:
- converting halftone-dot image data in a position space into data in a frequency space;
extracting data including low-frequency noise components whose frequency is lower than the fundamental frequency component of halftone dots from said data in the frequency space;
inversely converting the extracted data including the frequency noise components into image data in the position space; and
correcting pixels of said halftone-dot image data in positions which correspond to pixels including the noise components in the inversely converted image data such that the noise components will be reduced.
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Abstract
Halftone-dot image data in a position space is converted into data in a frequency space by fast Fourier transform unit, and data including low-frequency noise components whose frequency is lower than the fundamental frequency component of halftone dots is extracted from the data in the frequency space by a low-pass filter. The extracted data including the frequency noise components is inversely converted into image data in the position space by an inverse fast Fourier transform unit. Pixels of the halftone-dot image data in positions which correspond to pixels including the noise components in the inversely converted image data are corrected by a corrector such that the noise components will be reduced. The correcting process is in the position space, and can simply be carried out. A binary halftone-dot image generated from the corrected halftone-dot image data is substantially free of any moire pattern which would otherwise be caused by an interference between the output resolution of an image output device and the frequency of halftone dots.
47 Citations
22 Claims
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1. A method of correcting halftone-dot image data, comprising the steps of:
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converting halftone-dot image data in a position space into data in a frequency space;
extracting data including low-frequency noise components whose frequency is lower than the fundamental frequency component of halftone dots from said data in the frequency space;
inversely converting the extracted data including the frequency noise components into image data in the position space; and
correcting pixels of said halftone-dot image data in positions which correspond to pixels including the noise components in the inversely converted image data such that the noise components will be reduced. - View Dependent Claims (2, 3, 4, 5)
calculating the amount of exposure for each of the pixels of the halftone-dot image data in the position space on the basis of the shape of dots to be recorded on a recording material with a recording beam that is modulated based on the halftone-dot image data in the position space, and the diameter of the recording beam, referring to the density characteristics of the recording material on the basis of the calculated amount of exposure for each of the pixels, thereby determining the density for each of the halftone dots, and converting halftone-dot density data represented by the determined density for each of the halftone dots into data in the frequency space.
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3. A method according to claim 1, wherein said step of extracting data including low-frequency noise components, comprises the step of:
weighting the data including the low-frequency noise components with human vision characteristics.
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4. A method according to claim 2, wherein said step of extracting data including low-frequency noise components, comprises the step of:
weighting the data including the low-frequency noise components with human vision characteristics.
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5. A method according to claim 1, wherein said halftone-dot image data comprises binary halftone-dot image data, and said step of correcting pixels of said halftone-dot image data in positions which correspond to pixels including the noise components in the inversely converted image data such that the noise components will be reduced, comprises the steps of:
extracting a pair of a peak pixel at a maximum value of density and a peak pixel at a minimum value of density from the noise components in the inversely converted image data, and inverting one of a pair of pixels in said halftone-dot image data in positions corresponding to the pair of peak pixels from a blackened pixel to a non-blackened pixel and the other pixel from a non-blackened pixel to a blackened pixel, thereby to correct said halftone-dot image data.
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6. An apparatus for correcting halftone-dot image data, comprising:
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frequency converting means for converting halftone-dot image data in a position space into data in a frequency space;
a low-pass filter for extracting data including low-frequency noise components whose frequency is lower than the fundamental frequency component of halftone dots from said data in the frequency space;
inverse frequency converting means for inversely converting the extracted data including the frequency noise components into image data in the position space; and
correcting means for correcting pixels of said halftone-dot image data in positions which correspond to pixels including the noise components in the inversely converted image data such that the noise components will be reduced. - View Dependent Claims (7, 8)
means for extracting a pair of a peak pixel at a maximum value of density and a peak pixel at a minimum value of density from the noise components in the inversely converted image data, and inverting one of a pair of pixels in said halftone-dot image data in positions corresponding to the pair of peak pixels from a blackened pixel to a non-blackened pixel and the other pixel from a non-blackened pixel to a blackened pixel, thereby to correct said halftone-dot image data.
