This paper focuses in the interpretation of material properties of reflectivity and specularity assessed by the visual system under illumination consisting of both a focal and a diffuse component (the ‘sun-and-sky’ illumination assumption). This assumption provides for four kinds of luminance gradients: gradients of incident illumination, gradients of reflectivity, gradients of secondary self-illumination and gradients of shadowing. The analysis considers the dissociation of the material properties carried by specularity from the geometric properties of object shape, taking the sinusoidal surface as a canonical shape exemplar.

The bidirectional scatter distribution function (BSDF) characterizes the scattering properties of a material for any angle of illumination or viewing, and offers as such a complete description of the spatial optical characteristics of the surface. An accurate determination of the BSDF is important in many scientific domains, such as computer graphics, architectural and lighting design, and the field of material appearance characterization (e.g. the color and gloss properties). Many BSDF measuring instruments have been reported in the literature. The majority of these instruments are goniometric measurement devices, by use of which the BSDF is determined by scanning all incoming and outgoing light flux directions in sequence. For this, the sample, detector, and/or source perform relative individual movements. In result, the major restriction of this type of instruments constitutes the measurement time, which may run to the order of several hours depending on the accuracy (angular resolution) and the complexity (spectral coverage, absolute measurement capability, etc.) of the reported measurement data. This paper describes the results of a feasibility study, in which an alternative goniometric measurement system is designed, enabling to acquire the photometric BSDF in a full three-dimensional (3D) space, with a high mechanical angular resolution (0.1°) in a time efficient way (about 30 minutes). A near-field goniophotometer, originally intended to measure luminance intensity distributions and luminous fluxes of light sources and luminaires, was converted for this purpose. Besides a discussion of the design and the measurement procedure, test sample measurements are presented to illustrate the versatility of the device.

The European Metrology Research Program (EMRP) is a metrology-focused program of coordinated Research and Development (RD) funded by the European Commission and participating countries within the European Association of National Metrology Institutes (EURAMET). It supports and ensures research collaboration between them by launching and managing different types of project calls. Within the EMRP Call 2012 "Metrology for Industry", the joint research project (JRP) entitled "Multidimensional Reflectometry for Industry" (xD-Reflect) was submitted by a consortium of 8 National Metrology Institutes (NMIs) and 2 universities and was subsequently funded. The general objective of xD-Reflect is to meet the demands from industry to describe the overall macroscopic appearance of modern surfaces by developing and improving methods for optical measurements which correlate with the visual sensation being evoked. In particular, the project deals with the "Goniochromatism", "Gloss" and "Fluorescence" properties of dedicated artifacts, which will be investigated in three main work packages (WP). Two additional transversal WP reinforce the structure: "Modelling and Data Analysis" with the objective to give an irreducible set of calibration schemes and handling methods and "Visual Perception", which will produce perception scales for the different visual attributes. Multidimensional reflectometry involves the enhancement of spectral and spatial resolution of reference gonioreflectometers for BRDF measurements using modern detectors, conoscopic optical designs, CCD cameras, line scan cameras, and modern light sources in order to describe new effects like sparkle and graininess/coarseness. More information and updated news concerning the project can be found on the xD-Reflect website http://www.xdreflect.eu/.

Gloss is the second most relevant visual attribute of a surface beside its colour. While the colour originates from the wavelength repartition of the reflected light, gloss originates from its angular distribution. When an observer is asked to evaluate the gloss of a surface, he always first orientate his eyes along the specular direction before lightly tilting the examined sample. This means that gloss is located in and around the specular direction, in a peak that is called the specular peak. On the one hand, this peak is flat and broad on matte surfaces on the other hand, it is narrow and sharp on high gloss surfaces. For the late ones, the FWHM of the specular peak is less than 2° which can be quite difficult to measure. We developed a dedicated facility capable of measuring specular peak with a FWHM up to 0,1 °. We measured the evolution of the peak according to the angle of illumination and the specular gloss of the sample in the restricted field of very glossy surface. The facility and peaks measured are presented in the paper. The next step will be to identify the correlations between the peak and the roughness of the sample.

