Halftoning is a crucial part of image reproduction in print. For large format prints, especially at higher resolutions, it is important to have very fast and computationally feasible halftoning methods of good quality. The authors have already introduced an approach to obtain image-independent threshold matrices generating both first- and second-order frequency modulated (FM) halftones with different clustered dot sizes. Predetermined and image-independent threshold matrices make the proposed halftoning method a point-by-point process and thereby very fast. In this article, they report a comprehensive quality evaluation of first- and second-order FM halftones generated by this technique and compare them with each other, employing several quality metrics. These generated halftones are also compared with error diffusion (ED) halftones employing two different error filters. The results indicate that the second-order FM halftoning with small clustered dot size performs best in almost all studied quality aspects than the first- and second-order FM halftoning with larger clustered dot size. It is also shown that the first- and second-order FM halftones with small clustered dot sizes are of almost the same quality as ED halftones using Floyd–Steinberg error filter and of higher quality than halftones generated by ED employing Jarvis, Judice, and Ninke error filter.

Structure-aware halftoning algorithms aim at improving their non-structure-aware version by preserving high-frequency details, structures, and tones and by employing additional information from the input image content. The recently proposed achromatic structure-aware Iterative Method Controlling the Dot Placement (IMCDP) halftoning algorithm uses the angle of the dominant line in each pixel’s neighborhood as supplementary information to align halftone structures with the dominant orientation in each region and results in sharper halftones, gives a more three-dimensional impression, and improves the structural similarity and tone preservation. However, this method is developed only for monochrome halftoning, the degree of sharpness enhancement is constant for the entire image, and the algorithm is prohibitively expensive for large images. In this paper, we present a faster and more flexible approach for representing the image structure using a Gabor-based orientation extraction technique which improves the computational performance of the structure-aware IMCDP by an order of magnitude while improving the visual qualities. In addition, we extended the method to color halftoning and studied the impact of orientation information in different color channels on improving sharpness enhancement, preserving structural similarity, and decreasing color reproduction error. Furthermore, we propose a dynamic sharpness enhancement approach, which adaptively varies the local sharpness of the halftone image based on different textures across the image. Our contributions in the present work enable the algorithm to adaptively work on large images with multiple regions and different textures.

Structure-aware halftoning algorithms aim at improving their non-structure-aware version by preserving high-frequency details, structures, and tones and by employing additional information from the input image content. The recently proposed achromatic structure-aware Iterative Method Controlling the Dot Placement (IMCDP) halftoning algorithm uses the angle of the dominant line in each pixels neighborhood as supplementary information to align halftone structures with the dominant orientation in each region and results in sharper halftones, gives a more three-dimensional impression, and improves the structural similarity and tone preservation. However, this method is developed only for monochrome halftoning, the degree of sharpness enhancement is constant for the entire image, and the algorithm is prohibitively expensive for large images. In this paper, we present a faster and more flexible approach for representing the image structure using a Gabor-based orientation extraction technique which improves the computational performance of the structure-aware IMCDP by an order of magnitude while improving the visual qualities. In addition, we extended the method to color halftoning and studied the impact of orientation information in different color channels on improving sharpness enhancement, preserving structural similarity, and decreasing color reproduction error. Furthermore, we propose a dynamic sharpness enhancement approach, which adaptively varies the local sharpness of the halftone image based on different textures across the image. Our contributions in the present work enable the algorithm to adaptively work on large images with multiple regions and different textures. (C) 2022 Society for Imaging Science and Technology.

Two-and-a-half and 3D printing are becoming increasingly popular, and consequently the demand for high quality surface reproduction is also increasing. Halftoning plays an important role in the quality of the surface reproduction. Three dimensional halftoning methods, that adapt the halftone structures to the geometrical structure of 3D surfaces or to the viewing direction, could further improve surface reproduction quality. In this paper, a 3D adaptive halftoning method is proposed, that incorporates different halftone structures on the same 3D surface. The halftone structures are firstly adapted to the 3D geometrical structure of the surface. Secondly, the halftone structures are adapted based on the normal vector to the surface at a specific voxel. Two simple approaches to approximate the normal vector are also proposed. The problem of edge artefacts that might occur in the previously proposed 3D Iterative Method Controlling the Dot Placement (IMCDP) halftoning method is discussed and a solution to reduce these artefacts is given. The results show that the proposed adaptive halftoning can combine different halftone structures on the same 3D surface with no transition artefacts between different halftone structures. It is also shown that using second-order frequency modulation (FM) halftone, in comparison to first-order FM, can result in more homogeneous appearance of 3D surfaces with undesirable structures on them.

