Occlusion Textures for Plausible Soft Shadows

International audienceThis paper presents a new approach to compute plausible soft shadows for complex dynamic scenes and rectangular light sources. We estimate the occlusion at each point of the scene using prefiltered occlusion textures, which dynamically approximate the scene geometry. The algorithm is fast and its performance independent of the light's size. Being image-based, it is mostly independent of the scene complexity and type. No a priori information is needed, and there is no caster/receiver separation. This makes the method appealing and easy to use

A Proposal Concerning the Analysis of Shadows in Images by an Active Observer (Dissertation Proposal)

Shadows occur frequently in indoor scenes and outdoors on sunny days. Despite the information inherent in shadows about a scene\u27s geometry and lighting conditions, relatively little work in image understanding has addressed the important problem of recognizing shadows. This is an even more serious failing when one considers the problems shadows pose for many visual techniques such as object recognition and shape from shading. Shadows are difficult to identify because they cannot be infallibly recognized until a scene\u27s geometry and lighting are known. However, there are a number of cues which together strongly suggest the identification of a shadow. We present a list of these cues and methods which can be used by an active observer to detect shadows. By an active observer, we mean an observer that is not only mobile, but can extend a probe into its environment. The proposed approach should allow the extraction of shadows in real time. Furthermore, the identification of a shadow should improve with observing time. In order to be able to identify shadows without or prior to obtaining information about the arrangement of objects or information about the spectral properties of materials in the scene, we provide the observer with a probe with which to cast its own shadows. Any visible shadows cast by the probe can be easily identified because they will be new to the scene. These actively obtained shadows allow the observer to experimentally determine the number and location of light sources in the scene, to locate the cast shadows, and to gain information about the likely spectral changes due to shadows. We present a novel method for locating a light source and the surface on which a shadow is cast. It takes into account errors in imaging and image processing and, furthermore, it takes special advantage of the benefits of an active observer. The information gained from the probe is of particular importance in effectively using the various shadow cues. In the course of identifying shadows, we also present a new modification on an image segmentation algorithm. Our modification provides a general description of color images in terms of regions that is particularly amenable to the analysis of shadows

Fast Calculation of Soft Shadow Textures Using Convolution

International audienceThe calculation of detailed shadows remains one of the most difficult challenges in computer graphics, especially in the case of extended (linear or area) light sources. This paper introduces a new tool for the calculation of shadows cast by extended light sources. Exact shadows are computed in some constrained configurations by using a convolution technique, yielding a fast and accurate solution. Approximate shadows can be computed for general configurations by applying the convolution to a representative “ideal” configuration. We analyze the various sources of approximation in the process and derive a hierarchical, error driven algorithm for fast shadow calculation in arbitrary configurations using a hierarchy of object clusters. The convolution is performed on images rendered in an offscreen buffer and produces a shadow map used as a texture to modulate the unoccluded illumination. Light sources can have any 3D shape as well as arbitrary emission characteristics, while shadow maps can be applied to groups of objects at once. The method can be employed in a hierarchical radiosity system, or directly as a shadowing technique. We demonstrate results for various scenes, showing that soft shadows can be generated at interactive rates for dynamics environments

Customization of Discriminant Function Analysis for Prediction of Solar Flares

This research is an extension to the research conducted by K. Leka and G. Barnes of the Colorado Research Associates Division, Northwest Research Associates, Inc. in Boulder, Colorado (CORA) in which they found no single photospheric solar parameter they considered could sufficiently identify a flare-producing active region (AR). Their research then explored the possibility a linear combination of parameters used in a multivariable discriminant function (DF) could adequately predict solar activity. The purpose of this research is to extend the DF research conducted by Leka and Barnes by refining the method of statistical discriminant analysis (DA) with the goal of selecting those photospheric magnetic parameters most capable of identifying flare-producing active regions in hopes of increasing the reliability of short term flare warnings and the understanding of flare production. The data for this research were photospheric vector magnetograms captured by the Imaging Vector Magnetograph (IVM) at the University of Hawai`i Mees Solar Observatory at Haleakala and provided by CORA. Increasing the data set size was an essential task for this research in order to have a more statistically significant training sample for DA. This research also modified current DF procedures to enable the customization of the costs of flare false alarms and flare misses. Work was also done to expand the binary DF results to produce flare probability forecasts. The selection of the optimum combination of photospheric magnetic parameters to be used as predictors in a linear DF began with the elimination of redundant parameters and those parameters least likely to contribute to flare production. The selection of parameters was governed by maximizing the Mahalanobis distance in a step-up method. The DF results show a pre-flaring active region may be characterized by larger magnetic flux, an active region with a larger area of magnetic shear angle greater than 80°, larger current of heterogeneity, larger spatial vertical magnetic field gradient, and a larger kurtosis of the shear angle. With the optimum combination of parameters, DF flare probability forecasts were compared to the daily forecasts produced by the National Oceanic and Atmospheric Administration, Space Environment Center (NOAA SEC). The Chi-Squared values of each forecast show the objective DF based flare probability forecasting method performs as well as the subjective forecasting method employed by the SEC

