1,720,977 research outputs found
Ecological optics of natural materials and light fields
No full textThe appearance of objects in scenes is determined by their shape, material properties and by the light field, and, in contradistinction, the appearance of those objects provides us with cues about the shape, material properties and light field. The latter so-called inverse problem is underdetermined and therefore suffers from interesting ambiguities. Therefore, interactions in the perception of shape, material, and luminous environment are bound to occur. Textures of illuminated rough materials depend strongly on the illumination and viewing directions. Luminance histogram-based measures such as the average luminance, its variance, shadow and highlight modes, and the contrast provide robust estimates with regard to the surface structure and the light field. Human observers performance agrees well with predictions on the basis of such measures. If we also take into account the spatial structure of the texture it is possible to estimate the illumination orientation locally. Image analysis on the basis of second order statistics and human observers estimates correspond well and are both subject to the bas-relief and the convex-concave ambiguities. The systematic robust illuminance flow patterns of local illumination orientation estimates on rough 3D objects are an important entity for shape from shading and for light field estimates. Human observers are able to match and discriminate simple light field properties (e.g. average illumination direction and diffuseness) of objects and scenes, but they make systematic errors, which depend on material properties, object shapes and position in the scene. Moreover, our results show that perception of material and illumination are basically confounded. Detailed analysis of these confounds suggests that observers primarily attend to the low-pass structure of the light field. We measured and visualized this structure, which was found to vary smoothly in natural scenes in- and outdoors.Industrial Design Engineerin
The light field in natural scenes
No full textThis thesis focuses on the properties of light fields with respect to object appearance. More specifically, our interest was mainly directed to the structure and spatial variation of light fields in natural scenes. We approached the structure of light fields by means of spherical harmonics which allows one to divide the complicated spherical functions of local light fields in frequency bands and to analyze those separately. In chapter 2 we empirically studied the variation of different frequencies of light field approximations over natural scenes by means of panoramic photography and found that the low order components show systematic and stable spatial variations whereas the high order components vary rapidly and chaotically over most scenes. We showed how the ‘quality of light’ can be expressed by means of a light vector and a squash tensor which provide a formal mathematical but nevertheless very intuitive way of representation bridging the gap between scientific and artistic understandings of light. In chapter 3 we continued the study of the spatial behavior of light fields more thoroughly considering complicated scenes and focusing on the 2nd order structures which we measured by our custom made device. In Chapter 4 we presented a technique with which the 2nd order descriptions can be recovered for an entire three dimensional scene on the basis of a limited number of measurements and presented a visualization of the structure of light fields by means of light tubes. Chapter 5 was devoted to possible topological structures of light fields. In the Appendix we provided additional examples of methods, measurements and visualization of light fields. In chapter 2 we considered simple scenes such as typical ‘street’, ‘wall’ and ‘forest’ scenes and studied the spatial variation of light fields along the main axes of symmetry of the scenes. The measurements were performed photographically by utilizing panoramic imaging. We described the local light fields in terms of spherical harmonics up to the 10th order and analyzed the qualitative properties and physical meanings of the low order components. We took a first step in a further development of Gershun’s classical work on the light field by extending his description beyond the three-dimensional vector field, towards a more complete description of the light field using tensors. We showed that the three first components, namely the monopole (density of light), the dipole (light vector) and the quadrupole, which we named ‘squash tensor’, suffice to describe a wide range of qualitatively different light fields. The empirical analysis allowed us to conclude that the low order components dominate the structure of most light fields. The low order components are rather constant over the scenes whereas high order components are not. Using simple models, we found a strong relation between the low order components and the geometrical layouts of the scenes. In chapter 3 we presented a new technique to capture the global structure of the light field in terms of spherical harmonics functions. Our custom made device Plenopter allows to perform measurements of light fields up to the second order easily, quickly and with a high dynamic range. Using that device we continued the research presented in chapter 2 by considering measurements across the scenes, along the line orthogonal to the main axis of geometrical symmetry. The measurements clearly indicate that in scenes of similar geometry the light fields demonstrate characteristic variations of the light vector and the squash tensor over the scene. This happened despite the fact that the scenes possessed different reflective