Abstract. Halftoning is a crucial part of image reproduction in print. First-order frequency modulated (FM) halftones, in which the single dots are stochastically distributed, are 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., amplitude modulated halftones are commonly used in this type of printer, but they suffer from an 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 operate on the whole input image and require extensive computations and thus, are very slow when the input image is large. 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 sizes, shapes, and alignment. Using predetermined and image-independent threshold matrices makes the proposed halftoning method a point-by-point process and thereby very fast. 1 Introduction Many reproduction devices, e.g., printers, have a limited number of output states, leaving the choice of printed and nonprinted spots in order to reproduce a shade. Thus, continuous tone gray-scale or color images need to go through a process called halftoning before being printed. Because of the fact that the human eye is limited in its capacity to resolve small dots and dots close to each other, if the viewing distance is long or the dots are small enough, the human eye is not able to distinguish between the original image and the halftone one. Hence, since the human eye acts as a lowpass filter, the halftones appear pleasing if the difference between the original and the halftone is small in the low-frequency region.Halftoning algorithms are commonly categorized into two main subgroups, called amplitude modulated (AM) and frequency modulated (FM). In AM, i.e., periodic clustered-dot halftones, different shades of gray are reproduced by changing the size of the dots while keeping their spacing constant. In first-order FM, dispersed-dot halftones, on the other hand, the size of the dots is constant while their density (or frequency) is variable. There is also another type of halftones, which we call second-order FM in this paper, in which both the size and the frequency are variables. In these halftones, the clustered dots are stochastically distributed. In literature, this type of halftones is also referred to as stochastic clustered-dot halftones and even green-noise dither patterns. In Ref. 1, the radially averaged power spectrum (RAPS) curves for these three types of halftones, i.e., AM, firstorder...

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The most straightforward way of halftoning a color image is to halftone its cyan, magenta, and yellow channels independently. It has been shown in literature that halftoning the color channels dependently and using dot-off-dot strategy as much as possible will improve the print quality. Since a yellow dot on white substrate is much less visible than the other two colored dots in a number of methods only the C and M channels are halftoned dependently and the Y channel is halftoned independent of the two others. In this paper we will show that preventing the yellow dots from being printed on blue dots will improve the print quality and results in smoother textures. The color reproduction and the ink consumption for the proposed dependent color halftoning are also discussed.

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.

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.