Pushing the Limits of 3D Color Printing: Error Diffusion with Translucent Materials

Accurate color reproduction is important in many applications of 3D printing, from design prototypes to 3D color copies or portraits. Although full color is available via other technologies, multi-jet printers have greater potential for graphical 3D printing, in terms of reproducing complex appearance properties. However, to date these printers cannot produce full color, and doing so poses substantial technical challenges, from the shear amount of data to the translucency of the available color materials. In this paper, we propose an error diffusion halftoning approach to achieve full color with multi-jet printers, which operates on multiple isosurfaces or layers within the object. We propose a novel traversal algorithm for voxel surfaces, which allows the transfer of existing error diffusion algorithms from 2D printing. The resulting prints faithfully reproduce colors, color gradients and fine-scale details.Comment: 15 pages, 14 figures; includes supplemental figure

A study in how the ink set, solid ink density and screening method influence the color gamut in four color process printing

The number of possible reproducible colors in a printing method is called the color gamut. This gamut is the range of colors around the spectrum and from light to dark that are available in the process. Previous research has found that the three parameters, spectral reflectance of the ink, solid ink density level and screening method, all influence the gamut in process color printing. This thesis evaluated how much these parameters change the color gamut both as individual parameters and in combination. A test target with 168 patches in the most saturated combinations of cyan, magenta, yellow and black was printed and measured with a spectrophotometer. The three coordinates L*, a* and b* for each patch in the target was plotted in the CIELAB space. Connecting these points forms a three dimensional object, the color gamut volume. By calculating the volume of this object the number of CIELAB combinations that can be reproduced in the system was derived. The pressrun was made at three levels of solid ink density and the targets were screened using frequency and amplitude modulated screening. The print was made on the six color Heidelberg Speedmaster 72 sheet fed lithography press at School of Printing Management and Science, Rochester Institute of Technology. The inks that were used in the test print are the Naturalith process cyan, magenta, yellow and black from Sun Chemical. In addition to these, a fluorescent magenta and yellow ink from DayGlo\u27s ink series Starfire were used. These alternative magenta and yellow inks were printed with the Naturalith cyan and black. In this way two four color process combinations were evaluated with only six inks. The prints where made on a glossy coated paper, Centura Gloss from Consolidated Paper Inc. The result show that: Fluorescent inks can be used in process color printing and that they produced a color gamut that reproduce light and more colorful colors than normal process colors, while dark colors were reproduced with lower colorfulness. The fluorescent inks that were used are semi transparent and can not yield a dark black when printed on top of black. The solution is to use a high degree of under color removal and or to printing black as the last down ink. High solid ink densities was found to increase the color gamut for both ink sets. Frequency modulated screening was found to reproduce a color gamut with significantly higher gamut for the normal sheet fed inks, while no increase could be found for the fluorescent inks. By using frequency modulated screening and increasing the solid ink density when using normal sheet fed ink, the color gamut could be increased about 30 percent compared to amplitude modulated screening and SWOP solid ink densities. The Silicon Graphics based 3D software Explorer was found to be a very useful tool when determining the shape of the color gamut

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

A comparison study of concentric screening versus conventional AM screening and FM screening in offset printing

A new screening technology from Artwork Systems, Concentric screening, was evaluated against conventional AM and FM Staccato screening. The main feature of Concentric screening is to have very small concentric white circles inside AM halftone dots. Test targets were used in order to get information about different aspects of screening, such as: sensitivity to changes in ink film thickness; color gamut size, particularly for pastel colors; and detail rendering. A comparison was made between AM, FM, and Concentric screening. The results indicate that Concentric tints are more resistant to ink variation than conventional AM screening. Thus, the dot gain stability behavior was closer to FM Screening than to AM screening when ink levels are increased. Regarding color gamut, although there were differences in the expected direction, conclusions are uncertain because of inking variability between the press runs. However, micro-photographs from tints indicate that high resolution screens (FM and Concentric) have a thinner layer of ink on the dots when compared to AM screening. This results in improved color saturation and insensitivity to inking changes. Finally, in terms of contrast and resolution, results show no visually significant differences between the different screenings

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

A study of producing smoother gradients in the flexographic process on oriented polypropylene with UV ink by varying screening techniques, gradient lengths and the surrounding