In goniometric measurements, the acquisition of a material under different viewing and illumination angles gives access to its bidirectional reflectance distribution function (BRDF). If information is acquired not only from a point of the material but from the whole object, the bidirectional texture function (BTF) can even be measured. Depending on the camera utilized, the color accuracy of acquired information can vary. A multispectral camera with 19 bandpass channels is utilized here to sample the visible wavelength range. We first measure the transversal aberrations appearing in goniometric multispectral imaging. They are caused by the bandpass filters in the ray path. Utilizing the information from all the viewing angles can lead to a better correction of these aberrations. We compare separate correction of each acquired image and global correction over the whole set of goniometric images. Furthermore, angles vary for different points of image in the camera because of the image optics. We analyze these variations which can either bring additional aberrations to the measurement or additional information about the measured material.

A Line-scan camera based stereo method for high resolution 3D image reconstruction is proposed. The imaging model of a line scan camera is addressed in detail to describe the relationship between the coordinate of a physical object in space and the coordinate of its image captured by the scanner. Affine-SIFT feature detector is utilized for establishing dense stereo correspondence. Experimental result demonstrates the effectiveness and merit of this method to high resolution digitization of cultural heritages.

The objective is to create a micro-environment - an illuminated dome with a constant geometry of illumination - to undertake non invasive observation of the surface topography of artworks. The dome is attached to a stereo-microscope, which is able to gain - at the same time - both colour and texture features of the sample. By using a stereo-microscope, a more detailed observation is possible. Important elements including colour, texture, the morphology of the sample surface, in terms of specular and diffused components of reflected light, can be summarized/condensed in the measure of Bidirectional Texture Function.

Paintings are near-planar objects with material characteristics that vary widely. The fact that paint has a material presence is often overlooked, mostly because we often encounter these artworks in the form of two-dimensional reproductions. Capturing paintings in the third dimension is not only important for study, restoration and conservation, but it also inspires 3D printing methods1, particularly through the high demands it makes on reproducing color, gloss and texture. “A hybrid solution between fringe projection and stereo imaging is proposed as 3D imaging method, with a setup involving two cameras and a projector. Fringe projection is aided by sparse stereo matching to serve as image encoder. These encoded images processed by the stereo cameras solve the correspondence problem in stereo matching, leading to a dense and accurate topographical map, while simultaneously capturing the composition of the painting in full color”1. The topographical map and color data are used to make hardcopy 3D reproductions, using a specially developed printing system. Several paintings by Dutch masters Rembrandt and Van Gogh have been scanned and reproduced using this technique. These 3D printed reproductions have been evaluated by experts, both individually and in a side-by-side comparison with the original.

The rapid development of hardware and software in the digital image processing field has boosted research in computer vision for applications in industry. The development of new electronic devices and the tendency to decrease their prices makes possible new developments that few decades ago were possible only in the imagination. This is the case of 3D imaging technology which permits to detect failures in industrial products by inspecting aspects on their 3D surface. In search of an optimal solution for scanning textiles we present in this paper a review of existing techniques for digitizing 3D surfaces. Topographic details of textiles can be obtained by digitizing surfaces using laser line triangulation, phase shifting optical triangulation, projected-light, stereo-vision systems and silhouette analysis. Although we are focused on methods that have been used in the textile industry, we also consider potential mechanisms used for other applications. We discuss the advantages and disadvantages of the evaluated methods and state a summary of potential implementations for the textile industry.

