N-colour separation methods for accurate reproduction of spot colours

In packaging, spot colours are used to print key information like brand logos and elements for which the colour accuracy is critical. The present study investigates methods to aid the accurate reproduction of these spot colours with the n-colour printing process. Typical n-colour printing systems consist of supplementary inks in addition to the usual CMYK inks. Adding these inks to the traditional CMYK set increases the attainable colour gamut, but the added complexity creates several challenges in generating suitable colour separations for rendering colour images. In this project, the n-colour separation is achieved by the use of additional sectors for intermediate inks. Each sector contains four inks with the achromatic ink (black) common to all sectors. This allows the extension of the principles of the CMYK printing process to these additional sectors. The methods developed in this study can be generalised to any number of inks. The project explores various aspects of the n-colour printing process including the forward characterisation methods, gamut prediction of the n-colour process and the inverse characterisation to calculate the n-colour separation for target spot colours. The scope of the study covers different printing technologies including lithographic offset, flexographic, thermal sublimation and inkjet printing. A new method is proposed to characterise the printing devices. This method, the spot colour overprint (SCOP) model, was evaluated for the n-colour printing process with different printing technologies. In addition, a set of real-world spot colours were converted to n-colour separations and printed with the 7-colour printing process to evaluate against the original spot colours. The results show that the proposed methods can be effectively used to replace the spot coloured inks with the n-colour printing process. This can save significant material, time and costs in the packaging industry

A Study of effect of CRT gamma and white point on softcopy and hardcopy agreement

DTP (Desk Top Publishing) professionals rely on CRT displays to provide visual feedback to adjust or to proof color prior to hardcopy. Solutions are now appearing in the market to meet this need. But the proper CRT calibration is still not clear to the end users. The objective of this work is to study the CRT setting in terms of gamma and white point; and to explore gamma and white point\u27s effects on softcopy (CRT displayed image) and hardcopy (CMYK printed image) agreement. A number of CRT calibration experiments were performed. Two SCID (Standard Color Image Data) images were used in this study to test the agreement between a softcopy and a hardcopy image. A number of color measurement devices and color management software packages were used in this study. Specifically, ColorTron was used in this study as the tool to calibrate the CRT. Adobe Photoshop, with the ColorSync 2.0 plug-in module was used in this study to implement the printer CMYK to CRT RGB transformation. ColorBlind was used in this study to generate printer and monitor profiles. CA-100 was used in this study as a colorimetric measurement device for data collection and image gamma analysis. By means of observer experiment conducted under dark ambient light, it was found that the different CRT profiles do influence the color transformation between printer CMYK and CRT RGB; the system\u27s default CRT profile (gamma=1.8, whit point=D50, out of 6 CRT profiles tested) cannot achieve the best match between CRT and hardcopy. The optimum CRT profile for the best match was not to be found because of the influence of the keyness of the image itself

Modeling and Halftoning for Multichannel Printers: A Spectral Approach

Printing has been has been the major communication medium for many centuries. In the last twenty years, multichannel printing has brought new opportunities and challenges. Beside of extended colour gamut of the multichannel printer, the opportunity was presented to use a multichannel printer for ‘spectral printing’. The aim of spectral printing is typically the same as for colour printing; that is, to match input signal with printing specific ink combinations. In order to control printers so that the combination or mixture of inks results in specific colour or spectra requires a spectral reflectance printer model that estimates reflectance spectra from nominal dot coverage. The printer models have one of the key roles in accurate communication of colour to the printed media. Accordingly, this has been one of the most active research areas in printing. The research direction was toward improvement of the model accuracy, model simplicity and toward minimal resources used by the model in terms of computational power and usage of material. The contribution of the work included in the thesis is also directed toward improvement of the printer models but for the multichannel printing. The thesis is focused primarily on improving existing spectral printer models and developing a new model. In addition, the aim was to develop and implement a multichannel halftoning method which should provide with high image quality. Therefore, the research goals of the thesis were: maximal accuracy of printer models, optimal resource usage and maximal image quality of halftoning and whole spectral reproduction system. Maximal colour accuracy of a model but with the least resources used is achieved by optimizing printer model calibration process. First, estimation of the physical and optical dot gain is performed with newly proposed method and model. Second, a custom training target is estimated using the proposed new method. These two proposed methods and one proposed model were at the same time the means of optimal resource usage, both in computational time and material. The third goal was satisfied with newly proposed halftoning method for multichannel printing. This method also satisfies the goal of optimal computational time but with maintaining high image quality. When applied in spectral reproduction workflow, this halftoning reduces noise induced in an inversion of the printer model. Finally, a case study was conducted on the practical use of multichannel printers and spectral reproduction workflow. In addition to a gamut comparison in colour space, it is shown that otherwise limited reach of spectral printing could potentially be used to simulate spectra and colour of textile fabrics

