Philipp Urban

ACM Transactions on Graphics

We present an efficient and scalable pipeline for fabricating full-colored objects with spatially-varying translucency from practical and accessible input data via multi-material 3D printing. Observing that the costs associated with BSSRDF measurement and processing are high, the range of 3D printable BSSRDFs are severely limited, and that the human visual system relies only on simple high-level cues to perceive translucency, we propose a method based on reproducing perceptual translucency cues. The input to our pipeline is an RGBA signal defined on the surface of an object, making our approach accessible and practical for designers. We propose a framework for extending standard color management and profiling to combined color and translucency management using a gamut correspondence strategy we call opaque relative processing. We present an efficient streaming method to compute voxel-level material arrangements, achieving both realistic reproduction of measured translucent materials and artistic effects involving multiple fully or partially transparent geometries.

Journal of the Society for Information Display

The effect of observer metamerism induced by electronic displays depends to a large extent on their primary spectra (red, green, and blue in the most common case). In particular, for narrow-band primary spectra whose peak wavelength lies in the range of high variability of the observer's colormatching function, some observers can experience very large differences between actual surface colors (e.g. in a light booth) and displayed colors if the monitor is optimized for the International Commission on Illumination (CIE) 1931 standard observer. However, because narrow-band light-emitting diodes lead to larger color gamuts, more and more monitors with very narrow band primaries are coming onto the market without manufacturers taking into account the associated problem of observer variations. Being able to measure these variations accurately and efficiently is therefore an important objective. In this paper, we propose a new approach to predict the extent of observer metamerism for a particular multiprimary display. Unlike existing dedicated models, ours does not depend on a reference illuminant and a set of reflectance spectra and is computationally more efficient.

Journal of Imaging Science and Technology

We present a novel extended colour space for low- dimensional spectral colour management, coined LabAB. We discuss design goals and perform a comparison of sev- eral approaches to fuse the perceptual information engen- dered by several illuminants. Results indicate that, with only five dimensions, LabAB conveys significantly no less perceptual information on average than state-of-the-art rep- resentations such as the interim connection space LabPQR. We also demonstrate that LabAB can be used for spectral vector error diffusion.

Measuring, Modeling, and Reproducing Material Appearance 2016

Measuring, Modeling, and Reproducing Material Appearance (MMRMA) <2016, San Francisco, CA, USA>

The relation between perceived gloss levels and the texture height of a surface (sometimes referred to as bumpiness) has previously been investigated through several psychophysical experiments, which have suggested that a surface is perceived more glossy when the amount of texture is increased, and likewise, a surface is perceived more textured when the gloss level is increased. However, these studies have only been conducted using computer simulations as stimuli instead of physical surfaces or objects. The latter case is investigated in this paper, where physical samples of surfaces varying in surface gloss and texture levels were created by a 2.5D printing system. Psychophysical experiments were then conducted using these samples to investigate the influence of the macroscale texture characteristics on the perceived magnitude of surface glossiness. Although our results show that the influence of the gloss level on the perceived surface texture is negligible, they do confirm the existence of a slight influence of surface texture on the perception of surface glossiness.

IS&T

Measuring, Modeling, and Reproducing Material Appearance (MMRMA) <2016, San Francisco, CA, USA>

Electronic Imaging, 9

The main topics encountered on the papers of this document correspond to any of the following categories: Methods for measuring material properties: measurement of Bidirectional Reflectance Distribution Functions (BRDF), Bidirectional Texture Functions (BTF) and Bidirectional Surface Scattering Reflectance Distribution Function (BSSRDF); estimation of material difference perception; evaluation of metallic coatings/inks; measurement of glossiness; estimation of texture perception; data acquisition methods for different types of materials. Models for distinct characteristics of materials: modeling of Bidirectional Reflectance Distribution Functions (BRDF), Bidirectional Texture Functions (BTF) and Bidirectional Surface Scattering Reflectance Distribution Function (BSSRDF); modeling material difference perception; appearance modeling of glossiness and texture; modeling of varnish and special effects inks; softproofing methods for 2.5D and 3D printing. Material reproduction aspects: quality evaluation of 2.5D and 3D soft- and hardcopy reproductions (display and printing); estimation of effects of environmental aspects in material perception (lighting, observers' position, printing media); estimation of sensory input (visual, touch, audio) effect in material perception; evaluation of aesthetic aspects of 2.5D and 3D soft- and hard-copy reproductions (display and printing); saliency of 2.5D and 3D soft- and hard-copy reproductions (display and printing); imaging and perception of metallic and effect coatings/inks; saliency, quality, and aesthetics in appearance reproduction; spectral reproduction.