Multichannel printer modeling has been an active area of research in the field of spectral printing. The most commonly used models for characterization of such systems are the spectral Neugebauer (SN) and its extensions. This work addresses issues that can arise during calibration and testing of the SN model when modelling a 7-colorant printer. Since most substrates are limited in their capacity to take in large amount of ink, it is not always possible to print all colorant combinations necessary to determine the Neugebauer primaries (NP). A common solution is to estimate the nonprintable Neugebauer primaries from the single colorant primaries using the Kubelka-Munk (KM) optical model. In this work we test whether a better estimate can be obtained using general radiative transfer theory, which better represents the angular variation of the reflectance from highly absorbing media, and takes surface scattering into account. For this purpose we use the DORT2002 model. We conclude DORT2002 does not offer significant improvements over KM in the estimation of the NPs, but a significant improvement is obtained when using a simple surface scattering model. When the estimated primaries are used as inputs to the SN model instead of measured ones, it is found the SN model performs the same or better in terms of color difference and spectral error. If the mixed measured and estimated primaries are used as inputs to the SN model, it performs better than using either measured or estimated.

Artists often want to change the colors of an image to achieve a particular aesthetic goal. For example, they might limit colors to a warm or cool color scheme to create an image with a certain mood or feeling. Gamut masking is a technique that artists use to limit the set of colors they can paint with. They draw a mask over a color wheel and only use the hues within the mask. However, creating the color palette from the mask and applying the colors to the image requires skill. We propose an interactive tool for gamut masking that allows amateur artists to create an image with a desired mood or feeling. Our system extracts a 3D color gamut from the 2D user-drawn mask and maps the image to this gamut. The user can draw a different gamut mask or locally refine the image colors. Our voxel grid gamut representation allows us to represent gamuts of any shape, and our cluster-based image representation allows the user to change colors locally.

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.

We propose expanding the Murray-Davies formula by adding the effect of edges of solid inks in a halftoned image. The expanded formula takes into account the spectral reflectance of paper white, full tone ink and mixed area scaled by the fractional area coverages. Here, mixed area mainly refers to the edge of an inked dot where the density is very low, and lateral exchange of photons can occur. Also, in such area the paper micro components may have higher scattering power than ink, especially, in uncoated paper. Our methodology uses cyan, magenta and yellow separation ramps printed on different papers by impact and non-impact based printing technologies. The samples include both frequency and amplitude modulation halftoning methods of various print resolutions. Based on pixel values, the captured microscale halftoned image is divided into three categories: solid ink, mixed area, and unprinted paper between the dots. The segmented images are then used to measure the fractional area coverage that the model receives as parameters. We have derived the characteristic reflectance spectrum of mixed area by rearranging the expanded formula and replacing the predicted term with the measured value using half of the maximum colorant coverage. Performance has clearly improved over the Murray-Davies model with and without dot gain compensation, more importantly, preserving the linear additivity of reflectance of the classical physics-based model.

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.

Primary components in the colour reproduction of textured materials are firstly, the accurate rendering of the appearance of texture, and secondly the ability to print a surface topology that moves towards 2.5D printing. However, in order to render surfaces that contain no discernable pattern structure, unlimited variations in pattern can result in enormous file sizes. The paper explores how painters from the 15th to 21st century were absorbed in creating convincing representations of the attributes of materials. However, on close inspection, these paintings demonstrate a gestural, almost abstracted approach to capturing the appearance of the material, surface and object. The evolving question is: what are the key elements in paintings produced by artists that through the application of coloured brush marks, are able to create a verisimilitude of the material qualities of wood, metal, glass and fabric? The paper suggests that in order to create both a convincing visual appearance, a high level of detail is not necessary, and, that too much information possibly hinders the final appearance. It suggests that by using a more gestural approach, whereby the relationship of mark and colour, and by modulating the fluid dynamics of a mark through a textured surface, a more convincing rendering of texture can be achieved. Finally, by exploring analogue approaches to image making, and by addressing the complex way digital images are constructed, new methods could assist in reducing huge memory and image processing requirements.