properties and even were differently oriented with regard to the primary light sources. In chapter 4 we presented a method for measuring, reconstructing and visualizing the global structures of light fields in finite 3D spaces. We used the Plenopter to measure second order light fields at points over a regular grid and interpolate the spherical harmonics coefficients to calculate the light fields at all points of a closed 3D space. We presented a new way of visualizing the light field in 3D space by means of light tubes which indicate the radiant flux transfer and provide intuitive insights into the global structure of the light field through the entire space of the scene. In chapter 5 we considered possible topological structures of light fields. We studied singular points and showed that all generic topological structures that are possible for 2D vector fields may also occur in the case of light fields. We provided models which showed that light tubes can even be closed. The global structure of the light field may be described by means of the singular points.Human Information Communication DesignIndustrial Design Engineerin
Quantifying Natural Light for Lighting and Display Design
No full textIn natural scenes, the wavelength-dependent sunlight scattering in the atmosphere and the presence of occluders and mutual reflections cause variations in the local illumination's magnitude, direction, and spectral composition as a function of time and space. Yet, unlike the characterization of temporal changes of natural light, these spatial variations have received relatively limited attention. In this paper, we employed the light field concept to quantify those variations and empirically demonstrated that they are significant and how the different contributions of the diffuse and directed parts can be measured.Human Information Communication Desig
Light and Spectra in the Wild - Spectral Structures of Light Fields: Measurement, Simulation and Visualisation
No full textThe study of the light field has become a valuable framework for capturing and analysing the complex distribution of light in natural environments. The directional, spatial, temporal and spectral structure of light, collectively influence the optical information available to an observer and thus impact our perception of the surrounding world. The extended definition of the light field, which is equivalent to the plenoptic function in perceptual studies, incorporates radiance as a function of spectral energy, position, direction, and time in space, quantifying all the optical information available to an observer. However, there is a considerable gap in measuring, describing, and visualizing the properties of the light field in the chromatic domain, which this thesis aimed to address. The thesis focuses on the research question of how to effectively describe, measure, simulate, and visualize the spatiotemporal dynamics of the spectral structure of light fields. To address this research question, We outlined four main objectives in the thesis, which are addressed in separate chapters. The first objective is to investigate the interplay between the colours of surfaces and light sources in 3D indoor scenes, and its effects on the spatial and angular distribution of light. The second objective was to quantify the directional and spatial variations of chromatic light field effects on correlated colour temperature and colour rendering. The third objective was to explore the objective measurement, description, and visualization of the 7D light-field properties of outdoor illumination. Finally, the fourth objective was to examine the relationship between image statistics and perceived time of day in Western European paintings from the 17th to 20th centuries to determine if the representation of lighting in paintings serves as a contextual cue for the time of day
This is not a painting: Scanning and printing a painting's appearance
No full textThe appearance of a painting cannot solely be described by the depiction that it presents to the viewer. When viewing the artifact in real life, we find that the painted surface is in effect a three-dimensional landscape of paint. Paintings, “moveable, largely two-dimensional images created for the primary purpose of providing a visual experience”,1 can be created using a vast variety of materials on a range of supports. They are commonly built up as a complex stratigraphy of layers, generally consisting of a support, ground layer(s), one or multiple layers of (semi-) transparent paints, and in many cases a protective varnish layer. The current appearance of a painting is determined by the way a painter used and applied the materials, but also effects of aging, conservation and restoration treatments, which all continue to influence the physical state of a painting. Historically, cultural heritage (CH) reproductions were hand-crafted, and created for instance to disseminate or replace artworks, or to train in the skill of their creation. Also modern reproductions — or facsimiles — are still large hand-crafted, and for instance serve to provide access to (fragile) artworks or even complete (CH) sites, or to recreate their original appearance. Alternatively, reconstructions might reside only in the virtual domain. The continued development of digital imaging and digital fabrication technology (i.e. 3D printing) provides new opportunities for appearance reproduction, also suitable for application in the CH domain. If we want to replicate material appearance, we need to understand how (the appearance of) material is perceived. A material is, however, not observed directly, but has to be lighted, and via the light that is scattered by the material humans can perceive it. Appearance is therefore the light-material-confounded proximal stimulus for the human visual system (HVS). Even though we see, recognize and interact with a vast number of materials every day, and can effortlessly distinguish between them, it turns out that the perceptual mechanisms that underlie this, are still quite poorly understood, including linking individual appearance attributes to measurable and fabrication parameters. One of the consequences of this is that an integrated approach to (total) appearance reproduction, including color, topography/ texture/shape, gloss and transparency/translucency, is still lacking