AN APPRAISAL OF THE DEVELOPMENTS IN THE REPRODUCTION OF COLOUR IN COMPUTER PUBLISHING SYSTEMS

The plethora of coloured images that are reproduced in any printed media is facilitated through a variety of related processes that collectively constitute traditional printing techniques. The aim of this research is to appraise recent developments that have occurred within the colour prepress process. The colour prepress process involves the preparation of colour separated halftone films that are used in the production of offset lithographic printing plates. Over recent years the application of desktop publishing technology to perform many of the functions associated with the colour prepress process has raised a number of significant issues and debates. The reproduction of coloured images in the printed medium demands that certain fundamental criteria are adhered to in order to maintain professional standards of colour fidelity. Such criteria include: successful digital halftone production, the elimination of moiré patterns, and maintaining colour fidelity between the coloured original and the coloured reproduction. This research thesis shall therefore establish the principles and techniques involved in the reproduction of colour in a printed medium. It will also asses whether desktop publishing systems are able to facilitate successful professional colour reproduction by examining current debates that challenge the viability of desktop publishing solutions. Current debates concerning desktop publishing solutions are primarily concerned with assessing the value of Adobe PostScript level 2 solutions, computer interchange spaces for colour matching purposes, and rational supercell techniques that attempt to eliminate moiré patterns. The research also attempts to establish the validity of current debate findings by comparing them with statistics derived hom a questionnaire (undertaken as part of the research program) that seeks the opinions of system users on the effectiveness of their individual systems at processing and delivering acceptable colour separations

Digital Color Imaging

This paper surveys current technology and research in the area of digital color imaging. In order to establish the background and lay down terminology, fundamental concepts of color perception and measurement are first presented us-ing vector-space notation and terminology. Present-day color recording and reproduction systems are reviewed along with the common mathematical models used for representing these devices. Algorithms for processing color images for display and communication are surveyed, and a forecast of research trends is attempted. An extensive bibliography is provided

Innovative color management methods for RGB printing

Thesis (M. Eng.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2006.Includes bibliographical references (leaf 50).Re-calibrating a printer in response to systematic changes is measurement and labor intensive. In this study, a fast correction method with cycle-to-cycle control was proposed. The process includes two steps: the creation of look-up table using a characterization data set, and image color compensation in conjunction with Windows printing architecture. Several types of correction models for determining printer characterization were proposed and evaluated, including polynomial models and neural network models. The most successful of these methods was the quadratic spline interpolation model, which removed most errors introduced by the changes of colorant and printing substrate. A significant reduction in error was realized by incorporating this technique into the color management program.by Wei Dong.M.Eng

Clustered-dot periodic halftone screen design and ICC profile color table compression