Realistic images are a puzzle because they serve as visual representations of objects while also being objects themselves. When we look at an image we are able to perceive both the properties of the image and the properties of the objects represented by the image. Research on image quality has typically focused improving image properties (resolution, dynamic range, frame rate, etc.) while ignoring the issue of whether images are serving their role as visual representations. In this paper we describe a series of experiments that investigate how well images of different quality convey information about the properties of the objects they represent. In the experiments we focus on the effects that two image properties (contrast and sharpness) have on the ability of images to represent the gloss of depicted objects. We found that different experimental methods produced differing results. Specifically, when the stimulus images were presented using simultaneous pair comparison, observers were influenced by the surface properties of the images and conflated changes in image contrast and sharpness with changes in object gloss. On the other hand, when the stimulus images were presented sequentially, observers were able to disregard the image plane properties and more accurately match the gloss of the objects represented by the different quality images. These findings suggest that in understanding image quality it is useful to distinguish between quality of the imaging medium and the quality of the visual information represented by that medium.

Fabric is one of the most common materials in our everyday lives, and accurately simulating the appearance of cloth is a critical problem in graphics, design, and virtual prototyping. But modeling and rendering fabric is very challenging because fabrics have a very complex structure, and this structure plays an important role in their visual appearance—cloth is made of fibers that are twisted into yarns which are woven into patterns. Light interacting with this complex structure produce the characteristic visual appearance that humans recognize as silk, cotton, or wool. In this paper we present an end-to-end pipeline to model and render fabrics: we introduce a novel modality to create volume models of fabric at micron resolution using CT technology coupled with photographs; a new technique to synthesize models of user-specified designs from such CT scans; and finally, an efficient algorithm to render these complex volumetric models for practical applications. This pipeline produces the most realistic images of virtual cloth to date, and opens the way to bridging the gap between real and virtual fabric appearance.

To completely characterize the color of effect coatings, the spectral BRDF should be measured for a large number of illumination/detection geometries. This is possible with the GEFE, the gonio-spectrophotometer developed at Instituto de Óptica in CSIC (IO-CSIC). Twenty-four effect coating samples were prepared by AkzoNobel, containing metallic and/or interference flake pigments. The spectral BRDF of all samples was measured by GEFE using a normalized procedure under 448 different geometries. The results are presented in two-dimensional a*-b* diagrams and in three-dimensional CIELAB diagrams to show the color travel. Four different descriptors were used to quantify the color travel of these samples. We show that for many samples and geometries, the resulting colors fall outside the color gamut of a typical display. It was found that the reflection data do not vary considerably at reciprocal geometries, allowing a reduction of geometries to be done. Finally, we show that for 3D rendering applications, the reflection data from BRDF can be strongly reduced. For example, we show that using the concept of flake-based parameters it is possible to only use inplane geometries. The resulting rendered images were shown to be accurate enough for rendering on displays.

The appearance of translucent materials is strongly affected by bulk (or sub surface) scattering. For paper and carton board, lateral light propagation and angle-resolved reflection have been studied extensively but treated separately. The present work aims at modelling the BSSRDF of turbid media in order to study the angular variation of the reflectance as function of the lateral propagation within the medium. Monte Carlo simulations of the spatial- and angle resolved reflectance of turbid media are performed for different scattering and absorption coefficients, phase functions and surface topographies representative for uncoated paper grades. The angle-resolved radiance factor of turbid media is found to be function of the lateral light propagation within the substrate and both the reflected radiance factor and the fluorescence emission are found to be clearly non- Lambertian, although the latter clearly depends on the light absorption at the excitation wavelength. It is also suggested that the modelling of uncoated paper should not include surface scattering. The findings impact on the appearance of turbid media at different angles and make measurements of the lateral light propagation dependent on the instrument geometry.

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.

Commonly known lenticular prints use a lens-like system superimposed on a standard 2D print to control the light sent into each direction. Thanks to our 2.5D or relief printing system, we are capable of creating a lenticular effect embedded directly on the prints that does not require the use of a system of lenses. On a zigzag-shaped surface composed of continuous small triangles two source images are interlaced and printed on the sides of the triangular structures, each side corresponding to one of the two intended views. The effect of crosstalk or ghosting is often encountered in lenticular prints. Ghosting occurs when some parts of one source image remain visible for the illumination or viewing direction corresponding to the other source image. In this work, we use an image-content-driven technique that identifies the regions in the source images that are prone to cause ghosting for a given set of viewing angles. For the purpose of eliminating this artefact, a model of the ghosting effect appearance is implemented and used for compensation. We have observed improvements in the quality of the lenticular effect, however the impact on the quality of the prints still needs to be evaluated.

