Method and apparatus for implementing accurate angle screens
First Claim
1. A color document printing system for eliminating moire in printed output, the color document including a plurality of color separations wherein each separation is defined with a set of image signals representing optical density with m possible density levels, comprising:
- a printer, capable of rendering density with n density levels, adapted to print the color separations at distinct spatial resolutions, so that when the color separations are superposed on a substrate, a selected color is defined;
a source of image signals describing the color document with a plurality of color separations, each image signal representing optical density as one of m levels for a discrete area of a color separation;
a spatial resolution scaler, operatively connected to said source of image signals;
a first halftone processor, operatively connected to said spatial resolution scaler, for reducing the number of levels m representing optical density in a subset of the color separations to a number of levels n representing optical density, said processor generating a first periodic pattern at a first spatial resolution;
a second halftone processor, operatively connected to said spatial resolution scaler, for reducing the number of levels m representing optical density in at least one remaining color separation to a number of levels n representing optical density, said processor generating a second periodic pattern at a second spatial resolution; and
a video processor directing the distinct spatial resolution signals processed at each halftone processor to said printer to print the processed color image.
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Abstract
The present invention is a method and apparatus for processing a color document for moire-free printing, the color document including a plurality of color separations each defined with a set of image signals representing optical density with m possible density levels, while a printer is capable of rendering density with n density levels. The invention employs a spatial resolution scaler to produce m-bit signals at a spatial resolution that is a function of the halftone screen angle, base dot pitch, and desired halftone cell size. Subsequently, the m-bit signals at the desired spatial resolution (frequency and angle) are halftoned to produce n-bit signals that are printed with a raster output scanning-type printer suitable for printing at selectively variable spots per inch in both the fast and slow scan directions.
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Citations
16 Claims
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1. A color document printing system for eliminating moire in printed output, the color document including a plurality of color separations wherein each separation is defined with a set of image signals representing optical density with m possible density levels, comprising:
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a printer, capable of rendering density with n density levels, adapted to print the color separations at distinct spatial resolutions, so that when the color separations are superposed on a substrate, a selected color is defined; a source of image signals describing the color document with a plurality of color separations, each image signal representing optical density as one of m levels for a discrete area of a color separation; a spatial resolution scaler, operatively connected to said source of image signals; a first halftone processor, operatively connected to said spatial resolution scaler, for reducing the number of levels m representing optical density in a subset of the color separations to a number of levels n representing optical density, said processor generating a first periodic pattern at a first spatial resolution; a second halftone processor, operatively connected to said spatial resolution scaler, for reducing the number of levels m representing optical density in at least one remaining color separation to a number of levels n representing optical density, said processor generating a second periodic pattern at a second spatial resolution; and a video processor directing the distinct spatial resolution signals processed at each halftone processor to said printer to print the processed color image. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10)
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11. A method for preparing a color document for moire-free printing, the color document including a plurality of color separations wherein each separation is defined with a set of image signals representing optical density with m possible density levels, while a printer is capable of rendering density with n density levels, comprising:
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receiving a set of image signals describing the color document with a plurality of color separations, each image signal representing optical density as one of m levels for a discrete area of a color separation; spatially scaling the image signals of a first and second color separation; halftoning the m level image signals of the first color separation to n level image signals representing optical density in a manner generating a first periodic pattern having a fixed frequency and a distinct first angle for the first separation; halftoning the m level image signals of a second color separation to n level image signals representing optical density in a manner generating a second periodic pattern having the fixed frequency and a distinct second angle for the second separation; and directing the first and second periodic patterns to said printer to print the processed color image. - View Dependent Claims (12, 13, 14, 15, 16)
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Specification