This dissertation studies image quality problems associated with rendering images in devices like printing or displaying. It mainly includes two parts: clustered-dot periodic halftone screen design, and color table compression. Screening is a widely used halftoning method. As a consequence of the lower resolution of digital presses and printers, the number of printer-addressable dots or holes in each microcell may be too few to provide the requisite number of tone lev- els between paper white and full colorant coverage. To address this limitation, the microcells can be grouped into supercells. The challenge is then to determine the desired supercell shape and the order in which dots are added to the microcell. Using DBS to determine this order results in a very homogeneous halftone pattern. To simplify the design and implementation of supercell halftone screens, it is common to repeat the supercell to yield a periodically repeating rectangular block called the basic screen block (BSB). While applying DBS to design a dot-cluster growth order- ing for the entire BSB is simpler to implement than is the application of DBS to the single non-rectangular supercell, it is computationally very inefficient. To achieve a more efficient way to apply DBS to determine the microcell sequence, we describe a procedure for design of high-quality regular screens using the non-rectangular super- cell. A novel concept the Elementary Periodicity Set is proposed to characterize how a supercell is developed. After a supercell is set, we use DBS to determine the micro-cell sequence within the supercell. We derive the DBS equations for this situation, and show that it is more efficient to implement. Then, we mainly focus on the regular and irregular screen design. With digital printing systems, the achievable screen angles and frequencies are limited by the finite addressability of the marking engine. In order for such screens to generate dot clusters in which each cluster is identical, the elements of the periodicity matrix must be integer-valued, when expressed in units of printer-addressable pixels. Good approximation of the screen sets result in better printing quality. So to achieve a better approximation to the screen sets used for commercial offset printing, irregular screens can be used. With an irregular screen, the elements of the periodicity matrix are rational numbers. In this section, first we propose a procedure to generate regular screens starting from midtone level. And then we describe a procedure for design of high-quality irregular screens based on the regular screen design method. We then propose an algorithm to determine how to add dots from midtone to shadow and how to remove dots from midtone to highlight. We present experimental results illustrating the quality of the halftones resulting from our design procedure by comparing images halftoned with irregular screens using our approach and a template-based approach. We also present the evaluation of the smoothness and improvement of the proposed methods. In the next part, we study another quality problem: ICC profile color table compression. ICC profiles are widely used to provide transformations between different color spaces in different devices. The color look-up tables (CLUTs) in the profiles will increase the file sizes when embedded in color documents. In this chapter, we discuss compression methods that decrease the storage cost of the CLUTs. For DCT compression method, a compressed color table includes quantized DCT coefficients for the color table, the additional nodes with large color difference, and the coefficients bit assignment table. For wavelet-based compression method, a compressed color table includes output of the wavelet encoding method, and the additional nodes with large color difference. These methods support lossy table compression to minimize the network traffic and delay, and also achieves relatively small maximum color difference

Printing Beyond Color: Spectral and Specular Reproduction

For accurate printing (reproduction), two important appearance attributes to consider are color and gloss. These attributes are related to two topics focused on in this dissertation: spectral reproduction and specular (gloss) printing. In the conventional printing workflow known as the metameric printing workflow, which we use mostly nowadays, high-quality prints -- in terms of colorimetric accuracy -- can be achieved only under a predefined illuminant (i.e. an illuminant that the printing pipeline is adjusted to; e.g. daylight). While this printing workflow is useful and sufficient for many everyday purposes, in some special cases, such as artwork (e.g. painting) reproduction, security printing, accurate industrial color communication and so on, in which accurate reproduction of an original image under a variety of illumination conditions (e.g. daylight, tungsten light, museum light, etc.) is required, metameric reproduction may produce satisfactory results only with luck. Therefore, in these cases, another printing workflow, known as spectral printing pipeline must be used, with the ideal aim of illuminant-invariant match between the original image and the reproduction. In this workflow, the reproduction of spectral raw data (i.e. reflectances in the visible wavelength range), rather than reproduction of colorimetric values (colors) alone (under a predefined illuminant) is taken into account. Due to the limitations of printing systems extant, the reproduction of all reflectances is not possible even with multi-channel (multi-colorant) printers. Therefore, practical strategies are required in order to map non-reproducible reflectances into reproducible spectra and to choose appropriate combinations of printer colorants for the reproduction of the mapped reflectances. For this purpose, an approach called Spatio-Spectral Gamut Mapping and Separation, SSGMS, was proposed, which results in almost artifact-free spectral reproduction under a set of various illuminants. The quality control stage is usually the last stage in any printing pipeline. Nowadays, the quality of the printout is usually controlled only in terms of colorimetric accuracy and common printing artifacts. However, some gloss-related artifacts, such as gloss-differential (inconsistent gloss appearance across an image, caused mostly by variations in deposited ink area coverage on different spots), are ignored, because no strategy to avoid them exists. In order to avoid such gloss-related artifacts and to control the glossiness of the printout locally, three printing strategies were proposed. In general, for perceptually accurate reproduction of color and gloss appearance attributes, understanding the relationship between measured values and perceived magnitudes of these attributes is essential. There has been much research into reproduction of colors within perceptually meaningful color spaces, but little research from the gloss perspective has been carried out. Most of these studies are based on simulated display-based images (mostly with neutral colors) and do not take real objects into account. In this dissertation, three psychophysical experiments were conducted in order to investigate the relationship between measured gloss values (objective quantities) and perceived gloss magnitudes (subjective quantities) using real colored samples printed by the aforementioned proposed printing strategies. These experiments revealed that the relationship mentioned can be explained by a Power function according to Stevens' Power Law, considering almost the entire gloss range. Another psychophysical experiment was also conducted in order to investigate the interrelation between perceived surface gloss and texture, using 2.5D samples printed in two different texture types and with various gloss levels and texture elevations. According to the results of this experiment, different macroscopic texture types and levels (in terms of texture elevation) were found to influence the perceived surface gloss level slightly. No noticeable influence of surface gloss on the perceived texture level was observed, indicating texture constancy regardless of the gloss level printed. The SSGMS approach proposed for the spectral reproduction, the three printing strategies presented for gloss printing, and the results of the psychophysical experiments conducted on gloss printing and appearance can be used to improve the overall print quality in terms of color and gloss reproduction

