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

Image Mechanisms in Screenless Lithography

In conventional lithography, a halftone screen is used to photographically convert the continuous- tone image of the original into a grid of very small, closely spaced dots of constant density but varying areas. Screenless or continuous-tone lithography is a printing process capable of reproducing images with gradated tones without the use of a halftone screen. Screenless lithography has a high quality potential because it has few of the inherent problems that limit the quality of the halftone process. This thesis investigates the image mechanisms in screenless lithography and in particular the role of variable water receptivity. It was hypothesized that in screenless lithography which uses a particular image carrier known as the Association Products plate, the continuous-tone image is due to selective emulsification of the ink resulting in a varying ink layer thickness and therefore varying densities. It was theorized that the plate has a water receptivity which varies with exposure to light and that this results in varying degrees of water-in-ink emulsification when the plate is dampened and inked. When transferred to paper and dried this results in an ink layer of varying thickness, which in turn causes a variation in optical density. A mathematical relationship was developed relating optical density, percentage of emulsified water, and ink layer thickness. This relationship allowed prediction of the amount of water-in-ink emulsification necessary to produce a given density range and a prediction of the amount of emulsification expected on a given plate area associated with a particular printed density. Testing proved that these predicted values were not present in actual printing from the plates. It was concluded that the hypothesis is invalid

Effects of exposure on the printing characteristics of the association product process

The investigation examined the printing characteristics of the association product plate, a continuous tone lithographic process which utilizes a synthetic, light-sensitive coating variably ink-receptive according to the degree of exposure it receives. The methodology was designed to determine (1) the printing range of the plate, (2) the effective sensitivity of the coating to varying amounts of exposure within this range, and (3) the degree to which adherence to ideal tone reproduction could be achieved through systematic variation of exposure. The plate is not commercially available, therefore it was necessary to consult patent literature and prior research to obtain details of the coating composition and plate manufacture. A number of pre-trial tests were conducted to optimize the manufacturing process and achieve a desirable balance of coating resiliency and printability. After optimizing plate manufacture, equal-interval continuous tone gray scales were printed on a conventional lithographic press to determine the printing characteristics of the plate. The characteristic curves which were generated by plotting printed densities against negative densities revealed low highlight contrast and high midtone and shadow contrast within a relatively narrow log exposure range of .75-1.10. Faulty tone reproduction can often be corrected by modifying the contrast gradients of the film negative(s) used for plate exposure. In halftone lithography, this procedure selectively alters dot area to achieve desired print reflectance. In the case of continuous tone processes, tone corrected negatives selectively modify the ink receptivity of the plate coating. VIII In order to test the effects of exposure modulation on the tone reproduction characteristics of the plate, an optical matrix was sensitometrically generated from continuous tone panchromatic film. Various film development techniques were used to alter negative contrast and obtain gray scales with diverse equal-interval log E increments within the response range of the plate. The matrix was used for exposing plates of similar manufacture to generate print data for analysis. Printed densities obtained from the matrix were used to construct conventional characteristic curves, as well as response profiles relating printed density differences to log exposure increments. A mathematical model of the Jones tone reproduction diagram was used to calculate tone correction factors in terms of relative log exposure and to reconstruct tone corrected negative scales. Printed densities obtained from the test matrix were used to gauge the effect of the corrected scales on tone reproduction. This method, in essence, emulated the Jones-type approach to tone correction in a single print run, thereby reducing manufacturing and printing variables. The data revealed a high degree of success in correcting mid-tone and shadow reproduction through exposure modification. Although some areas of non-linear plate response were persistent, these were primarily due to shortcomings in the methodology rather than plate failure. Optical tone correction of the low highlight contrast was significantly more problematic. Given the relatively short printing range of the plate, the required negative gradient eliminates many original tonal values. Other means physical and/or chemical are required to lengthen the tonal range and raise highlight contrast in order to improve overall plate performance

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

Project Tech Top study of lunar, planetary and solar topography Final report

Data acquisition techniques for information on lunar, planetary, and solar topograph

The development of methods for the reproduction in continuous tone of digitally printed colour artworks

