Improving SLI Performance in Optically Challenging Environments

The construction of 3D models of real-world scenes using non-contact methods is an important problem in computer vision. Some of the more successful methods belong to a class of techniques called structured light illumination (SLI). While SLI methods are generally very successful, there are cases where their performance is poor. Examples include scenes with a high dynamic range in albedo or scenes with strong interreflections. These scenes are referred to as optically challenging environments. The work in this dissertation is aimed at improving SLI performance in optically challenging environments. A new method of high dynamic range imaging (HDRI) based on pixel-by-pixel Kalman filtering is developed. Using objective metrics, it is show to achieve as much as a 9.4 dB improvement in signal-to-noise ratio and as much as a 29% improvement in radiometric accuracy over a classic method. Quality checks are developed to detect and quantify multipath interference and other quality defects using phase measuring profilometry (PMP). Techniques are established to improve SLI performance in the presence of strong interreflections. Approaches in compressed sensing are applied to SLI, and interreflections in a scene are modeled using SLI. Several different applications of this research are also discussed

New 3D scanning techniques for complex scenes

This thesis presents new 3D scanning methods for complex scenes, such as surfaces with fine-scale geometric details, translucent objects, low-albedo objects, glossy objects, scenes with interreflection, and discontinuous scenes. Starting from the observation that specular reflection is a reliable visual cue for surface mesostructure perception, we propose a progressive acquisition system that captures a dense specularity field as the only information for mesostructure reconstruction. Our method can efficiently recover surfaces with fine-scale geometric details from complex real-world objects. Translucent objects pose a difficult problem for traditional optical-based 3D scanning techniques. We analyze and compare two descattering methods, phaseshifting and polarization, and further present several phase-shifting and polarization based methods for high quality 3D scanning of translucent objects. We introduce the concept of modulation based separation, where a high frequency signal is multiplied on top of another signal. The modulated signal inherits the separation properties of the high frequency signal and allows us to remove artifacts due to global illumination. Thismethod can be used for efficient 3D scanning of scenes with significant subsurface scattering and interreflections.Diese Dissertation präsentiert neuartige Verfahren für die 3D-Digitalisierung komplexer Szenen, wie z.B. Oberflächen mit sehr feinen Strukturen, durchscheinende Objekte, Gegenstände mit geringem Albedo, glänzende Objekte, Szenen mit Lichtinterreflektionen und unzusammenhängende Szenen. Ausgehend von der Beobachtung, daß die spekulare Reflektion ein zuverlässiger, visueller Hinweis für die Mesostruktur einer Oberfläche ist, stellen wir ein progressives Meßsystem vor, um Spekularitätsfelder zu messen. Aus diesen Feldern kann anschließend die Mesostruktur rekonstruiert werden. Mit unserer Methode können Oberflächen mit sehr feinen Strukturen von komplexen, realen Objekten effizient aufgenommen werden. Durchscheinende Objekte stellen ein großes Problem für traditionelle, optischbasierte 3D-Rekonstruktionsmethoden dar. Wir analysieren und vergleichen zwei verschiedene Methoden zum Eliminieren von Lichtstreuung (Descattering): Phasenverschiebung und Polarisation. Weiterhin präsentieren wir mehrere hochqualitative 3D-Rekonstruktionsmethoden für durchscheinende Objekte, die auf Phasenverschiebung und Polarisation basieren. Außerdem führen wir das Konzept der modulationsbasierten Signaltrennung ein. Hierzu wird ein hochfrequentes Signal zu einem anderes Signal multipliziert. Das so modulierte Signal erhält damit die separierenden Eigenschaften des hochfrequenten Signals. Dies erlaubt unsMeßartefakte aufgrund von globalen Beleuchtungseffekten zu vermeiden. Dieses Verfahren kann zum effizienten 3DScannen von Szenen mit durchscheinden Objekten und Interreflektionen benutzt werden

Clasificación automática de anomalías asociadas con ausencia de información en superficies tridimensionales de objetos de forma libre

En este trabajo se propone un método computacional para clasificar anomalías relacionadas con ausencia de información sobre modelos tridimensionales de forma libre. Para ello, se hizo una exploración descriptiva de las propiedades geométricas globales y locales de las anomalías y una evaluación posterior de distintos métodos de clasificación utilizados en visión artificial y aplicaciones de reconstrucción tridimensional. El método propuesto logra un nivel de clasificación cercano al 90% y un tiempo de ejecución de alrededor de 100 milisegundos. Restringir la clasificación de acuerdo a la aplicación en específico se propone como trabajo futuro./Abstract. In this work it is proposed a computational method to classify anomalies related with information absence over free-form tridimensional models. For that, it was made a descriptive exploration of global and local geometric properties of anomalies and a posterior evaluation of different classification methods widely used in artificial vision and tridimensional reconstruction applications. The proposed method achieved a classification level near to 90% and an execution time near to 100 miliseconds. Constrain classification according to the specific application is suggested as future work.Maestrí

Reconsidering light transport : acquisition and display of real-world reflectance and geometry

