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.

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.

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.

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.

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.

Gloss, as has long been known, is a far more complex visual concept than the present methods of instrumental gloss evaluation are able to characterize. The instrumental analyses are either highly oversimplified (standard gloss meters) or oversimplified but with results still difficult to interpret (goniophotometry). The dimensionality and power of the directed reflectance information measured by existing tools is lower and less expressive than the information gained from a direct visual examination of a surface. In this article, a new measurement principle for gloss characterization is presented, aimed to give more comprehensive gloss information, which at the same time has an intuitive interpretation. The integrated optical system is compact and has illumination and receptor devices in fixed positions, which facilitates a mechanically simple realization. The instrument is a goniophotometer with a spatial resolution, but it is restricted to a constant angle between the illumination and the receptor. The measurement yields a "ReflectanceVectorMap" (RVM) which is an approximate optical equivalent to the surface measured. The RVM simultaneously contains spatially resolved information about directed reflectance and surface apparent inclination. The resolution is high in both spatial and in angular coordinates. The measurement provides a complex massive data set, which when appropriately visualized is similar to the visual properties of the original surface and thus encourages further evaluation and interpretation. A homogeneity index called "GlossAngleSmoothness" (GAS) is introduced, derived from the RVM, by weighing perceptually "positive" and "negative" components of gloss. The index correlates well with results obtained by a panel of experienced gloss judges asked to rate gloss homogeneity for the limited but demanding set of black printed paper surfaces tested. The GAS index performs considerably better than a panel of inexperienced judges.

Gloss, as has long been known, is a visual concept far more complex than the present methods of instrumental gloss evaluation are able to characterize. A newly developed measurement principle for gloss characterization gives perceptually relevant gloss information. The characterization results in a Reflectance Vector Map (RVM) which simultaneously contains spatially resolved information about directed reflectance and apparent inclination. The experimental phase of the present study uses the RVM and a rudimentary model of a virtual optical environment to interactively visualize a simulated surface. The performance of the visualization environment has been evaluated by comparing results from two visual assessments of perceived gloss homogeneity for a limited but demanding set of black printed paper surfaces. Assessments were performed both on the physical surfaces, and on the computer generated visualizations of the same surfaces, reconstructed from the corresponding RVM’s. The results correlate well, with a less inter-judge variance in the visualization environment than with the physical surfaces. It is suggested that this visualization environment may be a powerful tool for gloss assessment, able to mediate perceptually important characteristics of gloss.

Gloss, as has long been known, is a far more complex visual concept than the present methods of instrumental gloss evaluation are able to characterize. The instrumental analyses are either highly over-simplified (standard gloss meters) or over-simplified but with results still difficult to interpret (goniophotometry). The dimensionality and power of the directed reflectance information measured by existing tools is lower and less expressive than the information gained from a direct visual examination of a surface. The purpose of this paper is to review important gloss measurement issues, in the context of perceptual evaluation. This work gives a background for defining necessary requirements for an evaluation system that can reveal the perceptually relevant gloss features of the surface measured.

Sensor interpixel correlation (IC), as caused by crosstalk, interpixel capacitance, the brighter–fatter effect, blooming or smear, lowers the sensor MTF and increases color distortion in color filter array sensors. This is generally overlooked, and challenging to mitigate. Recent findings regarding scientific grade (CCD or hybrid CMOS) sensors, describe non-linearities causing pixel charge accumulation dependent IC. This is relevant even for lower grade sensors. High dynamic range imaging (HDRI) is especially susceptible to IC, because the sensor is deliberately operated partly in saturation. Existing HDRI algorithms exclude saturated pixels but generally overlook IC. This review article motivates the need for a generalization of the point spread function (PSF) into an irradiance-exposure dependent (IED) PSF, also considering supersaturation. The IED PSF is challenging to characterize and apply, partly due to its non-linearity. However, doing so can improve the MTF irrespective of image sensor technology, and for conventional as well as HDRI imaging.

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.

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]

Recent advances in pigment production resulted in the possibility to print with RGBW primaries instead of CMYK and performing additive color mixing in printing. The RGBW pigments studied in this work have the properties of structural colors, as the primary colors are a result of interference in a thin film coating of mica pigments. In this work, we investigate the angle-dependent gamut of RGBW primaries. We have elucidated optimal angles of illumination and observation for each primary ink and found the optimal angle of observation under diffuse illumination. We investigated dot off dot halftoned screen printing with RGBW inks on black paper and in terms of angle-dependent dot gain. Based on our observations, optimal viewing condition for the given RGBW inks is in a direction of around 30◦ to the surface normal. Here, the appearance of the resulting halftoned prints can be estimated well by Neugebauer formula (weighted averaging of the individual reflected spectra). Despite the negative physical dot gain during the dot off dot printing, we observe angularly dependent positive optical dot gain for halftoned prints. Application of interference RGBW pigments in 2.5D and 3D printing is not fully explored due to the technological limitations. In this work, we provide colorimetric data for efficient application of the angle-dependent properties of such pigments in practical applications.

Distribution of ink jet ink in paper substrates and the consequences of ink penetration for printing color reproduction have been studied by combining microscopic image processing with spectroscopic analysis. The study focused on the effects of the composition of uncoated paper, for five laboratory papers plus two commercial products, all consisting of similar pulps but with different combinations of additives. In particular, it was observed that hydrophobizing internal size agents significantly reduced ink penetration, while their effect on paper optical properties was negligible. This observation thus made it possible to study experimentally the pure effects of ink penetration. Pairwise comparisons of prints on such laboratory papers with identical optical properties revealed remarkable impacts of ink penetration on optical density, causing color saturation reduction and color shift. These experimental observations confirmed the theoretical predictions.

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.