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Convincing stuff: Disclosing perceptually-relevant cues for the depiction of materials in 17th century paintings
No full textThis thesis explores convincing stuff depicted in 17th century paintings, with the primary aim of understanding their visual perception. ”Stuff” is the term first introduced by Edward Adelson in 2001 to differentiate materials from objects, and to call attention on the research gap in material perception. In an interesting parallel, the representation of materials in paintings constitutes a knowledge gap in art history as well. Both gaps have only recently been recognized and started to be addressed in their respective research fields. In this thesis, representation and perception come together to create a virtuous circle in which the knowledge of painters about the representation of materials is used to understand the mechanisms of the visual system for material perception, and this is in turn used to explain the pictorial features that make the representation of materials so convincing. The common thread used here to connect representation and perception, is ”The big world painted small”, a long-forgotten booklet of pictorial recipes written by the Dutch painter Willem Beurs in 1692. We argue that this book represents an index of key features for material perception, that means an index of image features that always work as perceptual cues regardless of the illumination and the viewing conditions of the depicted scene. The main research objective of this dissertation is: To understand the convincing depiction and perception of materials in 17th century paintings, connecting the image features found in paintings and listed by Beurs to their role as perceptual cues. In order to achieve this objective, we employed a novel, interdisciplinary research approach, merging science of human and computer vision, technical art history, and the historical textual source of Beurs.Human Information Communication Desig
On the Painterly Depiction of Materials: An Interdisciplinary Study on the Depiction and Perception of Materials within Paintings
No full textThe world around us is filled with materials. Our ability of visual material perception informs us how to navigate and interact with our environment. It tells us, for example, whether food is fresh, if a chair is strong enough to sit on, how much force to use to pick up a glass, etc. Painters have studied how to depict the world and the materials therein for thousands of years. We believe that the material depictions within paintings can be leveraged into insights for the scientific understanding of material perception. In this thesis, we studied the perception of painterly depictions of materials and aimed to make the study thereof accessible to other researchers with the release of the Materials In Paintings dataset. We collected a large set of paintings from museums and galleries. Then, we used an online crowd-sourcing approach to annotate material identity (fabrics, stone, etc.,) and gather spatial material segmentations (i.e., “cutting out” piece of the painting that depict the material). In the first study, we measured the perception of material attributes (soft, rough, fragile, etc.,) across a range of materials and found that painterly materials trigger distinct distributions of perceived attributes and we furthermore compared these distributions to those for photographic materials. In the second study, we continued crowd-sourcing annotations on material identity and material segmentations and combined these into the Materials In Paintings dataset. In a number of cross-disciplinary demonstrations we presented novel findings across art history, human perception, and computer vision. While these demonstrations are useful in their own right, the main focus here was the release of the dataset. Next, we used the dataset as a source of stimuli for two studies into specific materials. First, for fabrics, we studied the perception of satin and velvet and the effect of presenting only local or, both local and global information, and found that the perceptual distinction between these two fabrics becomes more ambiguous when removing global information. Furthermore, we showed that local image cues can affect perceptual responses for shininess but not for softness. Lastly, we studied the perception and depiction of pearls by identifying three image features that might trigger the perception of pearliness. In a series of experiments, we confirm the role of these image features but find that the presence of only one of these image features, highlights, is already sufficient for naive participants to trigger the perception of pearliness. Conversely, expert participants (art historians or pearl experts) perceive depictions with all three features as more pearly, which implies the existence of visual expertise for pearl perception. All in all, in this thesis we show the benefits of studying material perception through painterly depictions of materials and enable further study with the release of the MIP dataset.Human Information Communication Desig
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