In this thesis, we cover three scenarios that violate common simplifying assumptions about the nature of light transport. We begin with the first ingredient to any çD rendering: a geometry model. Most çD scanners require the object-of-interest to show diffuse refectance. The further a material deviates from the Lambertian model, the more likely these setups are to produce corrupted results. By placing a traditional laser scanning setup in a participating (in particular, fuorescent) medium, we have built a light sheet scanner that delivers robust results for a wide range of materials, including glass. Further investigating the phenomenon of fluorescence, we notice that, despite its ubiquity, it has received moderate attention in computer graphics. In particular, to date no datadriven reflectance models of fluorescent materials have been available. To describe the wavelength-shifling reflectance of fluorescent materials, we define the bispectral bidirectional reflectance and reradiation distribution function (BRRDF), for which we introduce an image-based measurement setup as well as an efficient acquisition scheme. Finally, we envision a computer display that showsmaterials instead of colours, and present a prototypical device that can exhibit anisotropic reflectance distributions similar to common models in computer graphics.In der Computergraphik und Computervision ist es unerlässlich, vereinfachende Annahmen über die Ausbreitung von Licht zumachen. In dieser Dissertation stellen wir drei Fälle vor, in denen diese nicht zutreffen. So wird die dreidimensionale Geometrie von Gegenständen oft mit Hilfe von Laserscannern vermessen und dabei davon ausgegangen, dass ihre Oberfläche diffus reflektiert. Dies ist bei den meisten Materialien jedoch nicht gegeben, so dass die Ergebnisse oft fehlerhaft sind. Indem wir das Objekt in einem fluoreszierenden Medium einbetten, kann ein klassischer CD-Scanner-Aufbau so modifiziert werden, dass er verlässliche Geometriedaten für Objekte aus verschiedensten Materialien liefert, einschließlich Glas. Auch die akkurate Nachbildung des Aussehens von Materialien ist wichtig für die photorealistische Bildsynthese. Wieder interessieren wir uns für Fluoreszenz, diesmal allerdings für ihr charakteristisches Erscheinungsbild, das in der Computergraphik bislang kaum Beachtung gefunden hat. Wir stellen einen bildbasierten Aufbau vor, mit dem die winkel- und wellenlängenabhängige Reflektanz fluoreszierender Oberflächen ausgemessen werden kann, und eine Strategie, um solche Messungen effizient abzuwickeln. Schließlich befassen wir uns mit der Idee, nicht nur Farben dynamisch anzuzeigen, sondern auch Materialien und ihr je nach Lichteinfall und Blickwinkel unterschiedliches Aussehen. Einer generellen Beschreibung des Problems folgt die konkrete Umsetzung in Formzweier Prototypen, die verschiedene Reflektanzverteilungen auf einer Oberfläche darstellen können

Surface scanning with uncoded structured light sources.

Structured Light Scanners measure the surface of a target object, producing a set of vertices which can be used to construct a three-dimensional model of the surface. The techniques are particularly appropriate for measuring the smoothly undulating, featureless forms which Stereo Vision methods find difficult, and the structured light pattern explicitly gives a dense graph of connected vertices, thus obviating the need for vertex-triangulation prior to surface reconstruction. In addition, the technique provides the measurements almost instantaneously, and so is suitable for scanning moving and non-rigid objects. Because of these advantages there is an imperative to extend the range of scannable surfaces to those including occlusions, which often reduce or prevent successful measurement.This thesis investigates ways of improving both the accuracy and the range of surface types which can be scanned using structured light techniques, extending current research by examining the role of occlusions and geometric constraints, and introducing novel algorithms to solve the Indexing Problem. The Indexing Problem demands that for every pattern element in the projected image, its counterpart, reflected from the surface of the target object, must be found in the recorded image, and most researchers have declared this problem to be intractable without resorting to coding schemes which uniquely identify each pattern element. The use of uncoded projection patterns, where the pattern elements are projected without any unique identification, has two advantages: firstly it provides the densest possible set of measured vertices within a single video timeframe, and secondly it allows the investigation of the fundamental problems without the distraction of dealing with coding schemes. These advantages educe the general strategy adopted in this thesis, of attempting to solve the Indexing Problem using uncoded patterns, and then adding some coding where difficulties still remain.In order to carry out these investigations it is necessary to precisely measure the system and its outputs, and to achieve this requirement two scanners have been built, a Single Stripe Scanner and a Multiple Stripe Scanner. The Single Stripe Scanner introduces the geometric measurement methods and provides a reference output which matches the industry standard; the Multiple Stripe Scanner then tests the results of the investigations and evaluates the success of the new algorithms and constraints. In addition, some of the investigations are tested theoretically, by using synthetic data and by the solution of geometric diagrams.These evaluations of success show that, if occlusions are not present in the recorded data, the Indexing Problem can often be completely solved if the new indexing algorithms and geometric constraints are included. Furthermore, while there are some cases where the Indexing Problem cannot be solved without recourse to a coding scheme, the addition of occlusion detection in the algorithms greatly improves the indexing accuracy and therefore the successful measurement of the target surface