Halftoning with gradient-based selection of dither matrices
First Claim
1. A method for obtaining halftone image data from corresponding gray scale image data, the gray scale image data being representative of pixels in an image and arbitrarily including values representative of both highlights and midtones for the image, said method comprising the steps of:
- calculating a local tonality gradient of gray scale image data for a target pixel, the tonality gradient being calculated based on a comparison between at least the gray scale image data for the target pixel and gray scale image data for a pixel adjacent the target pixel;
selecting one dither matrix from among plural dither matrices based on the calculated tonality gradient, each of the plural dither matrices containing threshold values which differ from others of the plural dither matrices; and
thresholding the gray scale image data for the target pixel using the selected dither matrix so as to obtain halftone image data for the target pixel.
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Abstract
Halftoning gray scale image data includes selection of one of plural dither matrices based on an analysis of local tonality gradient for each target pixel. Plural different but reciprocally compatible dither matrices are defined, and for each target pixel in the gray scale image data, a local gradient for tonality is calculated. One of the dither matrices is selected in accordance with the calculated gradient, and the target pixel is thresholded using the selected dither matrix. Enhanced dithering according to the invention improves smoothness of gradations present in the invention, diminishes the "banding" or "contouring" effect found in natural images, and improves visual appearance and stability of business graphics, particularly in cases where a complex image contains both natural and computer generated business graphics.
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Citations
25 Claims
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1. A method for obtaining halftone image data from corresponding gray scale image data, the gray scale image data being representative of pixels in an image and arbitrarily including values representative of both highlights and midtones for the image, said method comprising the steps of:
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calculating a local tonality gradient of gray scale image data for a target pixel, the tonality gradient being calculated based on a comparison between at least the gray scale image data for the target pixel and gray scale image data for a pixel adjacent the target pixel; selecting one dither matrix from among plural dither matrices based on the calculated tonality gradient, each of the plural dither matrices containing threshold values which differ from others of the plural dither matrices; and thresholding the gray scale image data for the target pixel using the selected dither matrix so as to obtain halftone image data for the target pixel. - View Dependent Claims (2, 3, 4, 5)
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6. A halftone printing method for printing a halftone image corresponding to a gray scale image, comprising the steps of:
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accepting a selection to halftone according to error diffusion or according to dithering; halftoning by error diffusion in a case where a selection to halftone according to error diffusion is selected; halftoning by dithering in a case where a selection to halftone according to dithering is selected; wherein in a case where halftoning according to error diffusion is selected (a) a local tonality gradient of gray scale image data for a target pixel is calculated based on a comparison between at least the gray scale image data for the target pixel and gray scale image data for a pixel adjacent the target pixel, (b) one dither matrix is selected from among plural dither matrices based on the calculated tonality gradient, each of the plural dither matrices containing threshold values which differ from others of the plural dither matrices, and (c) the gray scale image data is thresholded for the target pixel using the selected dither matrix so as to obtain halftone image data for the target pixel. - View Dependent Claims (7, 8, 9, 10)
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11. An apparatus for obtaining halftone image data from corresponding gray scale image data, the gray scale image data being representative of pixels in an image and arbitrarily including values representative of both highlights and midtones for the image, said apparatus comprising:
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a memory for storing computer executable process steps, and for storing plural dither matrices, each of the plural dither matrices containing threshold values which differ from others of the plural dither matrices; and a processor for executing the process steps stored in said memory; wherein said process steps include steps to (a) calculate a local tonality gradient of gray scale image data for a target pixel, the tonality gradient being calculated based on a comparison between at least the gray scale image data for the target pixel and gray scale image data for a pixel adjacent the target pixel, (b) select one dither matrix from among plural dither matrices based on the calculated tonality gradient, and (d) threshold the gray scale image data for the target pixel using the selected dither matrix so as to obtain halftone image data for the target pixel. - View Dependent Claims (12, 13, 14, 15)
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16. A halftone printing apparatus for printing a halftone image corresponding to a gray scale image, comprising:
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accepting means for accepting a selection to halftone according to error diffusion or according to dithering; halftoning means for halftoning by error diffusion in a case where a selection to halftone according to error diffusion is selected; halftoning means for halftoning by dithering in a case where a selection to halftone according to dithering is selected; wherein, in a case where halftoning according to dithering is selected, said process steps include steps to (a) calculate a local tonality gradient of gray scale image data for a target pixel, the tonality gradient being calculated based on a comparison between at least the gray scale image data for the target pixel and gray scale image data for a pixel adjacent the target pixel, (b) select one dither matrix from among plural dither matrices based on the calculated tonality gradient, each of the plural dither matrices containing threshold values which differ from others of the plural dither matrices, and (c) threshold the gray scale image data for the target pixel using the selected dither matrix so as to obtain halftone image data for the target pixel. - View Dependent Claims (17, 18, 19, 20)
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21. Computer-executable process steps stored on a computer readable medium, said process steps for obtaining halftone image data from corresponding gray scale image data, the gray scale image data being representative of pixels in an image and arbitrarily including values representative of both highlights and midtones for the image, said process steps comprising:
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a calculating step to calculate a local tonality gradient of gray scale image data for a target pixel, the tonality gradient being calculated based on a comparison between at least the gray scale image data for the target pixel and gray scale image data for a pixel adjacent the target pixel; a selecting step to select one dither matrix from among plural dither matrices based on the calculated tonality gradient, each of the plural dither matrices containing threshold values which differ from others of the plural dither matrices; and a thresholding step to threshold the gray scale image data for the target pixel using the selected dither matrix so as to obtain halftone image data for the target pixel. - View Dependent Claims (22, 23, 24, 25)
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Specification