Two-and-a-half and 3D printing are becoming increasingly popular, and consequently the demand for high quality surface reproduction is also increasing. Halftoning plays an important role in the quality of the surface reproduction. Three dimensional halftoning methods, that adapt the halftone structures to the geometrical structure of 3D surfaces or to the viewing direction, could further improve surface reproduction quality. In this paper, a 3D adaptive halftoning method is proposed, that incorporates different halftone structures on the same 3D surface. The halftone structures are firstly adapted to the 3D geometrical structure of the surface. Secondly, the halftone structures are adapted based on the normal vector to the surface at a specific voxel. Two simple approaches to approximate the normal vector are also proposed. The problem of edge artefacts that might occur in the previously proposed 3D Iterative Method Controlling the Dot Placement (IMCDP) halftoning method is discussed and a solution to reduce these artefacts is given. The results show that the proposed adaptive halftoning can combine different halftone structures on the same 3D surface with no transition artefacts between different halftone structures. It is also shown that using second-order frequency modulation (FM) halftone, in comparison to first-order FM, can result in more homogeneous appearance of 3D surfaces with undesirable structures on them. (C) 2022 Society for Imaging Science and Technology.

The appearance of 3D-printed objects is affected by numerous parameters. Specifically, the colour of each point on the surface is affected not only by the applied material, but also by the neighbouring segments as well as by the structure underneath it. Translucency of the 3D printing inks is the key property needed for reproduction of surfaces resembling natural materials. However, the prediction of colour appearance of translucent materials within the print is a complex task that is of great interest. In this work, a method is proposed for studying the appearance of translucent 3D materials in terms of the surface colour. It is shown how the thickness of the printed flat samples as well as the background underneath affect the colour. By studying diffuse reflectance and transmittance of layers of different thicknesses, apparent, spectral optical properties were obtained, i.e., extinction and scattering coefficients, in the case of commercially available polylactic acid (PLA) filaments for Fused Deposition Modelling (FDM) printers. The coefficients were obtained by fitting a simplistic model to the measured diffuse reflectance as a function of layer thickness. The results were verified by reconstructing reflected spectra with the obtained parameters and comparing the estimated colour to spectrophotometer measurements. The resulting colour differences in terms of the CIEDE2000 standard are all below 2.

Many image reproduction devices, such as printers, are limited to only a few numbers of printing inks. Halftoning, which is the process to convert a continuous-tone image into a binary one, is, therefore, an essential part of printing. An iterative halftoning method, called Iterative Halftoning Method Controlling the Dot Placement (IMCDP), which has already been studied by research scholars, generally results in halftones of good quality. In this paper, we propose a structure-based alternative to this algorithm that improves the halftone image quality in terms of sharpness, structural similarity, and tone preservation. By employing appropriate symmetrical and non-symmetrical Gaussian filters inside the proposed halftoning method, it is possible to adaptively change the degree of sharpening in different parts of the continuous-tone image. This is done by identifying a dominant line in the neighborhood of each pixel in the original image, utilizing the Hough Transform, and aligning the dots along the dominant line. The objective and subjective quality assessments verify that the proposed structure-based method not only results in sharper halftones, giving more three-dimensional impression, but also improves the structural similarity and tone preservation. The adaptive nature of the proposed halftoning method makes it an appropriate algorithm to be further developed to a 3D halftoning method, which could be adapted to different parts of a 3D object by exploiting both the structure of the images being mapped and the 3D geometrical structure of the underlying printed surface.

Managing the final appearance of 3D surfaces is an interesting and essential topic in 3D printing applications. Knowledge about the parameters which influence the 3D surface reproduction quality enables engineers to achieve the final appearance as accurately as designed. Many studies have been conducted to explore numerous parameters that affect the quality of 3D surface reproduction. This work contributes to verifying the role of halftoning in increasing the 3D surface visual quality and the control over the surface appearance of a 3D printed object. The results show that applying different halftones according to the geometrical characteristics of the 3D surface could emphasize or diminish the perceived 3D geometrical structures of a shape. The experimental results are in line with the simulated outputs reported in previous work. Our findings might introduce a new approach towards having more control over 3D appearance reproduction without changing the material or printer settings.

