The amount of trapping has a great impact on the gray balance and color reproduction of printed products. The conventional trapping models are print density based and give percentage values to estimate the effect of trapping. In an earlier paper (Hauck and Gooran, 2011), a spectral trapping model was proposed, that defines the trapping effect as the DE* ab colorimetric differences between the real ink overlap (measurements) and the ideal ink overlap. All the trapping models proposed so far, however, only calculate the trapping value for full-tone (solid) ink overlap. As the trapping value for full-tone ink overlap could be overestimating the actual ink trapping effect for halftones, it is important to be able to also approximate the trapping value of color halftones. Furthermore, for a detailed gray balance shift analysis, there is a need to estimate the trapping effect for specific color halftones.

In the present paper, we propose a novel spectral trapping model that delivers the trapping value as DE* ab color difference for color halftones taking into account secondary and tertiary ink overlap.

The results of the experiments show that the trapping value for color halftones are much smaller than their corresponding trapping value at full-tone, but trapping value of halftones, besides other common quality parameters, should still be considered if some quality inaccuracy, such as gray balance shift, occurs in a print production.

Although offset printing has been and still is the most common printing technology for color print productions, its print productions are subject to variations due to environmental and process parameters. Therefore, it is very important to frequently control the print production quality criteria in order to make the process predictable, reproducible and stable. One of the most important parts in a modern industrial offset printing is Computer to Plate (CtP), which makes the printing plate.

One of the most important quality criteria for printing is to control the dot gain level. It is crucial to have the dot gain level within an acceptable range, defined by ISO 12647-2/13. This is done by dot gain compensation methods in the Raster Image Processor (RIP). Dot gain compensation, which is also referred to as CtP calibration, is however a complicated task in offset printing because of the huge number of parameters affecting dot gain. The conventional CtP calibration methods for an offset printing process, which are very time and resource demanding and hence expensive, mostly uses one to five dot gain correction curves as maximum. The proposed CtP calibration method in this paper, calibrates the dot gain according to ISO 12647-2/13 recommendations fully automatically parallel to the print production.

Besides that, there is no limitation of the number of the needed dot gain correction curves. This method, which is much more efficient and economically beneficial compared to conventional CtP calibration methods, also makes the printing production very accurate in terms of dot gain value. This automated CtP calibration system for offset printing workflow is introduced and described in this paper.

The print quality of a printing machine highly depends on good register variation values. The measuring of register variation is very important for putting a multicolor press in operation or for its repair and service. The manufacturers of print presses also need the evaluation of register variation to develop new products. The current industry standard method for measuring the register variation is based on image processing, which is a very expensive method. It was a great demand to determine the register variation by an alternative and affordable technique. In the present paper we introduce a new method to determine the register variation based on densitometry. In order to create a new method, a special color test target has been designed. The input of the method is the densitometric measurement values, and its output is the register variation value. The results of the method have been compared with those of an image processing method and the correlation coefficient between the results is almost 0.9. Since in the proposed method only a densitometer is needed, it can be considered as a very inexpensive alternative to the image processing methods. The results were also demonstrated to different specialists of a manufacturer of print press and received very positive feedback.

 There is no doubt that printing has been one of the most important technological inventions for

human civilization. Books, magazines, news papers, and so on have been printed for different

purposes such as distributing knowledge, thoughts, and news and commercializing products.

Tone reproduction for images has been one of the challenging parts of the printing technology

because the printing devices are restricted to a few color inks, whereas the original image

may consist of millions of color tones. In this chapter, the basics of the tone reproduction

are introduced. We begin with a brief history of halftoning and a short introduction of digitalization.

It is followed by the description on visual acuity of human visual system and its

relationship with the screen resolution. Then the basic and general concepts of tone reproduction,

such as screen frequency, print resolution, screen angle and Moiré pattern, and dot gain

are described and illustrated. Dot gain is only briefl y described and illustrated in this chapter as

it is thoroughly discussed in Physical Evaluation of the Quality of Color Halftone . Finally,

technologies for color reproduction and color halftoning are discussed.

Halftoning is a crucial part of image reproduction in print. First-order FM halftones, in which the single dots are stochastically distributed, is widely used in printing technologies, such as inkjet, that are able to stably print isolated dispersed dots. Printers, such as laser printers, that utilize electrophotographic technology are not able to stably print the isolated dots and therefore use clustered-dot halftones. Periodic clustered-dot, i.e. AM, halftones are commonly used in this type of printers but they suffer from undesired periodic interference pattern called moiré. An alternative solution is to use second-order FM halftones in which the clustered dots are stochastically distributed. The iterative halftoning techniques, that usually result in well-formed halftones, are operating on the whole input image and require extensive computations and thereby are very slow when the input image is large. In this paper, we introduce a method to generate image independent threshold matrices for first and second-order FM halftoning. The first-order threshold matrix generates well-formed halftone patterns and the second-order FM threshold matrix can be adjusted to produce clustered-dots of different size, shape and alignment. Using predetermined and image independent threshold matrices makes the proposed halftoning method a point-by-point process and thereby very fast.

