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

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

Development of an image analysis system to produce a standardised assessment of print quality

A method has been developed using an image analysis system that simulates human print quality perception. Previous work in the area of print quality assessment has only produced methods that measure individual print quality variables, or assess small parts of an image. The image analysis system developed in this investigation is different from the previous work because it analyses the combined effects of different variables using neural network technology. In addition, measurements from an entire image can be obtained and the system can assess images irrespective of their shape. The image analysis system hardware consists of a monochrome CCD camera, a Matrox image acquisition board and a 200 MHz Pentium computer. A data pre-processing program was developed using Visual Basic version 5 to process the image data from the camera. The processed data was fed into a neural network so that empirical models of print quality could be formulated. The neural network code originated from the Matlab neural network toolbox. Backpropagation and radial basis neural network functions were used in the investigation. The hardware and software of the image analysis system were tested for non-impact printing techniques. Images of a square, a circle and text characters with dimensions of 1 cm or less were used as test images for the image analysis system. It was established that it was possible to identify the different printing processes that produced the simple shapes and text characters using the image analysis system. This was achieved by training the neural network using pre-processed image data. This produced multi-dimensional mathematical models that were used to classify the different printing processes. The classification of the different printing processes involved the objective measurement of print quality variables. Different printing processes can produce print that differs in print quality when assessed by observers. Therefore the successful classification of the printing processes demonstrated that the image analysis system could, in some cases, simulate human print quality perception. To consolidate on the preceding printing process identification result, a simulation of print quality perception was made. A neural network was trained using observer assessments of a simple pictorial image of a face. These face images were produced using a variety of different non-impact printing techniques. The neural network model was used to predict the outcomes of a further set of assessments of face images by the same observer. The accuracy of the predictions was 23 out of 24 for both the backpropagation and radial basis function neural network functions used in the test. The investigation also produced two possible practical applications for the system. Firstly, it was shown that the system has the potential to be used as a machine that can objectively assess the print quality from photocopiers. Secondly, it was demonstrated that the system might be used for forensic work, since it can identify different printing processes

An Investigation into the Effects of Dry Etching Techniques on Changing Halftone Dot-Sizes

The purpose of this research was to investigate the effects of dry etching techniques on changing halftone dotsizes. Dry etching, or photographic color correction, has emerged in recent years as an alternative to traditional dot etching for making local corrections to color separation halftones. This study examines six factors related to dry etching. First, three types of original halftone dots were considered: laser formed (hard) dots from a Hell DC-300B scanner, laser formed (semi-hard) dots from a P.D.I, scanner, and contact screen (soft) dots from a Crosfield 510 Magnascan scanner. Second, the original dot-sizes being treated were fixed as close as possible to five target dot-sizes (5$, 25$, 50$, 75$, and 95$). Third, three types of dry etching treat ments were applied a technique of overexposure; the use of a 4-mil clear spacer film; and a technique of exposing through the base. Fourth, sixteen levels of corrective treatment were applied ranging from 1 times dot-for-dot exposure to 100 times. Fifth, two types of graphic arts films were test ed lith film and rapid access. Sixth, two generations of contact printing were compared original positive-to-inter mediate negative and intermediate negative-to-duplicate positive

A Study utilizing halftone based digital proofing systems in the flexographic printing process