Occluder-aided non-line-of-sight imaging

Non-line-of-sight (NLOS) imaging is the inference of the properties of objects or scenes outside of the direct line-of-sight of the observer. Such inferences can range from a 2D photograph-like image of a hidden area, to determining the position, motion or number of hidden objects, to 3D reconstructions of a hidden volume. NLOS imaging has many enticing potential applications, such as leveraging the existing hardware in many automobiles to identify hidden pedestrians, vehicles or other hazards and hence plan safer trajectories. Other potential application areas include improving navigation for robots or drones by anticipating occluded hazards, peering past obstructions in medical settings, or in surveying unreachable areas in search-and-rescue operations. Most modern NLOS imaging methods fall into one of two categories: active imaging methods that have some control of the illumination of the hidden area, and passive methods that simply measure light that already exists. This thesis introduces two NLOS imaging methods, one of each category, along with modeling and data processing techniques that are more broadly applicable. The methods are linked by their use of objects (‘occluders’) that reside somewhere between the observer and the hidden scene and block some possible light paths. Computational periscopy, a passive method, can recover the unknown position of an occluding object in the hidden area and then recover an image of the hidden scene behind it. It does so using only a single photograph of a blank relay wall taken by an ordinary digital camera. We develop also a framework using an optimized preconditioning matrix to improve the speed at which these reconstructions can be made and greatly improve the robustness to ambient light. Lastly, we develop tools necessary to demonstrate recovery of scenes at multiple unknown depths – paving the way towards three-dimensional reconstructions. Edge-resolved transient imaging, an active method, enables the formation of 2.5D representations – a plan view plus heights – of large-scale scenes. A pulsed laser illuminates spots along a small semi-circle on the floor, centered on the edge of a vertical wall such as in a doorway. The wall edge occludes some light paths, only allowing the laser light reflecting off of the floor to illuminate certain portions of the hidden area beyond the wall, depending on where along the semi-circle it is illuminating. The time at which photons return following a laser pulse is recorded. The occluding wall edge provides angular resolution, and time-resolved sensing provides radial resolution. This novel acquisition strategy, along with a scene response model and reconstruction algorithm, allow for 180° field of view reconstructions of large-scale scenes unlike other active imaging methods. Lastly, we introduce a sparsity penalty named mutually exclusive group sparsity (MEGS), that can be used as a constraint or regularization in optimization problems to promote solutions in which certain components are mutually exclusive. We explore how this penalty relates to other similar penalties, develop fast algorithms to solve MEGS-regularized problems, and demonstrate how enforcing mutual exclusivity structure can provide great utility in NLOS imaging problems

