The appearance of an object is determined by its chromatic and geometric qualities in its surrounding environment using four optical parameters: color, gloss, translucency, and surface texture. Reconstructing the appearance of objects is of great importance in many applications, including creative industries, packaging, fine-art reproduction, medical simulation, and prosthesis-making. Printers are reproduction devices capable of replicating objects’ appearance in 2D and 3D forms. With the introduction of new printing technologies, new inks and materials, and demands for innovative applications, creating accurate reproduction of the desired visual appearance has become challenging. Thus, the appearance reproduction workflow requires improvements and adaptations. 

Accurate color reproduction is a critical quality measure in reproducing the desired appearance in any printing process. However, printers are devices with a limited number of inks that can either print a dot or leave it blank at a specific position on a substrate; hence, to reproduce different colors, optimal placement of the available inks is needed. Halftoning is a technique that deals with this challenge by generating a spatial distribution of the available inks that creates an illusion of the target color when viewed from a sufficiently large distance. Halftoning is a fundamental part of the color reproduction task in any full-color printing pipeline, and it is an effective technique to increase the potential of printing realistic and complex appearances. Although halftoning has been used in 2D printing for many decades, it still requires improvements in reproducing fine details and structures of images. Moreover, the emergence of new technologies in 3D printing introduces a higher degree of freedom and more parameters to the field of appearance reproduction. Therefore, there is a critical need for extensive studies to revisit existing halftoning algorithms and develop novel approaches to produce high quality prints that match the target appearance faithfully. This thesis aims at developing halftoning algorithms to improve appearance reproduction in 2D and 3D printing. 

Contributions of this thesis in the 2D domain is a dynamic sharpness-enhancing halftoning approach, which adaptively varies the local sharpness of the halftone image based on different textures in the original image for realistic appearance printing. The results show improvements in halftone quality in terms of sharpness, preserving structural similarity, and decreasing color reproduction error. The main contribution of this thesis in 3D printing is extending a high quality 2D halftoning algorithm to the 3D domain. The proposed method is then integrated with a multi-layer printing approach, where ink is deposited at variable depths to improve the reproduction of tones and fine details. Results demonstrate that the proposed method accurately reproduces tones and details of the target appearance. Another contribution of this thesis is studying the effect of halftoning on the perceived appearance of 3D printed surfaces. According to the results, changing the dot placement based on the elevation variation of the underlying geometry can potentially control the perception of the 3D printed appearance. It implies that the choice of halftone may prove helpful in eliminating unwanted artifacts, enhancing the object’s geometric features, and producing a more accurate 3D appearance. The proposed methods in this thesis have been evaluated using different printing techniques.    

The iridescent effect produced by structural color is difficult (if not impossible) to capture and print using traditional CMYK pigments. The so called RGB reflective pigments, nonetheless, generate angle-dependent colors by light interference. A layered surface structure generated by the pigments’ particles on a substrate reflects light waves of different wavelengths at different viewing angles according to the optical principle known as the Bragg Law. In this work, we have studied the influence of different halftone structures on printed images, produced with RGB reflective inks via screen printing. The main goal was to enhance the iridescence of a printed reproduction by studying the performance of different halftone algorithms on a screen printing process. We investigated the influence of different halftone structures in creating different spatial combinations of inks on a print to reproduce the image of an iridescent feathered headdress. We applied first-order, second-order, and structure-aware FM halftones to compare how they influence the reproduction of the material appearance of the object represented in the original image. The results show that the structure-ware halftones improve the representation of the image structures and details. Therefore, it could better convey the 3D surface features that produce iridescence in real feathers.

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.

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.

Today, power generation from clean and renewable resources such as wind and solar is of great salience. Smart grid technology efficiently responds to the increasing demand for electric power. Intelligent monitoring, control, and maintenance of wind energy facilities are indispensable to increase the performance and efficiency of smart grids (SGs). Integration of state-of-the-art machine learning algorithms and vision sensor networks approaches pave the way toward enhancing the wind farms performance. The generating power in a wind turbine farm is the most critical parameter that should be measured accurately. Produced power is highly related to weather patterns, and a new farm in a near area is also likely to have similar energy generation. Therefore, accurate and perpetual prediction models of the existing wind farms can be led to develop new stations with lower costs. The paper aims to estimate the angular velocity of turbine blades using vision sensors and signal processing. The high wind in the wind farm can cause the camera to vibrate in successive frames, and the noise in the input images can also strengthen the problem. Thanks to couples of solid computer vision algorithms, including FAST (Features from Accelerated Segment Test), SIFT (Scale-Invariant Feature Transform), SURF (Speeded Up Robust Features), BF (Brute-Force), FLANN (Fast Library for Approximate Nearest Neighbors), AE (Autoencoder), and SVM (support vector machines), this paper accurately localizes the Hub and track the presence of the Blade in consecutive frames of a video stream. The simulation results show that determining the hub location and the blade presence in sequential frames results in an accurate estimation of wind turbine angular velocity with 95.36% accuracy. (C) 2021 The Authors. Published by Elsevier Ltd.

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.