We propose a segmentation strategy for agricultural images in order to successfully distinguish between both soil and green parts, the last ones including weeds and crop plants, based on discrete wavelets transform. Vegetation indices have been commonly used for greenness image segmentation, but improvements are still possible. In agricultural images weeds and crops plants display high spatial variability with irregular and random distributions. Textures descriptors have the ability to capture this information, which conveniently combined with vegetation indices improve the greenness segmentation results. The proposed approach consists of the following steps: (a) greenness extraction based on vegetation indices; (b) application of the wavelets transform to the resulting image, allowing the extraction of spatial structures in three bands (horizontal, vertical and diagonal) containing detailed information; (c) use of texture descriptors to capture the spatial variability in the three bands; (d) combination of greenness and texture information, in the approximation coefficients of the wavelets transform, for enhancing plants (weeds and crops) identification; and (e) application of an image thresholding method for final image identification. The wavelets transform allows both capture of spatial texture and its fusion with the greenness information, making the main contribution of this paper. This approach is especially useful when the quality of imaging greenness is low. It has been favorably compared against existing strategies, obtaining better results, quantified by 4,5%. (C) 2015 Elsevier B.V. All rights reserved.

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.

A seemingly straightforward way to enhance the quality of printed images is to increase the number of colorants, beyond the four traditionally used, in multi-channel printing. Potential improvements to reproduced images include: increased colour accuracy, enhanced colour smoothness and reduced image graininess. Nevertheless, numerous challenges exist, one of them being the implementation of halftoning algorithms, which transform the original image into a binary one that is reproducible by the printing system. This thesis concerns the development, implementation and verification of halftoning algorithms suitable for an increased number of colorants in multi-channel printing.

The first focus in this thesis is on the implementation of an amplitude modulated (AM) halftoning method for seven-channel printing utilizing CMYKRGB colorants. The proposed AM halftoning method utilizes non-orthogonal halftone screens instead of orthogonal ones (dots), thus enabling a wider angle range for the channels that makes possible to accommodate multi-channel impressions. The performance of the non-orthogonal halftoning method was evaluated by computational simulation of channel misregistration for 1600 different scenarios and assessment of printed orthogonal and non-orthogonal patches. The simulated and printed results demonstrate that the proposed halftoning method utilizing non-orthogonal screens shows no visible moiré and produces smaller colour shifts in case of misregistration when compared to orthogonal halftoning.

However, the layer thickness of the combined colorants is not controlled by the aforementioned multi-channel AM halftoning approach. Therefore, the second focus in this thesis concerns the adjustment and implementation of a multilevel halftoning algorithm for achromatic and chromatic inks. In this algorithm, a channel is processed so that it can be printed using multiple inks of same hue value, achieving a single ink layer. Here, the thresholds for ink separation and dot gain compensation pose an interesting challenge. Since dot gain originates from the interaction between a specific ink and specific paper, compensating the original image for multilevel halftoning means expressing the dot gain of multiple inks in terms of the nominal coverage of a single ink. The applicability of the proposed multilevel halftoning workflow is demonstrated using multiple inks while avoiding dot-on-dot placement and accounting for dot gain. The results also show that the multilevel halftoned image is visually improved in terms of graininess and detailenhancement when compared to a bi-level halftoned image.

Printing using more than four ink channels visually improves the reproduction. Nevertheless, if the ink layer thickness at any given point exceeds a certain limit, ink bleeding and colour accuracy problems would occur. Halftoning algorithms that process channels dependently are one way of dealing with this shortcoming of multi-channel printing. A multilevel halftoning algorithm that processes a channel so that it is printed with multiple inks of same chromatic value was introduced in our research group. Here we implement this multilevel algorithm using three achromatic inks – photo grey, grey, black – in a real paper-ink setup. The challenges lay in determining the thresholds for ink separation and in dot gain compensation. Dot gain results in a darker reproduction and since it originates from the interaction between a specific ink and paper, compensating the original image for multilevel halftone means expressing dot gain of three inks in terms of the nominal coverage of a single ink. Results prove a successful multilevel halftone implementation workflow using multiple inks while avoiding dot-on-dot placement and accounting for dot gain. Results show the multilevel halftoned image is visually improved in terms of graininess and detail enhancement when compared to the bi-level halftoned image.

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.

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.