Measuring, Modeling, and Reproducing Material Appearance 2016

Measuring, Modeling, and Reproducing Material Appearance (MMRMA) <2016, San Francisco, CA, USA>

Electronic Imaging, 9

A research project is underway to develop a gonio imager particularly dedicated to sample the Bidirectional Reflectance Distribution Function (BRDF) of materials and material compositions employed and created by multimaterial 3D printers. It comprises an almost colorimetric RGB camera and a spectrally tunable light source. In this paper, we investigate an important part of this system, particularly the approach to estimate reflectances from RGB values acquired under multiple illuminants. We first characterize the system by estimating the spectral sensitivities of the camera. Then, we use the sensitivities, a set of illuminants produced by the tunable light source and the corresponding sensor responses to estimate reflectances. For evaluating this approach, we measure the Neugebauer primary reflectances of a polyjet printer employing highly translucent photo-polymer printing materials colored in cyan, magenta, yellow and white. Spectral and colorimetric deviations to spectroradiometric comparison measurements (average 0.67 CIEDE2000 units / 0.0286 spectral RMS) are within the inter-instrument variability of hand-held spectrophotometers used in graphic arts for prints on paper.

24th Color and Imaging Conference

Color Imaging Conference (CIC) <24, 2016, San Diego, CA, USA>

In order to reproduce translucent objects by 3D printers employing fully transparent (or clear) material, modeling the human visual perception of translucency is crucial. In this preliminary study, a set of 256 texture-less samples was created by mixing white and clear materials using multi-jet 3D printing. The samples differ in both lateral light transport properties and transmittance. Two psychophysical experiments were conducted to reveal the relationship between transmittance and a perceptually uniform scale for translucency. The results show that Stevens' power law describes well this relationship within the optically thin range of samples. Furthermore, the sensitivity to lateral light transport is small compared to transmittance for the texture-less sample set.

Measuring, Modeling, and Reproducing Material Appearance 2015

Measuring, Modeling, and Reproducing Material Appearance (MMRMA) <2, 2015, San Francisco, CA, USA>

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 proles. 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.

Measuring, Modeling, and Reproducing Material Appearance 2015

Measuring, Modeling, and Reproducing Material Appearance (MMRMA) <2, 2015, San Francisco, CA, USA>

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.

Image Quality and System Performance XII

IS&T/SPIE's Annual Symposium Electronic Imaging (EI) <27, 2015, San Francisco, CA, USA>

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.

Measuring, Modeling, and Reproducing Material Appearance 2015

Measuring, Modeling, and Reproducing Material Appearance (MMRMA) <2, 2015, San Francisco, CA, USA>

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.

Journal of the Optical Society of America A

For calculating color differences, the CIEDE2000 and CIE94 equations are widely used and recommended. These equations were derived more than a decade ago, based for a large part on the RIT-Dupont set of visual data. This data was collected from a series of psychophysical tests that use the method of constant stimuli. In this method, observers need to compare the color difference within a sample pair to that between a reference pair. In the current investigation, we show that the color difference equation significantly changes if reference pairs are chosen in the underlying visual experiments that differ from what was used when creating the RIT-Dupont dataset. The investigation is done using metallic paint samples representing two color centers, red and yellow-green. We show that the reproducibility differs for three different reference pairs, and that for modeling the visual data for the yellow-green color center, extra model terms are required as compared to the CIEDE2000 equation. Our results suggest that observers differ in their ability to mentally convert a color difference recognized in a sample pair into an equivalent color difference along the color difference direction represented by the reference pair. We also find that in these test the tolerance to lightness differences is widened by a factor of 1.3 to 1.6, and that for the red color center the tolerance ellipsoid is rotated by 30° as compared to the CIEDE2000 equation. The latter observations are possibly due to the metallic texture in the samples used for the current experiment.

Journal of the Optical Society of America A

For developing color difference formulas, there are several choices to be made on the psychophysical method used for gathering visual (observer) data. We tested three different psychophysical methods: gray scales, constant stimuli, and two-alternative forced choice (2AFC). Our results show that when using gray scales or constant stimuli, assessments of color differences are biased toward lightness differences. This bias is particularly strong in LCD monitor experiments, and also present when using physical paint samples. No such bias is found when using 2AFC. In that case, however, observer responses are affected by other factors that are not accounted for by current color difference formulas. For accurate prediction of relative color differences, our results show, in agreement with other works, that modern color difference formulas do not perform well. We also investigated if the use of digital images as presented on LCD displays is a good alternative to using physical samples. Our results indicate that there are systematic differences between these two media.