Daylight simulation with photon maps

Physically based image synthesis remains one of the most demanding tasks in the computer graphics field, whose applications have evolved along with the techniques in recent years, particularly with the decline in cost of powerful computing hardware. Physically based rendering is essentially a niche since it goes beyond the photorealistic look required by mainstream applications with the goal of computing actual lighting levels in physical quantities within a complex 3D scene. Unlike mainstream applications which merely demand visually convincing images and short rendering times, physically based rendering emphasises accuracy at the cost of increased computational overhead. Among the more specialised applications for physically based rendering is lighting simulation, particularly in conjunction with daylight. The aim of this thesis is to investigate the applicability of a novel image synthesis technique based on Monte Carlo particle transport to daylight simulation. Many materials used in daylight simulation are specifically designed to redirect light, and as such give rise to complex effects such as caustics. The photon map technique was chosen for its efficent handling of these effects. To assess its ability to produce physically correct results which can be applied to lighting simulation, a validation was carried out based on analytical case studies and on simple experimental setups. As prerequisite to validation, the photon map\u27s inherent bias/noise tradeoff is investigated. This tradeoff depends on the density estimate bandwidth used in the reconstruction of the illumination. The error analysis leads to the development of a bias compensating operator which adapts the bandwidth according to the estimated bias in the reconstructed illumination. The work presented here was developed at the Fraunhofer Institute for Solar Energy Systems (ISE) as part of the FARESYS project sponsored by the German national research foundation (DFG), and embedded into the RADIANCE rendering system.Die Erzeugung physikalisch basierter Bilder gilt heute noch als eine der rechenintensivsten Aufgaben in der Computergraphik, dessen Anwendungen sowie auch Verfahren in den letzten Jahren kontinuierlich weiterentwickelt wurden, vorangetrieben primär durch den Preisverfall leistungsstarker Hardware. Physikalisch basiertes Rendering hat sich als Nische etabliert, die über die photorealistischen Anforderungen typischer Mainstream-Applikationen hinausgeht, mit dem Ziel, Lichttechnische Größen innerhalb einer komplexen 3D Szene zu berechnen. Im Gegensatz zu Mainstream-Applikationen, die visuell überzeugend wirken sollen und kurze Rechenzeiten erforden, liegt der Schwerpunkt bei physikalisch basiertem Rendering in der Genauigkeit, auf Kosten des Rechenaufwands. Zu den eher spezialisierten Anwendungen im Gebiet des physikalisch basiertem Renderings gehört die Lichtsimulation, besonders in Bezug auf Tageslicht. Das Ziel dieser Dissertation liegt darin, die Anwendbarkeit eines neuartigen Renderingverfahrens basierend auf Monte Carlo Partikeltransport hinsichtlich Tageslichtsimulation zu untersuchen. Viele Materialien, die in der Tageslichtsimulation verwendet werden, sind speziell darauf konzipiert, Tageslicht umzulenken, und somit komplexe Phänomene wie Kaustiken hervorrufen. Das Photon-Map-Verfahren wurde aufgrund seiner effizienten Simulation solcher Effekte herangezogen. Zur Beurteilung seiner Fähigkeit, physikalisch korrekte Ergebnisse zu liefern, die in der Tageslichtsimulation anwendbar sind, wurde eine Validierung anhand analytischer Studien sowie eines einfachen experimentellen Aufbaus durchgeführt. Als Voraussetzung zur Validierung wurde der Photon Map bezüglich seiner inhärenten Wechselwirkung zwischen Rauschen und systematischem Fehler (Bias) untersucht. Diese Wechselwirkung hängt von der Bandbreite des Density Estimates ab, mit dem die Beleuchtung aus den Photonen rekonstruiert wird. Die Fehleranalyse führt zur Entwicklung eines Bias compensating Operators, der die Bandbreite dynamisch anhand des geschätzten Bias in der rekonstruierten Beleuchtung anpasst. Die hier vorgestellte Arbeit wurde am Fraunhofer Institut für Solare Energiesysteme (ISE) als teil des FARESYS Projekts entwickelt, daß von der Deutschen Forschungsgemeinschaft (DFG) finanziert wurde. Die Implementierung erfolgte im Rahmen des RADIANCE Renderingsystems