Recovering Neugebauer colorant reflectances and Ink-spreading curves from printed color images

Spectral reflection prediction models, although effective, are impractical for certain industrial applications such as self-calibrating devices and online monitoring because their calibration requires specific color-constant calibration patches. Using the CMYK Ink-Spreading enhanced Yule-Nielsen-modified Spectral Neugebauer model (IS-YNSN), we propose a method to recover the colorant reflectances (Neugebauer primaries), the ink-spreading curves, and the Yule-Nielsen n-value using only tiles extracted from printed color images. There is no prior knowledge about the reproduction device. Thanks to a set of constraints based on principal component analysis and the relationships between composed Neugebauer primaries and the ink transmittances, good approximations of the Neugebauer primaries are achieved. These approximations are then optimized, yielding an accurately calibrated IS-YNSN model comparable to the one obtained by classical calibrations

Spectrally stable ink variability in a multi-primary printer

It was shown previously that a multi-ink printer can reproduce spectral reflectances within a specified tolerance range using many distinct ink combinations. An algorithm was developed to systematically analyze a printer to determine the amount of multi-ink variability throughout its spectral gamut. The advantage of this algorithm is that any spectral difference metric can be used as the objective function. Based on the results of the analysis for one spectral difference metric, six-dimensional density map displays were constructed to illustrate the amount of spectral redundancy throughout the ink space. One CMYKGO ink-jet printer was analyzed using spectral reflectance factor RMS as the spectral difference metric and selecting 0.02 RMS as the tolerance limit. For these parameters, the degree of spectral matching freedom for the printer reduced to five inks because the chromatic inks were able to reproduce spectra within the 0.02 tolerance limit throughout the printer\u27s gamut. Experiments were designed to exploit spectrally stable multi-ink variability within the analyzed printer. The first experiment used spectral redundancy to visually evaluate spectral difference metrics. Using the developed database of spectrally similar samples allows any spectral difference metric to be compared to a visual response. The second experiment demonstrated the impact of spectral redundancy on spectral color management. Typical color image processing techniques use profiles consisting of sparse multi-dimensional lookup tables that interpolate between adjacent nodes to prepare an image for rendering. It was shown that colorimetric error resulted when interpolating between lookup table nodes that were inconsistent in digital count space although spectrally similar. Finally, the analysis was used to enable spectral watermarking of images. To illustrate the significance of this watermarking technique, information was embedded into three images with varying levels of complexity. Prints were made verifying that information could be hidden while preserving the visual and spectral integrity of the original image