SPIE Press

Measuring, Modeling, and Reproducing Material Appearance (MMRMA) <2, 2015, San Francisco, CA, USA>

Proceedings of SPIE, 9398

The main topics encountered on the papers of this document correspond to any of the following categories: Methods for measuring material properties: measurement of Bidirectional Reflectance Distribution Functions (BRDF), Bidirectional Texture Functions (BTF) and Bidirectional Surface Scattering Reflectance Distribution Function (BSSRDF); estimation of material difference perception; evaluation of metallic coatings/inks; measurement of glossiness; estimation of texture perception; data acquisition methods for different types of materials. Models for distinct characteristics of materials: modeling of Bidirectional Reflectance Distribution Functions (BRDF), Bidirectional Texture Functions (BTF) and Bidirectional Surface Scattering Reflectance Distribution Function (BSSRDF); modeling material difference perception; appearance modeling of glossiness and texture; modeling of varnish and special effects inks; softproofing methods for 2.5D and 3D printing. Material reproduction aspects: quality evaluation of 2.5D and 3D soft- and hardcopy reproductions (display and printing); estimation of effects of environmental aspects in material perception (lighting, observers' position, printing media); estimation of sensory input (visual, touch, audio) effect in material perception; evaluation of aesthetic aspects of 2.5D and 3D soft- and hard-copy reproductions (display and printing); saliency of 2.5D and 3D soft- and hard-copy reproductions (display and printing); imaging and perception of metallic and effect coatings/inks; saliency, quality, and aesthetics in appearance reproduction; spectral reproduction.

ACM Transactions on Graphics

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 article, 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.

Journal of Imaging Science and Technology

Spectral printing aims to achieve an illuminant-invariant match between the original and the reproduction. Due to limited printer spectral gamuts, an errorless spectral reproduction is mostly impossible, and spectral gamut mapping is required to reduce perceptual errors. The recently proposed paramer-mismatch-based spectral gamut mapping (PMSGM) strategy minimizes such errors. However, due to its pixel-wise processing, it may result in severely different tonal values for spectrally similar adjacent pixels, causing unwanted edges (banding) in the final printout. While the addition of some noise to the a* and b* channels of the colorimetric (e.g., CIELAB) image-rendered for the first illuminant-prior to gamut mapping solves the banding problem, it adversely increases the image graininess. In this article, the authors combine the PMSGM strategy with subsequent spectral separation, considering the spatial neighborhood within the tonal-value space and the illuminant-dependent perceptual spaces to directly compute tonal values. Their results show significant improvements to the PMSGM method in terms of avoiding banding artifacts.

23rd Color and Imaging Conference. Final Program and Proceedings

Color Imaging Conference (CIC) <23, 2015, Darmstadt, Germany>

Spectral printing aims to achieve an illuminant-invariant match between the original and the reproduction. Due to limited printer spectral gamuts, an errorless spectral reproduction is mostly impossible, and spectral gamut mapping is required to reduce perceptual errors. The recently proposed paramer-mismatch-based spectral gamut mapping (PMSGM) strategy minimizes such errors. However, due to its pixel-wise processing, it may result in severely different tonal values for spectrally similar adjacent pixels, causing unwanted edges (banding) in the final printout. While the addition of some noise to the a* and b* channels of the colorimetric (e.g., CIELAB) image-rendered for the first illuminant-prior to gamut mapping solves the banding problem, it adversely increases the image graininess. In this article, the authors combine the PMSGM strategy with subsequent spectral separation, considering the spatial neighborhood within the tonal-value space and the illuminant-dependent perceptual spaces to directly compute tonal values. Their results show significant improvements to the PMSGM method in terms of avoiding banding artifacts.

Measuring, Modeling, and Reproducing Material Appearance

Measuring, Modeling, and Reproducing Material Appearance (MMRMA) <1, 2014, San Francisco, CA, USA>

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.

IEEE Transactions on Image Processing

While image-difference metrics show good prediction performance on visual data, they often yield artifact-contaminated results if used as objective functions for optimizing complex image-processing tasks. We investigate in this regard the recently proposed color-image-difference (CID) metric particularly developed for predicting gamut-mapping distortions. We present an algorithm for optimizing gamut mapping employing the CID metric as the objective function. Resulting images contain various visual artifacts, which are addressed by multiple modifications yielding the improved color-image-difference (iCID) metric. The iCID-based optimizations are free from artifacts and retain contrast, structure, and color of the original image to a great extent. Furthermore, the prediction performance on visual data is improved by the modifications.

22nd Color and Imaging Conference. Final Program and Proceedings

Color and Imaging Conference (CIC) <22, 2014, Boston, MA, USA>

In order to assess the print quality, color and gloss are two important factors that should always be considered. In this paper, we investigate the impact of color on gloss using printed color samples varying between low and medium gloss levels. A psychophysical experiment was conducted to relate specular gloss measurements to perception. Results show that second order polynomials describe well this relationship independently of the underlying color. Following the same trend for all colors, the magnitude of perceived gloss decreases with increasing lightness.