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9. A method of correcting halftone-dot threshold data to correct the position of threshold values in halftone-dot threshold data which takes a screen ruling, a screen angle, and a dot shape into account and which takes a predetermined numerical range, comprising the steps of:
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(a) setting a threshold correcting range to correct the position of threshold values, within said numerical range represented by threshold values of said halftone-dot threshold data;
(b) comparing a central value of said threshold correcting range and each of the threshold values of said halftone-dot threshold data to generate halftone-dot image data;
(c) converting the halftone-dot image data in a position space into data in a frequency space;
(d) extracting data including low-frequency noise components whose frequency is lower than the fundamental frequency component of halftone dots from said data in the frequency space;
(e) inversely converting the extracted data including the frequency noise components into image data in the position space; and
(f) selecting a pair of threshold values to be replaced in said halftone-dot threshold data and replacing the selected threshold values to generate halftone-dot threshold data where the position of threshold values is corrected;
said step (f) comprising the steps of;
regarding one of the pair of threshold values as a threshold value in said halftone-dot threshold data in a position corresponding to a pixel having a minimum low-frequency noise component among the image data inversely converted in said step (e), in a region of non-blackened pixels of said halftone-dot image data produced in said step (b), on the assumption that threshold values in said halftone-dot threshold data which correspond to said non-blackened pixels are threshold values in said threshold correcting range; and
regarding the other of the pair of threshold values as a threshold value in said halftone-dot threshold data in a position corresponding to a pixel having a maximum low-frequency noise component among the image data inversely converted in said step (e), in a region of non-blackened pixels of said halftone-dot image data produced in said step (b), on the assumption that threshold values in said halftone-dot threshold data which correspond to said blackened pixels are threshold values in said threshold correcting range. - View Dependent Claims (10, 11, 12, 13, 14, 15, 16)
calculating the amount of exposure for each of the pixels of the halftone-dot image data in the position space on the basis of the shape of dots to be recorded on a recording material with a recording beam that is modulated based on the halftone-dot image data in the position space, and the diameter of the recording beam, referring to the density characteristics of the recording material on the basis of the calculated amount of exposure for each of the pixels, thereby determining the density for each of the halftone dots, and converting halftone-dot density data represented by the determined density for each of the halftone dots into data in the frequency space.
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11. A method according to claim 9, wherein said step (d) comprises the step of:
weighting the data including the low-frequency noise components with human vision characteristics.
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12. A method according to claim 10, wherein said step (d) comprises the step of:
weighting the data including the low-frequency noise components with human vision characteristics.
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13. A method according to claim 9, further comprising the steps of:
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(g) after said step (f), changing said threshold correcting range in said step (a) to a new threshold correcting range having a minimum value of said threshold correcting range in said step (b) as a minimum value thereof and said central value of said threshold correcting range in said step (b) as a maximum value thereof, and a new threshold correcting range having said central value of said threshold correcting range in said step (b) as a minimum value thereof and a maximum value of said threshold correcting range in said step (b) as a maximum value thereof;
said step (b) comprising the step of, after said step (g), comparing central values of the respective new threshold correcting ranges with each of the thresholds of the halftone-dot threshold data corrected in said step (f) thereby to generate the halftone-dot image data; and
(h) repeating said steps (b), (c), (d), (e), (f), and (g) until any pair of threshold values replaceable in said step (f) is no longer available, for thereby producing final corrected halftone-dot threshold data.
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14. A method according to claim 10, further comprising the steps of:
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(g) after said step (f), changing said threshold correcting range in said step (a) to a new threshold correcting range having a minimum value of said threshold correcting range in said step (b) as a minimum value thereof and said central value of said threshold correcting range in said step (b) as a maximum value thereof, and a new threshold correcting range having said central value of said threshold correcting range in said step (b) as a minimum value thereof and a maximum value of said threshold correcting range in said step (b) as a maximum value thereof;
said step (b) comprising the step of, after said step (g), comparing central values of the respective new threshold correcting ranges with each of the thresholds of the halftone-dot threshold data corrected in said step (f) thereby to generate the halftone-dot image data; and
(h) repeating said steps (b), (c), (d), (e), (f), and (g) until any pair of threshold values replaceable in said step (f) is no longer available, for thereby producing final corrected halftone-dot threshold data.