Printing using more than four ink channels visually improves the reproduction but causes challenges with the ink layer thickness that could lead to ink bleeding and color inaccuracy. A color image is commonly prepared for print by first being separated into the colorant channels the intended print device utilizes. The separations are usually halftoned independently, resulting in random dot overlap with possible spots where all colorants are printed. A multilevel halftoning algorithm that processes each channel so that it is printed with multiple inks of the same hue value has already been applied to three achromatic inks – photo gray, gray, black – in a real paper–ink setup. Results proved a successful multilevel halftone implementation workflow using multiple inks while avoiding dot-on-dot placement. However, in this approach, the gray inks were assumed to be neutral and lighter versions of black, an assumption that may cause a ΔE*ab color difference as high as 5. In the present paper an alternative approach, based on dot-off-dot halftoning avoiding dot overlap, is proposed andapplied to the same three inks. A look-up table driven separation procedure of the original image into the three channels is also proposed, which, combined with dot-off-dot halftoning, results in a ΔE*ab color difference not larger than 1.8. Results show that the dot-off-dot halftoned images are visually pleasant without any artifacts in tone transitions. The proposed approach has three main advantages to the commonly used independent halftoning. One being that dot overlap between different inks is completely avoided, i.e. photo gray, gray and black in the present work. The other one is that the results are less grainy compared to independent channel halftoning. The third one is that dot-off-dot halftoning consumes less ink than independent halftoning when reproducing the same color.

A multilevel halftoning algorithm can be used to overcome some of the challenges of multi-channel printing. In this algorithm, each channel is processed so that it can be printed using multiple inks of approximately the same hue, achieving a single ink layer. The computation of the threshold values required for ink separation and dot gain compensation pose an interesting challenge. Since the dot gain depends on the specific combination of ink, paper and print resolution, compensating the original image for multilevel halftoning means expressing the dot gain of multiple inks of same hue in terms of the coverage of a single ink. The applicability of the proposed multilevel halftoning workflow is demonstrated using chromatic inks while avoiding dot overlap and accounting for dot gain. The results indicate that the multilevel halftoned image is visually improved in terms of graininess when compared to bi-level halftoned images.

In printing, halftoning algorithms are applied in order to reproduce a continuous-tone image by a binary printing system. The image is transformed into a bitmap composed of dots varying in size and/or frequency. Nevertheless, this causes that the sparse dots found in light shades of cyan (C) and magenta (M) appear undesirably noticeable against white substrate. The solution is to apply light cyan (Lc) and light magenta (Lm) inks in those regions. In order to predict the color of CMYLcLm prints, we make use of the fact that Lc and Lm have similar spectral characteristics as C and M respectively. The goal of this paper is to present a model to characterize a five-channel CMYLcLm printing system using a three-channel color prediction model, where we treat the ink combinations Lc+C and Lm+M as new compound inks. This characterization is based on our previous three-channel CMY color prediction model that is capable of predicting both colorimetric tri-stimulus values and spectral reflectance. The drawback of the proposed model in this paper is the requirement of large number of training samples. Strategies are proposed to reduce this number, which resulted in expected larger but acceptable color differences.

Color vision deficiency (CVD) is the inability, or limited ability, to recognize colors and discriminate between them. A person with this condition perceives a narrower range of colors compared to a person with normal color vision. In this study we concentrate on recoloring digital images in such a way that users with CVD, especially dichromats, perceive more details from the recolored images compared to the original ones. During this color transformation process, the goal is to keep the overall contrast of the image constant, while adjusting the colors that might cause confusion for the CVD user. In this method, RGB values at each pixel of the image are first converted into HSV values and, based on pre-defined rules, the problematic colors are adjusted into colors that are perceived better by the user. Comparing the simulation of the original image, as it would be perceived by a dichromat, with the same dichromatic simulation on the recolored image, clearly shows that our method can eliminate a lot of confusion for the user and convey more details. Moreover, an online questionnaire was created and a group of 39 CVD users confirmed that the transformed images allow them to perceive more information compared to the original images.

The tone value increase in halftone printing commonly referred to as dot gain actually encompasses two fundamentally different phenomena. Physical dot gain refers to the fact that the size of the printed halftone dots differs from their nominal size, and is related to the printing process. Optical dot gain originates from light scattering inside the substrate, causing light exchanges between different chromatic areas. Due to their different intrinsic nature, physical and optical dot gains need to be treated separately. In this study, we characterize and compare the dot gain properties for offset prints on coated and uncoated paper, using AM and first and second generation FM halftoning. Spectral measurements are used to compute the total dot gain. Microscopic images are used to separate the physical and optical dot gain, to study ink spreading and ink penetration, and to compute the Modulation Transfer Function (MTF) for the different substrates. The experimental results show that the physical dot gain depends on ink penetration and ink spreading properties. Microscopic images of the prints reveal that the ink penetrates into the pores and cavities of the uncoated paper, resulting in inhomogeneous dot shapes. For the coated paper, the ink spread on top of the surface, giving a more homogenous dot shape, but also covering a larger area, and hence larger physical dot gain. The experimental results further show that the total dot gain is larger for the uncoated paper, because of larger optical dot gain. The effect of optical dot gain depends on the lateral light scattering within the substrate, the size of the halftone dots, and on the halftone dot shape, especially the dot perimeter.