Color Imaging XIX: Displaying, Processing, Hardcopy, and Applications

Color Imaging: Processing, Hardcopy, and Applications <19, 2014, San Francisco, CA, USA>

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.

IEEE Transactions on Image Processing

We propose a new strategy to evaluate the quality of multi and hyperspectral images, from the perspective of human perception. We define the spectral image difference as the overall perceived difference between two spectral images under a set of specified viewing conditions (illuminants). First, we analyze the stability of seven image-difference features across illuminants, by means of an information-theoretic strategy. We demonstrate, in particular, that in the case of common spectral distortions (spectral gamut mapping, spectral compression, spectral reconstruction), chromatic features vary much more than achromatic ones despite considering chromatic adaptation. Then, we propose two computationally efficient spectral image difference metrics and compare them to the results of a subjective visual experiment. A significant improvement is shown over existing metrics such as the widely used root-mean square error.

Measuring, Modeling, and Reproducing Material Appearance

Measuring, Modeling, and Reproducing Material Appearance (MMRMA) <1, 2014, San Francisco, CA, USA>

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.

Optics Express

Subsurface light transport can affect the visual appearance of materials significantly. Measuring and modeling this phenomenon is crucial for accurately reproducing colors in printing or for rendering translucent objects on displays. In this paper, we propose an apparatus to measure subsurface light transport employing a reference material to cancel out adverse signals that may bias the results. In contrast to other approaches, the setup enables improved focusing on rough surfaces (e.g. uncoated paper). We derive a measurement equation that may be used to deduce the point spread function (PSF) of subsurface light transport. Main contributions are the usage of spectrally-narrowband exchangeable LEDs allowing spectrally-resolved measurements and an approach based on quadratic programming for reconstructing PSFs in the case of isotropic light transport.

SPIE Press

Measuring, Modeling, and Reproducing Material Appearance (MMRMA) <1, 2014, San Francisco, CA, USA>

Proceedings of SPIE, 9018

The main topics encountered on the papers of this document correspond to any of the following categories: Methods for measuring material properties: measurement of Bidirectional Reflectance Distribution Functions (BRDF), Bidirectional Texture Functions (BTF) and Bidirectional Surface Scattering Reflectance Distribution Function (BSSRDF); estimation of material difference perception; evaluation of metallic coatings/inks; measurement of glossiness; estimation of texture perception; data acquisition methods for different types of materials. Models for distinct characteristics of materials: modeling of Bidirectional Reflectance Distribution Functions (BRDF), Bidirectional Texture Functions (BTF) and Bidirectional Surface Scattering Reflectance Distribution Function (BSSRDF); modeling material difference perception; appearance modeling of glossiness and texture; modeling of varnish and special effects inks; softproofing methods for 2.5D and 3D printing. Material reproduction aspects: quality evaluation of 2.5D and 3D soft- and hardcopy reproductions (display and printing); estimation of effects of environmental aspects in material perception (lighting, observers' position, printing media); estimation of sensory input (visual, touch, audio) effect in material perception; evaluation of aesthetic aspects of 2.5D and 3D soft- and hard-copy reproductions (display and printing); saliency of 2.5D and 3D soft- and hard-copy reproductions (display and printing); imaging and perception of metallic and effect coatings/inks; saliency, quality, and aesthetics in appearance reproduction; spectral reproduction.

Measuring, Modeling, and Reproducing Material Appearance

Measuring, Modeling, and Reproducing Material Appearance (MMRMA) <1, 2014, San Francisco, CA, USA>

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 artifacts 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 artifacts (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.

22nd Color and Imaging Conference. Final Program and Proceedings

Color and Imaging Conference (CIC) <22, 2014, Boston, MA, USA>

With the advent of multi-channel technologies, printers offer more and more possibilities for spectral reproduction. In order to print a specific color sensation, there are now more degrees of freedom when it comes to combining inks (i.e. more metamerism). In this paper, we take the example of a CMYKRGB printer and propose to visualize the extent of its spectral variability (or degree of metamerism) through the analysis of so-called paramermismatch gamuts. We then evaluate the suitability of the recently proposed LabAB interim connection space in the design of look-up tables for spectral color management. We demonstrate in particular that the spectral variability of this printer is small enough to drastically reduce the number of necessary grid points needed to sample the connection space without loss of perceived quality.

Eurographics Association

Workshop on Vision, Modeling, and Visualization (VMV) <19, 2014, Darmstadt, Germany>

VMV is a unique event that brings together scientists and practicioners interested in the interdisciplinary fields of computer vision and computer graphics, with special emphasis on the link between the disciplines. It offers researchers the opportunity to discuss a wide range of different topics within an open, international and interdisciplinary environment, and has done so successfully for many years.

Institut für Druckmaschinen und Druckverfahren

Workshop Farbbildverarbeitung <18, 2012, Darmstadt, Germany>