Fundamentally photographic : the art of Bill Culbert

This thesis applies photographic theory and approaches to the interpretation of Bill Culbert' s three-dimensional installations, elucidating the interdisciplinary nature of his oeuvre. It examines the writings of Roland Barthes, Rosalind Krauss and Susan Sontag, amongst others, and relates their discussions of photographic theory to the role that photography plays in Culbert' s work. Chapter one examines Culbert' s use of photography to explore themes further developed in his installations, investigates the role of the camera obscura in his break from painting, and identifies the continuous interaction between his three-dimensional work and his photographs. Chapter two explores Culbert's use of light and its interaction with space, relating the examination of light and space in his work to aspects of photography such as the index, time, movement, multiplicity and the photographic surface. It also identifies his site-specific practice as part of a broad challenge to Modernism's autonomy and the coded nature of the gallery that began in the 1960s. Chapter three examines Culbert's use of readymade objects and his manipulation of their original contexts, noting parallels to the photographic image's relationship with context. This chapter compares Culbert's use of found objects to Marcel Duchamp's readymades, the Surrealist found object, avantgarde photomontage and Minimalist sculpture. Chapter four investigates the binary states of reality and illusion - integral to photography - that inform Culbert's work. This chapter discusses Plato's allegory of the Cave and the indexical nature of photography, then goes on to explore Culbert' s use of shadow, reflection, metaphor, metonymy, pun and inversion, arguing that these devices resonate with photography's mediation of our perception of reality. The thesis concludes by arguing that Culbert's rejection of Modernism's self-referentiality and his fusion of painting, photography and installation locates his work within Postmodernist practice

