Design of Farthest-Point Masks for Image Halftoning
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Design of Farthest-Point Masks for Image Halftoning R. Shahidi Electrical & Computer Engineering, Faculty of Engineering and Applied Science, Memorial University of Newfoundland, St. John’s, NL, Canada A1B 3X5 Email: [email protected]
C. Moloney Electrical & Computer Engineering, Faculty of Engineering and Applied Science, Memorial University of Newfoundland, St. John’s, NL, Canada A1B 3X5 Email: [email protected]
G. Ramponi Image Processing Laboratory, DEEI, University of Trieste, 34127 Trieste, Italy Email: [email protected] Received 3 September 2003; Revised 5 January 2004 In an earlier paper, we briefly presented a new halftoning algorithm called farthest-point halftoning. In the present paper, this method is analyzed in detail, and a novel dispersion measure is defined to improve the simplicity and flexibility of the result. This new stochastic screen algorithm is loosely based on Kang’s dispersed-dot ordered dither halftone array construction technique used as part of his microcluster halftoning method. Our new halftoning algorithm uses pixelwise measures of dispersion based on one proposed by Kang which is here modified to be more effective. In addition, our method exploits the concept of farthest-point sampling (FPS), introduced as a progressive irregular sampling method by Eldar et al. but uses a more efficient implementation of FPS in the construction of the dot profiles. The technique we propose is compared to other state-of-the-art dither-based halftoning methods in both qualitative and quantitative manners. Keywords and phrases: image halftoning, ordered dither, irregular sampling, halftoning quality measures.
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INTRODUCTION AND BACKGROUND
1.1. Introduction Digital halftoning refers to transforming a many-toned image into a one with fewer tones, perhaps only two, for the purposes of either rendering or printing. In this paper, we consider only bi-level halftoning of gray-scale images using point-by-point comparison with a threshold array (ordered dither). Five techniques for doing this halftoning are now briefly described; the new farthest-point halftoning (FPH) algorithm presented in this paper is loosely based on the fifth, Kang’s dispersed-dot ordered dither. This section is concluded by a comment on a method for irregular sampling, which is at the root of our method. 1.2. Ordered dither halftoning techniques In this section, we review previous methods of ordered dither halftoning to which we compare our new FPH technique. We also review Kang’s dispersed-dot ordered dither algorithm which is the basis for FPH.
1.2.1. The modified blue noise mask The modified blue noise mask (MBNM) technique [1] starts by creating an initial pattern of “ink” dots using an algorithm called binary pattern power spectrum matching algorithm (BIPPSMA). This algorithm converts a white noise pattern at a given gray level gi to a blue noise pattern at the same gray level. The initial white noise pattern is filtered with a level-dependent Gaussian in the frequency domain and then converted back to the spatial dom
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