Printers find that producing smooth gradients in the highlight area is a great challenge for flexographic printing. Screening technology vendors claim that hybrid screening technologies produce smoother gradients and enhance reproducible dots in the highlight areas. This study was designed to investigate if hybrid screening technologies can achieve better gradient results than other screening technologies—conventional screening and FM screening—with the flexographic process. A single test form was printed on oriented polypropylene with UV ink, as these are common materials used in flexible packaging. The first objective was to see how different pressure settings impact tone reproduction of each screening technique. There were three pressure settings—kiss impression, moderate pressure and high pressure. Tone reproduction curves of all three screening techniques were evaluated to see the change due to the different pressure settings. The results show that FM screening had a high sensitivity to change in pressure, while AM and hybrid screenings were more forgiving to variations in pressure settings. In the highlight areas, hybrid screening is the least sensitive to changes in pressure. The second objective was to study whether smoother gradients can be produced by altering three variables: screening techniques, gradient lengths and the impact of the surrounding. These variables were used to create a gradient matrix. Printed sheets from different points in the press run were collected for data analysis. There were two types of data analysis, measurement based evaluation and visual evaluation. Because of difficulty in the methodology for analyzing the measured data, the conclusions were then based on the results from the visual evaluation. There are three aspects to the problems with gradient smoothness: highlight breaking in AM screening, graininess of FM screening, and a disjunction at the transition point of hybrid screening. When minimum dot size, transition point, and transfer curve are set correctly, hybrid screening would be the best selection to use with the flexographic process. The surrounding, or solid frames around the gradients, did not truly enhance gradient smoothness at kiss impression

An Investigation of amplitude & frequency modulated screening on dot gain and variability

This study examines the effect of screening on dot gain and variability. Essentially two hypotheses are examined: first that as screen ruling is increased, the level of dot gain is increased; and second that as screen ruling is increased the level of variability is increased. These predictions are also applied to stochastic or frequency modulated screens which are presumed to behave as very screens due to their fine dot structure. A mathematical model referred to as the border zone is proposed to describe the nature of the gain and the variability for each of the screening systems. The hypotheses were tested by printing tint patches from a variety of screening systems (various screen rulings in addition to a FM screen) on a web press. The tint patches were then measured for gain characteristics. This data were statistically ana lyzed in relation to average gain and variability. The study determined that a correlation does exist between screen ruling and dot gain, but little evidence was developed to support the idea that screening relates to variability

A comparative analysis between the RIT Contrast Resolution Test Target And the Gutenberg Test Target

In addition to color, the other two most important properties that determine quality of reproduction of a printing/imaging system are contrast and resolution. Contrast and resolution limitations of an output device partially define output quality. These limitations can be attributed to various factors such as the screening method used by the RIP, the image transfer method of the output device, the substrates used, and capabilities of the postscript interpreter or a combination of all these factors. This study introduces a new test target for quality evaluation called the Gutenberg Test Target. A comparative analysis between the Gutenberg Test Target and the RIT Contrast Resolution Target has been performed. Both targets have been developed collectively by Dr. Edward Granger and Franz Sigg and both are used to measure the contrast and resolution limitations of an imaging system. The Gutenberg target uses a visual subjective comparison to evaluate overall output quality of an imaging/printing system. On the other hand, the RIT Contrast Resolution Target uses a method of analysis to calculate a Contrast-Resolution-Volume (CRV) value, which then is used as a relative indicator of the reproduction quality of the imaging/printing system. For this study both test targets were printed on a selected imaging /printing system with varying degrees of image deteriorations (Gaussian Blur) applied on the images. The image deteriorations simulated different Modulation Transfer Functions (MTF) for different devices. The printed test targets were analyzed by performing two visual experiments, one for each target, using a number of observers. The generated data from the experiment was used for mathematical analysis and a comparison was made between the two targets. The final analysis and results showed that both targets do a good job in measuring the resolution contrast limitation of the system

Spectral printing of paintings using a seven-color digital press

The human visual system is trichromatic and therefore reduces higher dimensional spectral data to three dimensions. Two stimuli with different spectral power curve shapes can result in the same cone response and therefore match each other. Color reproduction systems take advantage of this effect and match color by creating the same cone response as the original but with different colorants. ICC color management transforms all colors into a three-dimensional reference color space, which is independent from any input or output devices. This concept works well for a single defined observer and illumination conditions, but in practice, it is not possible to control viewing conditions leading to severe color mismatches, particularly for paintings. Paintings pose unique challenges because of the large variety of available colorants resulting in a very large color gamut and considerable spectral variability. This research explored spectral color reproduction using a seven-color electrophotographic printing process, the HP Indigo 7000. Because of the restriction to seven inks from the 12 basic inks supplied with the press, the research identified both the optimal seven inks and a set of eight artist paints which can be spectrally reproduced. The set of inks was Cyan, Magenta, Yellow, Black, Reflex Blue, Violet and Orange. The eight paints were Cadmium Red Medium, Cadmium Orange, Cadmium Yellow Light, Dioxazine Purple, Phthalo Blue Green Shade, Ultramarine Blue, Quinacridone Crimson and Carbon Black. The selection was based on both theoretical and experimental analyses. The final testing was computational indicating the possibility of both spectral and colorimetric color reproduction of paintings