The Connection between solar magnetic fields and photospheric dynamics

La convezione rappresenta il meccanismo principale di trasporto dell’energia negli strati sottostanti la superficie solare. La dinamica dei flussi fotosferici associati a tale meccanismo determina la formazione e l’evoluzione del campo magnetico globale e di una grande varietà di strutture presenti nelle regioni più esterne del Sole. In particolare l’interazione tra i flussi di plasma e il campo magnetico determina la configurazione spaziale e l’evoluzione delle regioni attive e degli elementi magnetici superficiali, importanti ad esempio nel determinare la variabilità solare. La convezione solare può essere studiata o mediante lo sviluppo di simulazioni di magnetoconvezione (simulazioni MHD) o attraverso osservazioni spettrali della superficie solare. In questo lavoro il problema della connessione tra campi magnetici solari e dinamica fotosferica è stato affrontato seguendo un approccio sperimentale. In particolare abbiamo lavorato sui sistemi di acquisizione per la spettroscopia solare bidimensionale, sulla pipeline di riduzione di dati spettroscopici solari e infine sull’analisi dei dati. Uno degli strumenti principali della fisica solare sperimentale è la spettroscopia, che permette di derivare informazioni su molti parametri dell’atmosfera solare, quali velocità, temperatura e campo magnetico. Inoltre, l’analisi spettroscopica permette di ricavare la velocità verticale delle strutture emergenti sulla superficie solare. In questo modo, poiché ogni lunghezza d’onda può essere associata ad una determinata quota nell’atmosfera, è possibile trasformare un’immagine bidimensionale in un campo 3D. Al fine di studiare la dinamica dell’atmosfera solare, sono necessarie osservazioni ad alta risoluzione spettrale e spaziale. Inoltre, la rapida evoluzione delle strutture solari osservate richiede monocromatori con un’elevata trasparenza per acquisire spettri multi-riga in un tempo molto breve. Uno strumento che soddisfa tutte queste richieste è IBIS (Interferometric BIdimensional Spectrometer), uno spettrometro bidimensionale installato presso il Dunn Solar Telescope-DST. IBIS produce dati con elevata risoluzione spaziale (0.2” al DST), spettrale (Dl/l~200000) e temporale (tempo di esposizione 10 ms, rate di acquisizione 5 immagini al secondo). Le immagini acuiqiste con IBIS sono registrare da un sensore CCD. Il Capitolo 1 della tesi fornisce un’introduzione alla spettroscopia solare e all’uso delle immagini spettroscopiche per ottenere informazioni sulla dinamica degli strati fotosferici solari. Lo schema dello strumento IBIS, utilizzato in questa tesi per l’acquisizione delle immagini spettroscopiche, è descritto. Nel Capitolo 2 sono riportate le misure e le calibrazioni, effettuate in laboratorio attraverso la Tecnica della Photon Transfer, di due sensori: il sensore CMOS Si-1920-HD e il sensore EMCCD Andor Ixon DV885. Il nostro interesse in questi sensori nasce dalla necessità di sostituire il sensore attualmente installato sul canale spettrale di IBIS, al fine di incrementare l’efficienza di acquisizione dei dati. In particolare, i miglioramenti al sistema di acquisizione di IBIS riguardano diversi aspetti: aumento della sensibilità/efficienza quantica, riduzione del tempo di lettura, incremento della dimensione dell’array e aumento del guadagno del sensore. Nel Capitolo 3 sono descritti i vari passi della pipeline di riduzione dei dati IBIS, che include sia una correzione standard delle immagini sia un software scritto in IDL per l’analisi di immagini solari ad elevata risoluzione. Nel Capitolo 4 riportiamo i risultati scientifici legati allo studio dell’emersione e dell’organizzazione del campo magnetico sulla superficie solare sia come struttura isolata sia come cluster. Tipiche strutture magnetiche isolate sono le macchie solari e i “pore”. E’ stata studiata la dinamica, su piccola scala, di una regione di intenso campo magnetico (pore), con struttura brillante interna. I pore rappresentano una delle tante strutture formate dall’emersione del campo magnetico sulla superficie solare. Essi rappresentano un link tra i più piccoli elementi di flusso e le regioni magnetiche associate alle macchie. I light bridge, in un pore o in una macchia, sono strutture brillanti che dividono la regione di ombra in una strutture più o meno complessa. Comunemente, i light bridge indicano la presenza di un processo in corso all’interno della regione attiva: l’emersione di regioni magnetiche o, al contrario, il disfacimento dell’intera struttura. In entrambi i casi ci si aspetta una riconfigurazione topologica del campo magnetico emergente. Un altro modo per studiare l’interazione del campo magnetico con i moti del plasma consiste nell’andare ad investigare le proprietà oscillatorie della cromosfera solare, sia quieta che attiva, in relazione alla fotosfera sottostante, ponendo particolare riguardo alla topologia del capo magnetico. Nell’atmosfera solare esiste una frequenza di cut-off acustica che produce una riflessione delle onde a bassa frequenza verso gli strati più bassi dell’atmosfera e la regione convettiva. Dunque solo le onde con frequenza maggiori della frequenza di cut-off possono propagarsi verso gli strati più alti dell’atmosfera. Il campo magnetico modifica dunque le proprietà delle oscillazioni acustiche. In particolare, in presenza di un campo magnetico inclinato la frequenza di cut-off acustica si abbassa, permettendo così la propagazione verso l’alto di onde a frequenza maggiore. Questo risultato è stato confermato dalle mappe dei picchi di potenza relative alla fotosfera e alla cromosfera, ottenute utilizzando i dati acquisiti con IBIS. Lo studio della dinamica della fotosfera solare può essere intrapreso anche con metodi statistici, analizzando le proprietà topologiche degli elementi di origine convettiva e magnetica, come la distribuzione spaziale delle strutture presenti nei magnetogrammi. A tal proposito è stato sviluppato un algoritmo in grado di determinare, in maniera automatica, i “vuoti” in una fissata distribuzione di particelle. Questo metodo, applicato ad una serie temporale di magnetogrammi solari con un ampio campo di vista, ha permesso di identificare i vuoti tra strutture magnetiche e di studiarne la distribuzione sulla superficie solare.Convection is the chief mode of heat transport in the outer envelopes of cool stars such as the Sun. Convective effects are recognizable in large-scale features, such as the global differential rotation and meridional circulation flows, as well as smaller scale phenomena such as granulation, mesogranulation, and supergranulation. Moreover, convective flows widely determine the evolution and organization of tiny magnetic elements observed in the solar surface responsible for small scale irradiance solar variations. Our understanding of the solar convection derives from numerical simulations of compressible convection (MHD approach) and from spectral observations of the solar surface (velocity and center line maps, helioseismological data, etc.). In this work we face the problem of connection between solar magnetic fields and photospheric dynamics through an experimental approach. In particular we worked on acquisitions systems for solar imaging spectroscopy, on a pipeline for the spectroscopic data reduction and on the data analysis. One of the basic tools of observational solar physics is spectroscopy, which allows us to derive information on several physical parameters of solar atmosphere such as velocity, temperature, magnetic field strength etc. Spectroscopic analysis allows us to determine the vertical velocity of solar surface structures. Moreover, as wavelength can be somehow associated to depth in the solar atmosphere, it is possible to transform a bidimensional image in a 3-D field. In order to study solar atmosphere dynamics, observations of adequate spectral purity, together with high spatial resolution to resolve small-scale structures are necessary. Moreover, the rapid evolution of observed solar features requires monochromators with high transparency to acquire multiple-line spectra in a comparatively short time. In order to meet all these requirements, suitable instruments and techniques have to be used. An instrument which satisfies all these constraints is IBIS, an Interferometric Bidimensional Spectrometer, installed at the Dunn Solar Telescope/NSO (Sac Peak, USA). IBIS produces data with high spectral (Dl/l~200000), spatial (0.2’’ at DST telescope) and temporal resolution (exposure time 10 ms; acquisition rate 5 frames s-1). Images acquired with IBIS are currently recorded by a CCD camera. Chapter 1 introduces the reader to the solar spectroscopy and to the use of spectroscopic imaging to retrieve information on solar photospheric layers dynamics. The basic concept and the layout of the IBIS spectrograph, used in this thesis to acquire spectroscopic images, is described. Chapter 2 reports laboratory measurements and calibrations, derived through the application of the Photon Transfer Technique, of two sensors: the SI-1920 HD CMOS sensor and the Andor DV885 EMCCD sensor. Our interest in these sensors is related to the necessity to replace the CCD camera, now installed on the IBIS spectral channel. Improvements in the IBIS camera system concern an increased sensitivity/quantum efficiency, a decreased detector readout time, a larger array size and an increased full well/programmable detector gain. Chapter 3 describes the various steps of the pipeline developed for the IBIS data reduction. The pipeline includes both the standard image processing and a high performance IDL software package written specifically for high resolution solar images. In Chapter 4 we report some results related to the study of the emergence and the organization of the magnetic field on the solar surface both as isolated structures and as clusters. More in detail, typical isolated magnetic features are pores or sunspots. We investigated the small scale dynamics of a strong magnetic field region (pore) with a light bridge inside it, observed with the IBIS spectrometer. An analysis of the intensity and velocity maps revealed the presence, inside the light bridge, of elongated structures showing a kind of reversal in intensity and velocity. More in detail, in the intensity images we observed a narrow central dark lane running along the axis of the light bridge, that we explain proposing an analytical model. Regarding the velocity structure, its topology resembles a convective roll and may indicate a modification of the photospheric convective flows. By adopting the IBIS dataset, we studied the oscillatory properties of the solar atmosphere, in the photosphere and the chromosphere, with particular regard to the influence of the magnetic topology. In particular, we analyzed the propagation of waves in the atmosphere in correspondence of a pore, of a magnetic network area and of a quiet Sun region. Studying the generation and propagation of waves in the solar atmosphere provides information about the atmospheric structure and dynamics and it helps to identify the key mechanism of chromospheric and coronal heating. Finally, by using large FoV MDI magnetograms we analyzed the spatial distribution of reticular clusters of magnetic features, such as the magnetic network. For this purpose, we developed a numerical algorithm able to detect voids between magnetic fragments. We computed Void Probability Functions which describe, in a uniform and objective way, the assessment of the void structure of different magnetic elements distributions

