Method and system for reducing auto-moire artifacts in periodic halftone screening
The design of periodic screens for halftone patterns is based upon a series of constrained optimization problems. For each of a number of input levels, a pattern is found that minimizes a cost function. The cost function is a spatial-frequency weighted difference between an ideal, continuous-space pattern and a predicted reconstructed pattern. The frequency weighting is a parameterized trade-off between the preservation of periodic components and the introduction of non-harmonic components.
1 . A method for generating halftone patterns for reproduction of multi-tone images, comprising steps
(a) selecting desired levels for a tone to be reproduced;
(b) generating for each of said levels an ideal halftone pattern;
(c) optimizing based on said ideal halftone patterns; and
(d) storing optimized patterns obtained in a computer-readable medium for access during conversion of a multi-tone image to a halftone image for reproduction.
2 . The method of claim 1 , wherein said optimizing of (c) is conducted for each level of said levels iteratively.
3 . The method of claim 2 , wherein said optimizing of (c) for a level of said levels is also based on a pattern resulting from at least one previous level.
4 . The method of claim 1 , wherein said optimizing of (c) is conducted by minimizing a cost function.
5 . The method of claim 1 , wherein said optimizing of (c) is conducted by an optimization algorithm.
6 . The method of claim 1 , wherein said optimizing of (c) iteratively reduces differences.
7 . The method of claim 1 , wherein said optimizing of (c) does not find a global minimum.
8 . The method of claim 1 , wherein said optimizing of (c) comprises filtering to reconstruct a trial pattern on a sampling grid to determine differences from said ideal halftone pattern as weighted non-harmonic components and weighted harmonic components.
9 . The method of claim 8 , wherein said optimizing reduces the differences between said trial pattern as reconstructed and said ideal halftone pattern.
10 . The method of claim 8 , wherein said filtering is performed with a filter that includes a model of a human visual system.
11 . The method of claim 10 , wherein said filter comprises a low-pass filter.
12 . The method of claim 1 , wherein said optimized patterns are quantized prior to storing said optimized patterns.
13 . The method of claim 1 , wherein said optimized patterns are quantized during said optimizing of (c).
14 . The method of claim 1 , wherein a number of halftone patterns greater than a number of multi-tone levels to be reproduced are subjected to said optimizing, and further including a step of assigning selected ones of optimized patterns to respective levels.
15 . The method of claim 1 , further comprising a step of estimating, based on said optimized halftone patterns, intermediate halftone patterns for at least one level not subjected to said optimizing.
16 . The method of claim 1 , wherein said optimized patterns are converted into a threshold array.
17 . A system for converting a multi-tone image to a halftone image for reproduction, comprising:
a memory storing a plurality of halftone patterns, each of which is generated by obtaining a halftone pattern for a tone to be reproduced, by generating an ideal halftone pattern for said tone, and optimizing based on said ideal halftone patterns;
a halftone processor that receives a value indicative of a level of a pixel in an image to be reproduced and, in response thereto, retrieves a pattern from said memory that relates to the level received, and outputs a signal indicating whether a mark is to be printed at a location corresponding to said pixel; and
a frame buffer for storing signals output by said halftone processor.
18 . The system of claim 17 , wherein the halftone patterns are stored as represented by threshold arrays.
19 . The system of claim 17 , further comprising a filter to reconstruct a trial pattern on a sampling grid to determine differences from said ideal halftone pattern as weighted non-harmonic components and weighted harmonic components.
20 . The system of claim 19 , wherein said differences between said trial pattern as reconstructed and said ideal halftone pattern are reduced during said optimizing.
21 . The system of claim 19 , wherein said filter includes a model of a human visual system.
22 . The system of claim 21 , wherein said halftone pattern is convolved with said filter.
23 . The system of claim 21 , wherein said filter comprises a low-pass filter.