Dithering by Differences of Convex Functions

  • Authors:
  • T. Teuber;G. Steidl;P. Gwosdek;C. Schmaltz;J. Weickert

  • Affiliations:
  • tteuber@kiwi.math.uni-mannheim.de and steidl@math.uni-mannheim.de;-;gwosdek@mia.uni-saarland.de and schmaltz@mia.uni-saarland.de and weickert@mia.uni-saarland.de;-;-

  • Venue:
  • SIAM Journal on Imaging Sciences
  • Year:
  • 2011

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Abstract

Motivated by a recent halftoning method which is based on electrostatic principles, we analyze a halftoning framework where one minimizes a functional consisting of the difference of two convex functions. One describes attracting forces caused by the image's gray values; the other one enforces repulsion between points. In one dimension, the minimizers of our functional can be computed analytically and have the following desired properties: The points are pairwise distinct, lie within the image frame, and can be placed at grid points. In the two-dimensional setting, we prove some useful properties of our functional, such as its coercivity, and propose computing a minimizer by a forward-backward splitting algorithm. We suggest computing the special sums occurring in each iteration step of our dithering algorithm by a fast summation technique based on the fast Fourier transform at nonequispaced knots, which requires only $\mathcal{O}(m\log m)$ arithmetic operations for $m$ points. Finally, we present numerical results showing the excellent performance of our dithering method.