Computational mechanisms for colour and lightness constancy

Attributes of colour images have been found which allow colour and lightness constancy to be computed without prior knowledge of the illumination, even in complex scenes with three -dimensional objects and multiple light sources of different colours. The ratio of surface reflectance colour can be immediately determined between any two image points, however distant. It is possible to determine the number of spectrally independent light sources, and to isolate the effect of each. Reflectance edges across which the illumination remains constant can be correctly identified.In a scene illuminated by multiple distant point sources of distinguishalbe colours, the spatial angle between the sources and their brightness ratios can be computed from the image alone. If there are three or more sources then reflectance constancy is immediately possible without use of additional knowledge.The results are an extension of Edwin Land's Retinex algorithm. They account for previously unexplained data such as Gilchrist's veiling luminances and his single- colour rooms.The validity of the algorithms has been demonstrated by implementing them in a series of computer programs. The computational methods do not follow the edge or region finding paradigms of previous vision mechanisms. Although the new reflectance constancy cues occur in all normal scenes, it is likely that human vision makes use of only some of them.In a colour image all the pixels of a single surface colour lie in a single structure in flux space. The dimension of the structure equals the number of illumination colours. The reflectance ratio between two regions is determined by the transformation between their structures. Parallel tracing of edge pairs in their respective structures identifies an edge of constant illumination, and gives the lightness ratio of each such edge. Enhanced noise reduction techniques for colour pictures follow from the natural constraints on the flux structures