Realistic appearance reproduction is of great importance in 3D printing’s applications. Halftoning as a necessary process in printing has a great impact on creating visually pleasant appearance. In this article, we study the aspects of adapting and applying Iterative Method Controlling Dot Placement (IMCDP) to halftone three-dimensional surfaces. Our main goal is to extend the 2D algorithm to a 3D halftoning approach with minor modifications. The results show high-quality reproduction for all gray tones. The 3D halftoning algorithm is not only free of undesirable artifacts, it also produces fully symmetric and wellformed halftone structures even in highlight and shadow regions.

As 3D printing is becoming increasingly popular, the demand for high quality surface reproduction is also increasing. Like in 2D printing, halftoning plays an important role in the quality of the surface reproduction. Developing advanced 3D halftoning methods for 3D printing and adapting them to the structure of the surface is therefore essential for improving surface reproduction quality. In this paper, an extension of an iterative 2D halftoning method to 3D is used to apply different halftone structures on 3D surfaces. The results show that using different halftones based on the 3D geometrical structure of the surface and/or the viewing angle in combination with the structure of the texture being mapped on the surface can potentially improve the quality of the appearance of 3D surfaces.

The spatial resolution of 3D printing is finite. The necessary discretisation of an object before printing produces a step-like surface structure that influences the appearance of the printed objects. To study the effect of this discretisation on specular reflections, we print surfaces at various oblique angles. This enables us to observe the step-like struc- ture and its influence on reflected light. Based on the step-like surface structure, we develop a reflectance model describing the redistribution of the light scattered by the surface, and we study dispersion effects due to the wavelength dependency of the refractive index of the material. We include preliminary verification by comparing model predictions to photographs for different angles of observation.

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.

To improve color reproduction, many printers today use extra colorants, in addition to the traditional four inks (Cyan, Magenta, Yellow and Black). Adding the complementary colorants (Red, Green and Blue) increases the gamut of reproducible colors, while lighter versions of the primary inks can be added to reduce graininess and dot visibility. Using more than three inks introduces colorimetric redundancy in the color separation process, because different ink combinations can reproduce the same target color. When additional inks are introduced, this redundancy rapidly increases, and it is thus crucial to introduce additional constraints in the color separation process, to improve determinacy and to optimize different aspects of print quality. This study focuses on an analysis of the redundancy in the color separation process for an 11-ink printer. It is investigated how the extensive colorimetric redundancy can be utilized to select optimal ink combinations to meet the, sometimes contradictory, criteria of color accuracy, graininess and ink consumption. Analysis of the results of applying different criteria in the color separation process shows that the result heavily depends on the selected criterion. For example, prioritizing graininess will improve print quality by reducing dot visibility, imposing the use of lighter inks, but it will also increase ink consumption.

Multi-channel printing employs additional inks to improve the perceived image quality by reducing the graininess and augmenting the printer gamut. It also requires a color separation that deals with the one-to-many mapping problem imposed when using more than three inks. The proposed separation model incorporates a multilevel halftoning algorithm, reducing the complexity of the print characterization by grouping inks of similar hues in the same channel. In addition, a cost function is proposed that weights selected factors influencing the print and perceived image quality, namely color accuracy, graininess and ink consumption. The graininess perception is qualitatively assessed using S-CIELAB, a spatial low-pass filtering mimicking the human visual system. By applying it to a large set of samples, a generalized prediction quantifying the perceived graininess is carried out and incorporated as a criterion in the color separation. The results of the proposed model are compared with the separation giving the best colorimetric match, showing improvements in the perceived image quality in terms of graininess at a small cost of color accuracy and ink consumption. (c) 2016 Wiley Periodicals, Inc.