Advances in printing technologies in the late 19th century led to the development of half-toning techniques enabling the economical reproduction of photographic images in print. Whilst undoubtedly successful in low cost high volume image reproduction, half-toning representations are less faithful in detail when compared to continuous tone photomechanical methods in use at that time. This thesis asks the question: can the creative application of 21st century digital fabrication technologies enable the qualities of continuous tone imaging to be regained? In the 21st-century, printmaking may be seen as the interchange of ideas, experimental practice and interdisciplinary thinking. Printmaking has always been a means of combining modern technology and methods with existing traditional and commercial imaging processes. Technological advancement in print however does not always provide a finer quality of print. Qualities often attributed to pre-digital continuous tone printing can be lost in the transition to a digital half tone print workflow. This research project examines a near obsolete 19th century print process, the continuous tone Woodburytype, developed to address the issue of permanence in photography. Through a methodological approach analyses of the Woodburytype an empirical reconstruction of the process provides a comprehensive critique of its method. The Woodburytype’s surface qualities are not found in other photomechanical printing methods capable of rendering finely detailed photographic images. Its method of image translation results in the printed tonal range being directly proportional to the deposition thickness of the printing ink, however it never successfully developed into a colour process. By examining and evaluating digital imaging technology this study identifies, current computer aided design and manufacturing techniques and extends upon known models of Woodburytype printing through the development of this deposition height quality enabling a new digital polychromatic colour printing process

Scanline calculation of radial influence for image processing

Efficient methods for the calculation of radial influence are described and applied to two image processing problems, digital halftoning and mixed content image compression. The methods operate recursively on scanlines of image values, spreading intensity from scanline to scanline in proportions approximating a Cauchy distribution. For error diffusion halftoning, experiments show that this recursive scanline spreading provides an ideal pattern of distribution of error. Error diffusion using masks generated to provide this distribution of error alleviate error diffusion "worm" artifacts. The recursive scanline by scanline application of a spreading filter and a complementary filter can be used to reconstruct an image from its horizontal and vertical pixel difference values. When combined with the use of a downsampled image the reconstruction is robust to incomplete and quantized pixel difference data. Such gradient field integration methods are described in detail proceeding from representation of images by gradient values along contours through to a variety of efficient algorithms. Comparisons show that this form of gradient field integration by convolution provides reduced distortion compared to other high speed gradient integration methods. The reduced distortion can be attributed to success in approximating a radial pattern of influence. An approach to edge-based image compression is proposed using integration of gradient data along edge contours and regularly sampled low resolution image data. This edge-based image compression model is similar to previous sketch based image coding methods but allows a simple and efficient calculation of an edge-based approximation image. A low complexity implementation of this approach to compression is described. The implementation extracts and represents gradient data along edge contours as pixel differences and calculates an approximate image by performing integration of pixel difference data by scanline convolution. The implementation was developed as a prototype for compression of mixed content image data in printing systems. Compression results are reported and strengths and weaknesses of the implementation are identified

Laser driven variable dot size thermal wax transfer printing

A digital image hardcopy device has been designed using a laser exposure mechanism, a pigmented wax /resin donor ink sheet, and an opaque receiver sheet. The writing system relies on image-wise thermal mass transfer of molten ink to the receiver in order to produce high resolution output. With the receiver media and a specially designed donor ribbon being held to a platen through vacuum pressure, a pulsed solid-state diode pumped near IR NdrYAG laser provides the energy necessary to complete the thermal transfer process. By varying pulse width, dot size variation is possible. A mathematical model was developed to explain the physics of the imaging process and aid further experimentation. In order to maximize photothermal conversion and transfer efficiency while maintaining environmental friendliness, a water/ alcohol based multi-layer donor ribbon was designed. Digital image analysis techniques and processing algorithms were developed specifically to provide a reliable quantification scheme for all variables. A randomized four factor central composite design provided a statistically robust means by which to map measured image quality. Response surface methods of factorial experimental design afforded a means to model the ribbon design space. Utilization of the Downhill Simplex Method (Nelder and Mead, 1965) yielded the optimum point on the estimated image quality response surface. The optimum point represented the final donor ribbon composition. It is primarily the continuously variable dot size capability and high thermal efficiency of the developed system that sets this research apart from other published works related to laser driven thermal transfer

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