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We introduce a class of linear filters called ``donut filters'' for
the design of halftone screens that enable robust printing with
stochastic clustered dots. The donut filter approach is a simple, yet
efficient method to produce pleasing stochastic clustered-dot
halftone patterns (a.k.a AM-FM halftones) suitable for systems with
poor isolated dot reproduction and/or significant dot-gain. The
radial profile of a donut filter resembles the radial cross section
of a donut shape, with low impulse response at the center that rises
to a peak and drops off rapidly as the pixel distance from the center
is increased. A simple extension for the joint design of any number
of colorant screens is given. This extension makes use of several
optimal linear filters that may be treated as a single donut
multi-filter having matrix-valued coefficients. A key contribution is
the design of the parametric donut filters to be used at each
graylevel. We show that given a desired spatial pair-correlation
profile (a.k.a. spatial halftone statistics), optimum donut filters
may be generated, such that the donut filter based screen design
produces patterns possessing the desired profile in the
maximum-likelihood sense. The donut filter based screen design method
is virtually identical to the void-and-cluster method of
dither array generation, with the empirically chosen kernels replaced
by optimal donut filters. This shows that the void and cluster
approach can be optimally tuned (not empirically as was done in the
past) to produce desired halftone statistics.
 
Example AM-FM halftone patches produced by the donut filter
based screen design
A slide set describing donut
filter theory is available here
References
N. Damera-Venkata and Q. Lin, "AM-FM
screen design using donut filters", Proc. SPIE Electronic
Imaging: Color Imaging IX: Processing, Hardcopy,
and Applications, vol. 5293, pp.
469-480, January 2004.
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