Multichannel printing employs additional colorants to achieve higher quality reproduction, assuming their physical overlap restrictions are met. These restrictions are commonly overcome in the printing workflow by controlling the colorant choice at each point. Our multilevel halftoning algorithm bundles inks of same hues in one channel with no overlap, separating them into eight channels, consequentially benefitting of increased ink options at each point. In this article, implementation and analysis of the algorithm is carried out. Color separation is performed using the cellular Yule‐Nielsen modified spectral Neugebauer model. The channels are binarized with the multilevel halftoning algorithm. The workflow is evaluated with an eight-channel inkjet at 600 dpi resulting in mean and maximum ΔE ₉₄ color differences around 1 and 2, respectively. The halftoning algorithm is analyzed using S-CIELAB, thus involving the human visual system, in which multilevel halftoning showed improvement in terms of image quality compared to the conventional approach.

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.

A color separation model which separates a target color inside the gamut of a printing system into the combination of process inks used in the system is a crucial part of the printing procedure. A simple color separation model for CMY printing is presented in this paper. It is based on a color prediction model proposed in our previous papers. This color prediction model, which was based on CIEXYZ (CIELAB) values, is extended in this paper to work also for spectral data. Therefore, the color separation model is executable for target colors represented by both CIELAB and spectral data. Two experiments are designed and carried out to evaluate the accuracy of the proposed models. The first experiment proves the accuracy and stability of the forward (color prediction) model. The second experiment shows that our simple inverse model (color separation) has a satisfying accuracy for different target colors in terms of giving less ink consumption and small CIELAB (ΔE94) or small spectrum difference (ΔRMS). The proposed color separation model has the potential to be applied to practical printing systems due to its simplicity and accuracy.

The color of a print is affected by ink spreading and lateral light scattering in the substrate, making printed dots appear larger. Characterization of physical and optical dot gain is crucial for the graphic arts and paper industries. We propose a novel approach to separate physical from optical dot gain by use of a high-resolution camera. This approach is based on the histogram of microscale images captured by the camera. Having determined the actual physical dot shape, we estimate the modulation transfer function (MTF) of the paper substrate. The proposed method is validated by comparing the estimated MTF of 11 offset printed coated papers to the MTF obtained from the unprinted papers using measured and Monte Carlo simulated edge responses.

The amount of trapping has a great impact on the gray balance and color reproduction of printed products. Therefore, conventional print density based models to estimate the effect of trapping have been created, which only give percentage values. In an earlier paper (Hauck and Gooran, 2011) we have proposed a trapping model based on reflectance spectra, which defines the trapping effect as the DE^*_ab colorimetric differences between the measurements and the calculated values. Therefore, this model is more useful and meaningful for the press machine operators than the conventional trapping models. The surface (gloss) and ground (ink penetration) effect may have an impact on the print results depending on the substrate and inks but these effects have mainly been ignored in all previous trapping models. In the present paper, we extend our earlier model to investigate the impact of both effects for high quality glossy coated paper and a set of sheet-fed offset inks. An ink mileage test was carried out to find the surface and ink penetration effects. The results of our investigation demonstrates that these two effects compensate each other and their total impact is almost negligible for the tested materials. This means that our previously proposed model can successfully be used for high quality glossy coated papers to determine the trapping value.

Multi-channel printing with more than the conventional four colorants brings numerous advantages, but also challenges, like implementation of halftone algorithms. This paper concentrates on amplitude modulated (AM) halftoning for multi-channel printing. One difficulty is the correct channel rotation to avoid the moiré effect and to achieve colour fidelity in case of misregistration. 20 test patches were converted to seven-channel images and AM halftoning was applied using two different approaches in order to obtain a moiré-free impression. One method was to use orthogonal screens and adjust the channels by overlapping the pairs of complimentary colours, while the second was to implement non-orthogonal halftone screens (ellipses). By doing so, a wider angle range is available to accommodate a seven-channel impression. The performance was evaluated by simulating misregistration in both position and angle for a total of 1600 different scenarions. ΔE values were calculated between the misregistered patches and the correct ones, for both orthogonal and non-orthogonal screens. Results show no visible morié and improvement in colour fidelity when using non-orthogonal screens for seven-channel printing, producing smaller colour differences in case of misregistration.

