Research

The project focused on four key scientific areas:

  • Spectral modelling of the printer/paper/ink combination.
  • Spectral gamut prediction and gamut mapping.
  • The effect of paper optical and surface properties on the colour reproduction of multi-channel devices.
  • Optimal halftoning algorithms and tonal reproduction characteristics of multi-channel printing.
We have had funding for 252 researcher months for ESR positions and 48 researcher months of experienced researcher (ER) positions in Germany (24 months) and Norway (24 months). The work performed in this project is summarised below in brief.

Project Summary

Colour Printing 7.0: Next Generation Multi-Channel Printing (CP7.0) was an Initial Training Network funded by EU’s Seventh Framework programme, within the Marie Curie Actions (Project number N-290154). Led by Gjøvik University College, it involved a strong consortium of six full network partners and six associated partners from European academia and industry. Nine Early Stage Researchers (ESR) and two Experienced Researchers (ER) were employed in the project.

The project addressed a significant need for research, training and innovation in the printing industry. The main objectives of were to train a new generation of printing scientists who would be able to assume science and technology leadership in this traditional technological sector, and to perform research in the colour printing field by fully exploring the possibilities of using more than the conventional four colorants (CMYK) in printing; focusing particularly on the spectral properties.

A significant amount of high quality research has been performed and published throughout the project. The research contributions may be classified into four thematic areas, spectral modeling and surface properties; spectral reproduction workflow algorithms and evaluation, relief and gloss printing, and applications.

In the area of spectral modeling Slavuj et al. reviewed the state-of-the-art in spectral characterization models and multichannel ink-jet printing, and analysed the integration of current and novel halftoning strategies into spectral reproduction workflows. The evaluation was performed on the basis of criteria such as computational simplicity and model accuracy. A method to estimate Neugebauer primaries from the reflectance of single ink patches using radiative transfer theory was then proposed and the effect of ink spreading and ink amount in the Yule-Nielsen modified spectral Neugebauer model was later studied.

Coppel et al. studied the effect of illumination on the appearance of fluorescing substrates.

Focusing on paper optics and surface properties, Rahaman and Norberg proposed an experi- mental setup and method to analyse micro-scale halftoning image with the aim of relating the model parameters with the measured values by calculating the actual physical and optical enlargement of the dots.

Also, the dot gain effect in the context of spectral printing was investigated thoroughly by in Namedanian et al. and Coppel.

Lateral light propagation in paper and angle-resolved reflectance was studied to predict the reflectance of multi-layer and relief prints which require modeling the light scattering in multilayer materials and the lateral propagation in different layers.

Breaking with on the conventional paradigm of colour reproduction based on metamerism, spectral reproduction aims to reproduce the exact spectral reflectance of the original object or scene.

A key enabler for spectral reproduction is standardization and compatibility with current workflows and processes; studies are being carried out with regards to the feasibility of using a workflow based on an extension of the industry standard colour management workflow standardized by the International Color Consortium (ICC, http://www.color.org). Furthermore we have addressed the important fundamental aspect of observer variability, which contributes strongly to the problems of colour matching in current workflows, particularly for proofing. Spectral reproduction has the potential of alleviating these problems.

Extending the current workflow, Le Moan and Urban proposed an efficient design of a Profile Connection Space (PCS) which will allow a high-dimensional spectral image representations using limited features, which is efficiently used in the design of look-up tables for spectral colour management.

A key problem in spectral reproduction is spectral gamut mapping. Samadzadegan and Urban proposed a novel strategy to reduce the banding artifacts that may occur in gamut-mapped images. A spatio-spectral gamut mapping method was further proposed. A spectral gamut mapping and separation algorithm was extended to include fluorescence; this model was also used to evaluate the impact of fluorescence variation on spectral image quality. Application of the model in spectral proofing successfully minimising colour differences due to varying UV content was also studied.

A core element of a reproduction workflow is halftoning. In the area of halftoning of multi- channel images, several options have been investigated. The conventional approach has been to implement the AM halftoning algorithm in multi-channel printing, specifically, in seven-channel printing. Extending this work, Qu et al. proposed a CMYLmLc colour separation model since the multi-channel printers also use inks at different saturation levels (for example, light cyan (Lc) and light magenta (Lm)). A multi-level halftoning approach was also introduced to optimally use three black ink levels and to control ink overlap. Additionally, the direct binary search algorithm, previously used for greyscale and color halftoning was successfully extended to spectral halftoning by Slavuj and Pedersen.

To evaluate a spectral reproduction workflow, the problem of quantifying image quality must be addressed. Research has been carried out in the project concerning how to measure the quality of spectral images for spectral modeling. The impact of fluorescence on perceived image quality in spectral hard proofing has also been studied. Finally, a database of spectral images with subjective ratings was published by Le Moan et al.

In one part of the project, an extra dimension was added to the print in the form of relief, with the goal of controlling the angular dependent reflection properties. For such prints, called 2.5D prints, a surface texture is created by printing multiple layers of ink on desired locations. The study led us to create prints with relief that closely resemble the original, with a focus on aspects such as improving the print quality, reducing the print costs and discovering new market opportunities. Following a literature study on the topic of 2.5D (and 3D) print quality, Baar and Ortiz Segovia proposed a method for capturing surface texture and reflection properties using a flatbed scanner.

Work has also been done on producing the gloss appearance of 2.5D prints by manipulating the way the ink is deposited in a layer-by-layer basis. This novel way of controlling gloss to investigate the impact of colour on gloss for at low to medium gloss level was then used by Samadzadegan et al.

Several application areas have been investigated throughout the project.

Specific emphasis has been given to the area of fine art. By mixing inkjet inks in similar ways to traditional artistic print processes, the potential to create colours that could not be made by customary means of inkjet printing have been explored by Olen and Parraman. By controlling the order of inks printed on top of each other, Olen and Parraman showed significant print density increase and significant colour difference between colour overlays using multilayer inkjet printing. This process is similar to the application of layering pigments as demonstrated in old master paintings and allows reproducing a painted original with high dynamic range by improving colour variation in the shadow region.

In the context of fine art, gloss was also studied by Baar et al.

Slavuj et al. analysed the feasibility of textile colour reproduction with multichannel printing systems. The experimental results indicate that with a CMYKRGB ink system and by controlling carefully the ink limits, we are able to include over 90 % of a sample set of textile colours into the printable gamut, as compared to only 60% with conventional four-colour printing.

Another core application area where spectral reproduction has much potential is spectral proof- ing, that is, simulating the rendering of a print under various viewing conditions. The feasibility of this application was studied in detail by Coppel et al., and further elaborated by Coppel and Le Moan.

In addition to the research activity outlined above, the project provided a targeted profes- sional and technical skills development programme for the involved researchers, focusing on multi- disciplinary theoretical and practical understanding of the field of multi-colorant printing. A num- ber of training events and special sessions in key international conferences have been organised. The project also focused on outreach and networking, see http://cp70.org/outreach/ for more information.

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