Halftoning for Multi-Channel Printing

Paula Žitinski ElíasPartner: LiU, Sweden

Fellow researcher:

Paula Žitinski Elías

(paulazitinski@gmail.com)

Supervisor: Dr. Daniel Nyström

Work Summary

In the Marie Curie project CP7.0: Next generation multi-channel printing, several researchers from multicultural backgrounds joined effort in a consortium to face the challenges presented when additional ink channels are used in printing. A part of these challenges is in relation to halftoning algorithms and has been the main research objective conducted at Linköping University, as part of the on-going PhD studies.

Halftoning algorithms are applied to an image before printing, altering it so that it is reproducible by printing devices. This is necessary because printing devices form the image by applying ink droplets on the paper. A halftoned image is a dotted representation of the initial image that tells the printer where to place ink droplets. By altering the amount of droplets, the illusion of lighter or darker shades is formed. Therefore, halftoning algorithms convert an initial, continuous-tone, image to a binary one.

In multi-channel printing, inks are added to the four usual ones in order to enhance the reproduced image quality. Nevertheless, since paper substrates have a limit on ink absorbance, uncontrolled ink addition could lead to smudging, color inaccuracy, etc. To avoid these problems, control over ink overlap is a prerequisite in multi-channel printing and one way to achieve it is with halftoning algorithms. The aim of this research is to investigate and develop halftoning algorithms that can be applied to multi-channel printing.

The first focus in the research carried out is on the implementation of an amplitude modulated halftoning method for seven-channel printing. The particular challenge is that this type of halftoning may lead to undesirable optical grid-like phenomena, caused by the halftone dots, especially if multi-channel is used. In the research carried out, this optical grid was avoided by changing the round dot shape of the halftone dots. A non-orthogonal element shape was used instead, thus breaking the undesirable optical grid effect, showing very good results when compared to the usual method. However, the layer thickness of the combined colorants is not controlled with this approach.

Therefore, the second focus concerns a different type of halftoning, in an alteration from binary (bilevel) to multilevel halftoning algorithms. In it, a channel is processed so that it can be printed using multiple inks, with no overlap between them. Control over the physical paper-ink interactions pose an interesting challenge. Since these interactions are specific to each ink, control over the interactions means expressing them in terms of one joined interaction. The applicability of the proposed multilevel halftoning is demonstrated using multiple inks while avoiding ink overlap placement and was successfully implemented in eight-channel printing. The results of carried image quality metrics suggest that the multilevel halftoned image is visually improved when compared to the usual methods.

Publication and Dissemination

  • Zitinski Elias, P., Nystr ̈om, D. and Gooran, S. (2013), Multi channel printing by or- thogonal and non orthogonal AM halftoning, in ’AIC Colour 2013’, International Colour Association (AIC), Newcastle upon Tyne, UK, pp. 597 − 600.
  • Qu, Y. Y., Zitinski Elias, P. and Gooran, S. (2014), Colour prediction modeling for five-channel CMYLcLm printing, SPIE Electronic Imaging, San Francisco, CA, USA.
  • Namedanian, M., Nystr ̈om, D., Zitinski Elias, P. and Gooran, S. (2014), Physical and optical dot gain: Characterization and relation to dot shape and paper properties, SPIE Electronic Imaging, San Francisco, CA, USA.
  • Zitinski Elias, P.; Gooran, S.; Nystr ̈om, D., (2014), Multilevel halftoning applied to monochromatic inks in multi-channel printing, ‘Advances in Printing and Media Tech- nology, Print and Media Research for the Benefit of Industry and Society’, IARIGAI print and media research, Swansea, UK.
  • Coppel, L. G.; Le Moan, S.; Zitinski Elias, P.; Slavuj, R.; Hardeberg, J. Y., (2014), Next generation printing – Towards spectral proofing, ‘Advances in Printing and Media Technology, Print and Media Research for the Benefit of Industry and Society’, IARIGAI print and media research, Swansea, UK.
  • Zitinski Elias, P., (2014), Halftoning for Multi-Channel Printing: Algorithm Develop- ment, Implementation and Verification, Vol. 1694, Link ̈oping University Electronic Press, DOI: 10.3384/lic.diva-112271.
  • Zitinski Elias, P.; Gooran, S.; Nystr ̈om, D., (2015), Multilevel halftoning as an algorithm to control ink overlap in multi-channel printing, Colour and Visual Computing Symposium (CVCS), IEEE, Norway.

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