The goal of the present work is to reduce the number of the training samples used in our color prediction model based on CIEXYZ using an Effective Coverage Map while keeping satisfying prediction. A general approach is proposed in this paper to choose the best reference combination for the training samples. The approach is based on the dot gain behavior of each primary ink, which is characterized by three curves using CIEXYZ tri-stimulus values. The proposed approach is built in our model to predict the color values for the color prints using two different devices, i.e. a laser printer and an inkjet printer. For the laser printer the number of the training samples is reduced from 125 to 64 while still giving quite good result. The approach also shows that for the test laser printer it is possible to further cut this number to 53 with a satisfying result. For the inkjet printer the number of training samples for our model is reduced from 125 to 79 or 64, both giving satisfying results.

The goal of the present work is to reduce the number of thetraining samples used in our color prediction model based onCIEXYZ using an Effective Coverage Map while keepingsatisfying prediction. A general approach is proposed in thispaper to choose the best reference combination for the trainingsamples. The approach is based on the dot gain behavior ofeach primary ink, which is characterized by three curves usingCIEXYZ tri-stimulus values. The proposed approach is built inour model to predict the color values for the color prints usingtwo different devices, i.e. a laser printer and an inkjet printer.For the laser printer the number of the training samples isreduced from 125 to 64 while still giving quite good result. Theapproach also shows that for the test laser printer it is possibleto further cut this number to 53 with a satisfying result. For theinkjet printer the number of training samples for our model isreduced from 125 to 79 or 64, both giving satisfying results.

Most of the color prediction models use a single dot gain curve for each primary ink. In the proposed model, the behaviour of the dot gain of each primary ink is characterized by three curves based on CIEXYZ tri-stimulus values. In our previous works, it was shown that the usage of three characterization curves for each primary ink reduced the color difference between the predicted and measured data compared with the simple Yule-Nielsen model. In the present paper, an effective coverage map based on CIEXYZ is created. This map presents the effective coverage values of the primary inks corresponding to different ink combinations. Given any reference ink combination, the effective coverage values of the involved primary inks are estimated by cubic interpolation. Compared to our previous models, the proposed model gives significant reduction in the color difference between the predicted and the measured data. 

One of the most important targets on the graphic market is to realize standardization. The standardization defines the target value of solid Lab and dot gain in printing process. In addition the tolerance range of these two values will be described by standardization. The correct dot gain will be achieved during the measuring of the dot gain in printing process by the additional calculation of a correction curve well known as Print Characteristic Curve (PCC) [1]. The Raster Image Processor (RIP) needs the PCC for the imaging of printing plate with the correct tone value. In this paper we will propose a Networked Workflow (figure 1) with the Workflow Control System (or alternatively a MIS Management Information System). This Networked Workflow is necessary for the realization of a Fully Automated CtP Calibration System.

In a multicolor offset press the process inks (kcmy) will be printed consecutively on the substrate from oneprinting unit to the other. The printing dots and elements in different process color will be printed either isolated, partly orcompletely overprinted depending on the halftoning. In a multicolor press the inks will be printed wet on wet. That meansthat in the area where process inks overlap each other one or more inks will be printed on another ink which is not dryenough. The adhesive power between the wet inks is different and less compared to the adhesive power between one inkprinted on the top of a less wet ink or even a completely dry ink. The adhesive power between the substrate and printed ink isalso different from the adhesive power of one ink on the top of another one. Depending on this adhesive power and the inks’inner cohesion power the thickness of the second printed ink varies. The thickness or amount of the second printed ink on thefirst one can be determined; its value is called trapping. The amount of the trapping will also be changed due to differentparameters such as ink temperature, dampening, printing speed etc. An important outcome is that the gray balance and thecolor appearance (secondary and tertiary colors) of the printed product also depend very much on the trapping’s amount. Thatshows how important it is to have an explicit value for the trapping. The amount of the second printed inks will bedetermined by trapping formulas. This value should be useful for the printer at the press. Unfortunately, the conventionaltrapping formulas are only useful for the “relative” comparison of trapping between two print products. All the conventionalformulas for trapping only deliver the amount of the second printed ink on the top of the first one in percent. This value for itsown (e.g. 63%) is not really useful and meaningful for print machine operators. There are three different formulas fordetermining trapping. These are Preucil, Ritz, and Brunner. All of them are based on density only. The results of theseformulas are different to each other, especially the Brunner formula differs to the other ones. Here a method will beintroduced which is based on spectrometry and will complement the conventional formulas.