Contract proofing for printing has been traditionally done by press proofing. This is costly and wasteful, not just in terms of equipment and labor but also in terms of expendables. The advent of off-press proofing was greeted with some degree of uncertainty by the printing industry. With press proofing, the proof was literally a preview of what would happen on a press. The press proof, although often printed on a different press, generally used the same type of inks, plates and substrate that would characterize the final print. With offpress proofing, printers were comparing apples to oranges; instead of comparing a press sheet to a press sheet they were comparing a press sheet to an approximation of a press sheet. However, over time as printers learned to read off-press proofs, they became accepted as contract proofs. The same situation has now befallen digital proofs. In the particular case of flexography, the proofing problem is a bit different. Off-press analog proofs were designed with lithography in mind, they were characterized to simulate lithographic dot gain. In order to make a proof that looks like a flexographic press sheet, two sets of films are required; one which compensates for flexographic dot gain (this is the set from which the job would be printed) and one which has extra dot gain built into the highlight and quarter- tones (this is the set that the proof would be made from). This extra set of films is wasteful and time consuming to generate. Digital proofing seems to be well suited for flexography, because the dot gain can be built into the proofing system and no extra film is required to create the proof. At the most basic level there are two types of digital proofers available; those which simulate halftone dots and those that do not. Whether or not the dots are necessary is open to discussion, however, in the case of flexography the dots appear to be crucial. For this research document it was decided that the halftone dots were preferred. The reason for this is that at about 133 lpi the rosette patterns formed by halftone dots are at the threshold of resolution by the human eye. For more course screen rulings this is even more critical. Much of flexography is printed at screen rulings of 133 lpi or lower, so very often the dots can be resolved by the eye. Therefore, the mindset at the beginning of the research was that if the dots can be resolved on the press sheet then the dots should be resolved on the proof. The major thrust of this research was to observe whether or not a halftone-based digital proofer can simulate the appearance of a flexographic press sheet. A flexographic test form was created and printed on a film based substrate. A press sheet was sent to two vendors who manufacture halftone proofers. The proofing systems are not mentioned by name; they are instead referred to as Digital Proof A and B. They then attempted to match the press sheet as closely as possible. Thus, through reverse engineering, the vendors created a device profile for this set of printing conditions. Upon receipt of the proofs, they were compared to the press sheet in terms of optical density, hue (AE) and halftone dot size. Later, a visual assessment was executed to observe how closely the digital proofs matched the press sheet using a 3M Matchprint, that had been altered to approximate flexography, as the reference or control proof. The results showed that there were significant differences between proof and press sheet in some instances and insignificant differences in others. In terms of the physical structure of the halftone dots, the 3M Matchprint had the closest match to the press sheet dot structure. In terms of physical dot size; digital proof A best matched the 50 and 75% dots and the Matchprint matched the 5% dots the best. In terms of optical density; digital proof A best matched the density of the 25% dots, digital proof B best matched the density of the 5% dots and the Matchprint best matched the 50 and 75% dot patches. In terms of AE values (color or hue difference); the Matchprint most closely matched the press sheet, digital proof B was next, and digital proof A was last. In terms of a visual match, the three proofs were found to be statistically equal in their ability to visually match the press sheet. The visual match being the most powerful of the criteria; shows that the measurable differences in the proofs did not directly affect their ability to match the press sheet. The results show that either of the two halftone digital proofs could have been used in place of the 3M Matchprint. The results also question the need for halftone dots in a proof. This is primarily because the two halftone digital proofs utilized a different RIP than the Agfa generated films for the 3M Matchprint and flexographic press sheet. Yet the visual observations made by the judges could not, at a normal viewing distance, discern this difference. The conclusion is that there is no visual difference between the halftone digital proofs and the 3M Matchprint proof in terms of visually matching the press sheet

A Study using a high-addressability Inkjet Proofer to produce amhalftone p roofs matching Kodak approval in color, screening, and subject moiré

An investigation of the feasibility of using a high-addressability inkjet printer as an alternative to the traditional proofing systems, such as Kodak Approval, was completed. The inkjet proofs must match the press sheet in terms of color, screening, screen angle, screen ruling, and screen dot size. The relatively low cost per print and the ability to incorporate color management makes inkjet technology a potential candidate also for use as a proofer for these requirements. Existing software and patents on halftone inkjet proofing were analyzed. A test form was designed to test the proofing models for screening, screen angle, and screen ruling observed in the proofs. Several workflow models were developed to generate proofs that matched the press sheet in color and moiré. Solutions for the encountered problems were tested until, finally, an optimized model was obtained that was capable of generating halftone inkjet proofs matching the press sheet visually in terms of color and moiré. This model is simple, cost effective and does not require any special software. However it is limited by the constraints of file size of Photoshop

Color Reproduction of Metallic-Ink Images

We study the color reproduction of full-color metallic-ink images. Full-color metallic-ink images are prints whose contributing colorants are exclusively made of colored metallic inks. Due to the presence of metallic particles, metallic inks show a metal-like luster. These particles are opaque and hide the underlying ink or substrate. In order to obtain predictable halftone colors, we need a juxtaposed halftoning method to create halftone dots of different colors side by side without overlapping. Juxtaposed halftoning invalidates many assumptions generally made for the color reproduction workflow. For printing metallic-ink images, one needs a color separation system creating surface coverages for the 8 metallic inks that correspond to the 8 Neugebauer primaries. For this purpose, we introduce a simple and fast method for N-color separation that relies either on Demichel’s or on a variant of Kueppers’ ink-to-colorant separations. Thanks to a unique set of ink-to-colorant formulas, pseudo-CMY ink values are separated into amounts of printable colorants. We also describe color separation procedures that are able to optimize different properties of the resulting metallic-ink images

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

Currency security and forensics: a survey

By its definition, the word currency refers to an agreed medium for exchange, a nation’s currency is the formal medium enforced by the elected governing entity. Throughout history, issuers have faced one common threat: counterfeiting. Despite technological advancements, overcoming counterfeit production remains a distant future. Scientific determination of authenticity requires a deep understanding of the raw materials and manufacturing processes involved. This survey serves as a synthesis of the current literature to understand the technology and the mechanics involved in currency manufacture and security, whilst identifying gaps in the current literature. Ultimately, a robust currency is desire