Current research at the Centre for Fine Print Research (CFPR) at the University of the West of England, Bristol, is exploring the potential of creating coloured pictorial imagery from a continuous tone relief surface. To create the printing matrices the research team have been using CNC milled images where the height of the relief image is dictated by creating a tone curve and then milling this curve into a series of relief blocks from which the image is cast in a silicone ink. A translucent image is cast from each of the colour matrices and each colour is assembled - one on top of another - resulting is a colour continuous tone print, where colour tone is created by physical depth of colour. This process is a contemporary method of continuous tone colour printing based upon the Nineteenth Century black and white printing process of Woodburytype as developed by Walter Bentley Woodbury in 1865. Woodburytype is the only true continuous tone printing process invented, and although its delicate and subtle surfaces surpassed all other printing methods at the time. The process died out in the late nineteenth century as more expedient and cost effective methods of printing prevailed. New research at CFPR builds upon previous research that combines 19th Century Photomechanical techniques with digital technology to reappraise the potential of these processes.

Relief printing technology developed by Océ allows the superposition of several layers of colorant on different types of media which creates a variation of the surface height defined by the input to the printer. Evaluating the reproduction accuracy of distinct surface characteristics is of great importance to the application of the relief printing system. Therefore, it is necessary to develop quality metrics to evaluate the relief process. In this paper, we focus on the third dimension of relief printing, i.e. height information. To achieve this goal, we define metrics and develop models that aim to evaluate relief prints in two aspects: overall fidelity and surface finish. To characterize the overall fidelity, three metrics are calculated: Modulation Transfer Function (MTF), difference and root-mean-squared error (RMSE) between the input height map and scanned height map, and print surface angle accuracy. For the surface finish property, we measure the surface roughness, generate surface normal maps and develop a light reflection model that serves as a simulation of the differences between ideal prints and real prints that may be perceived by human observers. Three sets of test targets are designed and printed by the Océ relief printer prototypes for the calculation of the above metrics: (i) twisted target, (ii) sinusoidal wave target, and (iii) ramp target. The results provide quantitative evaluations of the printing quality in the third dimension, and demonstrate that the height of relief prints is reproduced accurately with respect to the input design. The factors that affect the printing quality include: printing direction, frequency and amplitude of the input signal, shape of relief prints. Besides the above factors, there are two additional aspects that influence the viewing experience of relief prints: lighting condition and viewing angle.

Graphic prints and manuscripts constitute main part of the cultural heritage objects created by the most of the known civilizations. Their presentation was always a problem due to their high sensitivity to light and changes of external conditions (temperature, humidity). Today it is possible to use an advanced digitalization techniques for documentation and visualization of mentioned objects. In the situation when presentation of the original heritage object is impossible, there is a need to develop a method allowing documentation and then presentation to the audience of all the aesthetical features of the object. During the course of the project scans of several pages of one of the most valuable books in collection of Museum of Warsaw Archdiocese were performed. The book known as "Great Dürer Trilogy" consists of three series of woodcuts by the Albrecht Dürer. The measurement system used consists of a custom designed, structured light-based, high-resolution measurement head with automated digitization system mounted on the industrial robot. This device was custom built to meet conservators' requirements, especially the lack of ultraviolet or infrared radiation emission in the direction of measured object. Documentation of one page from the book requires about 380 directional measurements which constitute about 3 billion sample points. The distance between the points in the cloud is 20 μm. Provided that the measurement with MSD (measurement sampling density) of 2500 points makes it possible to show to the publicity the spatial structure of this graphics print. An important aspect is the complexity of the software environment created for data processing, in which massive data sets can be automatically processed and visualized. Very important advantage of the software which is using directly clouds of points is the possibility to manipulate freely virtual light source.