Many 3D printing applications require the reproduction of an object's color in addition to its shape. One
3D printing technology, called multi-jetting (or poly-jetting), allows full color 3D reproductions by arranging
multiple colored materials (UV curing photo-polymers) on a droplet level in a single object. One property of
such printing materials is their high translucency posing new challenges for characterizing such 3D printers to
create ICC profiles.
In this paper, we will first describe the whole color-managed 3D printing workflow and will then focus
on measuring the colors of highly translucent printing materials. We will show that measurements made by
spectrophotometers used in the graphic arts industry are systematically biased towards lower reflection. We will
then propose a trichromatic camera-based approach for measuring such colors. Error rates obtained in comparison
with spectroradiometric measurements for the same viewing conditions are within the interinstrument-variability
of hand-held spectrophotometers used in graphic arts.
Special effect coatings have been increasingly used in many industries (e.g. automotive, plastics industry) over
the past two decades. The measurement of perceived color differences on such coatings cannot be done by means
of traditional color-difference formulas (e.g. CMC(l:c), CIEDE2000, AUDI2000) as they lack to consider distinct
optical properties such as coarseness, glint and goniochromatism. However, there is a need to ensure quality
and colorimetric accuracy when designing and processing special effect coatings. In this paper, we present a
psychophysical experiment intended to serve as a basis for future work on a new generation of color-difference
formula(s) for multiple viewing geometries (viewing and illumination angle). We are especially interested in
assessing whether judging under a single geometry can lead to different results as judging under several (two)
geometries, i.e. whether the sum is more than its part.
Printing appearance effects beyond colour - such as gloss - is an active research topic in the scope of multi-layer printing (2.5D or 3D printing). Such techniques may enable a perceptually more accurate reproduction of optical material properties and are required to avoid appearance related artefacts sometimes observed in regular colour printing - such as bronzing and differential gloss. In addition to technical challenges of printing such effects, a perceptual space that describes the related visual attributes is crucial; particularly to define perceptually meaningful tolerances and for appearance gamut mapping. In this paper, we focus on spatially-varying gloss created by varnish-halftones. This enables us to print specular gloss effects covering a large portion of the NCS gloss scale from full matte to high gloss. We then conduct a psychophysical experiment to find the relationship between measured specular gloss and a perceptually uniform gloss scale. Our results show that this relationship can be well described by a power function according to Stevens Power Law.
In this study, we investigate the extent to which an image-difference metric based on structural similarity can
correlate with human judgment. We introduce a modified version of the recently published iCID metric and
present new results over two large image quality databases. It is particularly noteworthy that the proposed
metric yields a correlation of 0.861 with mean opinion scores on the 2013 version of the renowned Tampere
Image Database, without dedicated parameter optimization.
KEYWORDS: Color management, Visible radiation, Ranging, Reflectivity, Error analysis, Color reproduction, Electronic imaging, Current controlled current source, Databases, Multispectral imaging
Multi- or hyper-spectral pixels are usually represented as vectors with high dimensionality. For many applications,
not all of these dimensions are actually necessary, and a few values are enough to measure and/or process the
pixel with a very good accuracy. In this work, we introduce a new strategy to reduce the dimensionality of
spectral images ranging in the visible wavelengths, for purposes of color management. We define a new Interim
Connection Space (ICS) that contains only five dimensions, and show that it has numerous advantages over
state-of-the-art ICS such as LabPQR. In particular, it allows for a better spectral reconstruction accuracy.
The grey scale method is commonly used for investigating differences in material appearance. Specifically, for testing
color difference equations, perceived color differences between sample pairs are obtained by visually comparing to
differences in a series of achromatic sample pairs. Two types of grey scales are known: linear and geometric. Their
instrumental color differences vary linearly or geometrically (i.e., exponentially), respectively. Geometric grey scales are
used in ISO standards and standard procedures of the textile industries.
We compared both types of grey scale in a psychophysical study. Color patches were shown on a color-calibrated
display. Ten observers assessed color differences in sample pairs, with color differences between ΔEab = 0.13 and 2.50.
Assessments were scored by comparison to either a linear or a geometric grey scale, both consisting of six achromatic
pairs. For the linear scale we used color differences ΔEab = 0.0, 0.6, 1.2,..., 3.0. For the geometric scale this was
ΔEab=0.0, 0.4, 0.8, 1.6, 3.2, 6.4. Our results show that for the geometric scale, data from visual scores clutter at the low
end of the scale and do not match the ΔEab range of the grey scale pairs. We explain why this happens, and why this is
mathematically inevitable when studying small color differences with geometric grey scales. Our analysis explains why
previous studies showed larger observer variability for geometric than for linear scales.
An important aspect for print quality assessment is the perceived gloss level across the printout. There exists a strong
relationship between the surface roughness of a printout and the amount of specular reflection which is perceived as
gloss variations. Different print parameters influence the surface roughness of the printouts such as the paper substrate,
the type of inks and the print method. The lack of control over the print’s surface roughness may result in artefacts such
as bronzing and differential gloss.
Employing a 2.5D or relief printing system, we are able to control the printout roughness by manipulating the way the
ink is deposited in a layer-by-layer basis. By changing the deposition time in between two layers of white ink and the
order on which the pixels are printed, we achieve different gloss levels from a matte to a glossy appearance that can be
controlled locally. Understanding the relationship between different printing parameters and the resulting gloss level
allows us: to solve differential gloss artefacts (to obtain a print with a full gloss or matte finish) and to use the local gloss
variations to create reflection effects in the printouts. Applications related to security printing have also been explored.
Our results showed a reduced level of gloss toward a matte appearance as the ink deposition time between the layers was
increased, allowing more time for the ink to dry between passes. We measured the gloss levels using a gloss meter and a
psychophysical experiment was conducted to validate our measurements and observations.
KEYWORDS: High dynamic range imaging, Time multiplexed optical shutter, Visualization, Colorimetry, Associative arrays, Space operations, Visual process modeling, Optimization (mathematics), Computer graphics, Image quality standards
In this paper, we present a novel approach of tone mapping as gamut mapping in a high-dynamic-range (HDR) color space. High- and low-dynamic-range (LDR) images as well as device gamut boundaries can simultaneously be represented within such a color space. This enables a unified transformation of the HDR image into the gamut of an output device (in this paper called HDR gamut mapping). An additional aim of this paper is to investigate the suitability of a specific HDR color space to serve as a working color space for the proposed HDR gamut mapping. For the HDR gamut mapping, we use a recent approach that iteratively minimizes an image-difference metric subject to in-gamut images. A psychophysical experiment on an HDR display shows that the standard reproduction workflow of two subsequent transformations – tone mapping and then gamut mapping – may be improved by HDR gamut mapping.
Spectral separation is the process of obtaining printer control values to reproduce a given spectral reflectance. Given a multispectral image where each pixel represents a spectral reflectance, separation could be implemented by inverting a physical printer model on a pixel-by-pixel basis. Such a process would obviously need to be very fast to handle high-resolution images in a reasonable time. For a printer whose spectral response is characterized by the Yule–Nielsen spectral Neugebauer model, the linear regression iteration (LRI) method can be used to invert the model. We introduce the subspace linear regression iteration (SLRI) method, a modification of LRI shown to be significantly accelerated due to performing its calculations within the subspace determined by the Neugebauer primaries. Using this subspace approach, the number of multiplications becomes independent of the spectral sampling rate. Using a standard six color printer and a common spectral sampling rate, the number of multiplications can be decreased by about two-thirds without changing the convergence behavior.
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