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15. A method according to claim 11, further comprising the steps of:
- (g) after said step (f), changing said threshold correcting range in said step (a) to a new threshold correcting range having a minimum value of said threshold correcting range in said step (b) as a minimum value thereof and said central value of said threshold correcting range in said step (b) as a maximum value thereof, and a new threshold correcting range having said central value of said threshold correcting range in said step (b) as a minimum value thereof and a maximum value of said threshold correcting range in said step (b) as a maximum value thereof;
said step (b) comprising the step of, after said step (g), comparing central values of the respective new threshold correcting ranges with each of the thresholds of the halftone-dot threshold data corrected in said step (f) thereby to generate the halftone-dot image data; and
(h) repeating said steps (b), (c), (d), (e), (f), and (g) until any pair of threshold values replaceable in said step (f) is no longer available, for thereby producing final corrected halftone-dot threshold data.
- (g) after said step (f), changing said threshold correcting range in said step (a) to a new threshold correcting range having a minimum value of said threshold correcting range in said step (b) as a minimum value thereof and said central value of said threshold correcting range in said step (b) as a maximum value thereof, and a new threshold correcting range having said central value of said threshold correcting range in said step (b) as a minimum value thereof and a maximum value of said threshold correcting range in said step (b) as a maximum value thereof;
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16. A method according to claim 12, further comprising the steps of:
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(g) after said step (f), changing said threshold correcting range in said step (a) to a new threshold correcting range having a minimum value of said threshold correcting range in said step (b) as a minimum value thereof and said central value of said threshold correcting range in said step (b) as a maximum value thereof, and a new threshold correcting range having said central value of said threshold correcting range in said step (b) as a minimum value thereof and a maximum value of said threshold correcting range in said step (b) as a maximum value thereof;
said step (b) comprising the step of, after said step (g), comparing central values of the respective new threshold correcting ranges with each of the thresholds of the halftone-dot threshold data corrected in said step (f) thereby to generate the halftone-dot image data; and
(h) repeating said steps (b), (c), (d), (e), (f), and (g) until any pair of threshold values replaceable in said step (f) is no longer available, for thereby producing final corrected halftone-dot threshold data.
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17. A method of correcting threshold values in threshold matrix data used in obtaining halftone-dot image data by processing continuous-tone image data to be binary or multivalued data, comprising the steps of:
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preparing threshold values in threshold matrix data in a conventional and arbitrary method for generating halftone-dot gradation data having a desired screen ruling, screen angle, and dot shape;
converting halftone-dot image data in a position space generated in a predetermined gradation value into data in a frequency space;
extracting data including low-frequency noise components whose frequency is lower than the fundamental frequency component of halftone dots from said data in the frequency space;
inversely converting the extracted data including the low-frequency noise components into image data in the position space; and
correcting said threshold values for correcting pixels of said halftone-dot image data in positions which correspond to pixels including the noise components in the inversely converted image data such that the noise components will be reduced. - View Dependent Claims (18, 19, 20, 21, 22)
reducing the noise components by exchanging two of said threshold values with each other.
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20. A method according to claim 17, wherein said step of converting halftone-dot image data in a position space generated in a predetermined gradation value into data in a frequency space comprises the steps of:
calculating the amount of exposure for each of the pixels of the halftone-dot image data in the position space on the basis of the shape of dots to be recorded on a recording material with a recording beam that is modulated based on the halftone-dot image data in the position space, and the diameter of the recording beam, referring to the density characteristics of the recording material on the basis of the calculated amount of exposure for each of the pixels, thereby determining the density for each of the halftone dots, and converting halftone-dot density data represented by the determined density for each of the halftone dots into data in the frequency space.
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21. A method according to claim 17, wherein said step of extracting data including low-frequency noise components in the frequency space comprises the step of:
weighting the data including the low-frequency noise components with human vision characteristics.
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22. A method according to claim 20, wherein said step of extracting data including low-frequency noise components in the frequency space comprises the step of:
weighting the data including the low-frequency noise components with human vision characteristics.
Specification
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Current AssigneeFujifilm Corporation (Fujifilm Holdings Corporation)
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Original AssigneeFuji Photo Film Co Limited (Fujifilm Holdings Corporation)
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InventorsInoue, Yoshiaki
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Primary Examiner(s)Rogers, Scott
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Assistant Examiner(s)Behpour, Gholam A.
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Application NumberUS09/165,805Time in Patent Office830 DaysField of Search358/1.9, 358/454, 358/533, 358/298, 358/299, 358/463, 282/275, 282/456, 382/264, 382/260, 382/261US Class Current358/3.06CPC Class CodesH04N 1/4051 producing a dispersed dots ...