Most of the color prediction models use single dot gain curve, few assume that dot gain changes when ink superposition happens, but still, use single dot gain curve for each ink to compensate the effective ink coverage. Considering the fact that optical dot gain is the effect of light scattering in paper, it is reasonable that light with different wavelength might produce different optical dot gain for each ink. In this study, for each primary ink we utilized three different curves obtained by CIEX, Y and Z, which approximately stand for three special wavelength bands, to calculate color coordinates. In addition, we noticed that dot gain curves obtained from the print samples with single ink printed on paper do not work well for the prints where ink is printed on another, or others. Therefore, dot gain curves for different ink superposition situations are optimized by matching the calculated tri-stimulus values of training patches to their measurement counterpoints. For each ink, dividing the dot gain into several dot gain actions responding to different ink superposition situations, we got the final dot gain as a group of multiple curves that takes into account all possible 'dot gain actions' with certain probability coefficients.

Characterization of total dot gain gives a good insight to the study of paper and print. In this article, we propose three approaches based on the Murray-Davies model to obtain total dot gain. In the first approach, the total gain is approximated by minimizing the root-mean-square between the calculated spectrum and the reflected spectrum measured by the spectrophotometer. The other two approaches are based on microscale images captured by a high resolution camera. These two approaches differ in their schemes on how to obtain the gray tone of the full-tone ink. By the use of microscale images, the authors also illustrate the shape of the effective dot area for the investigated paper substrate. They also study the histograms of the reflected and transmitted microscale images. This comparison shows that although the transmitted image has less optical dot gain compared to the reflected image, the transmittance also incorporates some small amount of optical dot gain. (C) 2011 Society for Imaging Science and Technology [DOI: 10.2352/J.ImagingSci.Technol.2011.55.4.040501]

By separating the optical dot gain from the physical dot gain, it is possible to study different behaviors of color inks on different papers. In this study we are investigating the dependency of dot gain and wavelength in color print. Microscopic images have been used to separate optical and physical dot gain from each other. The optical behavior of primary color inks in different absorbing wavelength bands has been studied. It has been illustrated that the light scattering in the paper is wavelength independent, and therefore the Point Spread Function which indicates the probability of light scattering of the paper does not change in visible wavelengths (380 nm -700 nm). We have shown that it is possible to separate two printed color inks on one specific wavelength, due to the filtering behavior of the color inks. By considering the fact that light scattering in the paper is wavelength independent, it was possible to separately analyze the dot gain of each color.

In commercial prints the halftone dots are seldom placed exactly at their corresponding positions in the digital bitmap, mostly due to the imprecise transportation of the printing substrate. In this study, we firstly present an image processing model to measure the displacement of the dots in different color separations by using a high resolution camera. By using a filter wheel equipped with a set of interference filters and sending light in different wavelength bands, it is possible to separate the different color inks. For example, for the combination of cyan and magenta inks, only cyan will be visible in the captured image if the wavelength band of the incoming light is concentrated around 700 nm. On the other hand, for the wavelength band concentrated around 500 nm only magenta will be visible. By comparing the positions of the dots in the captured images with those in the original bitmap we can measure register shift. Secondly in this study, we investigate how miss-registration affects the color appearance of the final print for different halftoning techniques. We use AM, FM first generation and FM 2nd generation halftoning methods and investigate and compare their sensitivity to register shift.

In the present work we measure the register shift for color patches printed in offset by the proposed image processing model. In order to study the effect of miss-registration on the resulting color appearance we first simulate the miss-registration in the digital bitmap. Then we print the simulated bitmap using an office laser printer. Since the miss-registration is usually negligible for digital prints, especially in lower resolution, we can examine the accuracy of our model for measuring dot displacement by comparing the simulated displacement with the measured one. Finally, by using a spectrophotometer to measure color coordinates, we can study the effect of miss-registration on the resulting color appearance for different halftoning methods by calculating the ΔElab color difference.

The printed dots appear bigger than the dots in the original digital bitmap. This is partly because of the spreading and penetrating of the ink on and in the paper, called physical dot gain, and partly because of the diffusion of the light in paper, which is referred to as optical dot gain. Dot gain is often approximated by measurements obtained by densitometer or spectrophotometer. In this study we use a very high-resolution scanner with a resolution of 2 μm/pixel and with a field of view of 2.7×2 mm, which makes it possible to clearly see the small halftone dots and their surroundings. In this camera it is also possible to illuminate the paper surface both from above and below, which means that the camera can capture both the reflected and transmitted lights. Since the transmitted light does not scatter in paper the optical dot gain has no effect on the transmitted image. In this paper we investigate the behavior of physical and optical dot gain for print on coated paperin offset, by using the micro scale images. We also present a method to separate optical and physical dot gain by using the reflected and the transmitted images. By comparing the reflectance and transmittance histograms it is possible to understand that, there is no optical dot gain in transmitted image. We also compare our results with the result obtained by a spectrophotometer, which also measures both reflected and transmitted light. Previously the physical and optical dot gains were mostly analyzed numerically, however in this paper we will also graphically illustrate and compare these two concepts by using microscale images.

Printed dots appear bigger than their reference size in the original bitmap. This is because of the physical and optical dot gain. In order to overcome the problem original images are compensated for dot gain. The compensation is usually done by using a dot gain curve for each colour separation. In this paper we firstly show that using only one dot gain curve works well for black, but not for any of the other three colours, i.e. cyan, magenta or yellow. We also present a new approach to calculate colour values where three different curves are used for each colour separation. In order to evaluate the proposed approach we compare the results of our method with the results when only one dot gain curve is used for each colour, both for Murray-Davies and Yule-Nielsen models. In the case of only one dot gain curve for each separation we use the curve that gives a minimized ∆ELab using least squares method. The experiments and calculations show that our approach gives a better approximation of the resulting colour coordinates.

A digital gray scale image generally consists of 256 different gray tones. Printers and image setters normally generate much fewer levels and mostly only two levels. Therefore, in order to be able to display a digital cone-tone image by a multilevel device it has to be transformed into an image with fewer levels. The technique doing this transformation is called multi-level halftoning, and in the case of bilevel devices it is simply called halftoning. In this paper we propose a novel (bilevel) halftoning technique that is based on multilevel halftoning. The proposed method can also be categorized as belonging to hybrid amplitude modulated (AM)/ frequency modulated (FM) techniques. In this method the original digital image is firstly halftoned by a multilevel FM halftoning. Each level in the multilevel halftoned image is then replaced by a halftone table (microcell). An approach for extending any bilevel FM halftoning method to a multilevel method is also presented in this paper. The performance of the proposed method is examined by a number of illustrations where nonmodified error diffusion and our FM method are used. The problem with maze-like artifacts that occur when our FM halftoning, or similar methods such as DBS, are used as the macroscreen is discussed and a simple solution is introduced. An approach for extending the proposed method to be used in situations where the halftone dots cannot be produced smaller than a specific size is also proposed and examined. This modified version of the method can be useful for flexography, where the dots normally cannot be produced smaller than a critical size.

The underline idea of grey component replacement (GCR) is to replace a mixture of primary colors (cyan, magenta, and yellow) by a black. Current algorithms of GCR are mainly based on the concept of equal-tone-value-reduction or mixing equal amount (tone value) of primary colors generating gray, which in turn can be represented by the same amount of black. As the colors used are usually non-ideal, such a replacement can result in remarkable color deviation.    

We proposed an algorithm of maximal GCR based on color matching, i.e. the black is introduced in a way that preserves the color (before and after GCR). In the algorithm, the primary with smallest tonal value is set to be zero (tone value) while the other two are reduced according to the color matching calculations. To achieve a real color matching of print, dot gain effects have been considered in the calculation. The proposed algorithm has been tested successfully for FM halftoning using an electrophotographic printer.   

Most printing devices, such as laser and ink jet printers and many print presses, are restricted to very few colors. The contone images should therefore be transformed into binary ones before being printed. The techniques doing this transformation are referred to as halftoning methods. Halftoning methods can be divided into two main categories, namely AM (Amplitude Modulated) and FM (Frequency Modulated). Some printing methods, such as Flexography, are not able to produce dots sufficiently small in order to handle the highlights and the shadows of the original image by using just an AM halftoning method. In this article we propose a hybrid halftoning method that incorporates AM and FM technologies in order to overcome this problem. The strategy is to use an FM method in the highlights (and the shadows) of the image and an AM method in the rest of the image.