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

Microgeometry capture using an elastomeric sensor

We describe a system for capturing microscopic surface geometry. The system extends the retrographic sensor [Johnson and Adelson 2009] to the microscopic domain, demonstrating spatial resolution as small as 2 microns. In contrast to existing microgeometry capture techniques, the system is not affected by the optical characteristics of the surface being measured---it captures the same geometry whether the object is matte, glossy, or transparent. In addition, the hardware design allows for a variety of form factors, including a hand-held device that can be used to capture high-resolution surface geometry in the field. We achieve these results with a combination of improved sensor materials, illumination design, and reconstruction algorithm, as compared to the original sensor of Johnson and Adelson [2009].National Science Foundation (U.S.) (Grant 0739255)National Institutes of Health (U.S.) (Contract 1-R01-EY019292-01

Material Recognition Meets 3D Reconstruction : Novel Tools for Efficient, Automatic Acquisition Systems

For decades, the accurate acquisition of geometry and reflectance properties has represented one of the major objectives in computer vision and computer graphics with many applications in industry, entertainment and cultural heritage. Reproducing even the finest details of surface geometry and surface reflectance has become a ubiquitous prerequisite in visual prototyping, advertisement or digital preservation of objects. However, today's acquisition methods are typically designed for only a rather small range of material types. Furthermore, there is still a lack of accurate reconstruction methods for objects with a more complex surface reflectance behavior beyond diffuse reflectance. In addition to accurate acquisition techniques, the demand for creating large quantities of digital contents also pushes the focus towards fully automatic and highly efficient solutions that allow for masses of objects to be acquired as fast as possible. This thesis is dedicated to the investigation of basic components that allow an efficient, automatic acquisition process. We argue that such an efficient, automatic acquisition can be realized when material recognition "meets" 3D reconstruction and we will demonstrate that reliably recognizing the materials of the considered object allows a more efficient geometry acquisition. Therefore, the main objectives of this thesis are given by the development of novel, robust geometry acquisition techniques for surface materials beyond diffuse surface reflectance, and the development of novel, robust techniques for material recognition. In the context of 3D geometry acquisition, we introduce an improvement of structured light systems, which are capable of robustly acquiring objects ranging from diffuse surface reflectance to even specular surface reflectance with a sufficient diffuse component. We demonstrate that the resolution of the reconstruction can be increased significantly for multi-camera, multi-projector structured light systems by using overlappings of patterns that have been projected under different projector poses. As the reconstructions obtained by applying such triangulation-based techniques still contain high-frequency noise due to inaccurately localized correspondences established for images acquired under different viewpoints, we furthermore introduce a novel geometry acquisition technique that complements the structured light system with additional photometric normals and results in significantly more accurate reconstructions. In addition, we also present a novel method to acquire the 3D shape of mirroring objects with complex surface geometry. The aforementioned investigations on 3D reconstruction are accompanied by the development of novel tools for reliable material recognition which can be used in an initial step to recognize the present surface materials and, hence, to efficiently select the subsequently applied appropriate acquisition techniques based on these classified materials. In the scope of this thesis, we therefore focus on material recognition for scenarios with controlled illumination as given in lab environments as well as scenarios with natural illumination that are given in photographs of typical daily life scenes. Finally, based on the techniques developed in this thesis, we provide novel concepts towards efficient, automatic acquisition systems

Image based surface reflectance remapping for consistent and tool independent material appearence

Physically-based rendering in Computer Graphics requires the knowledge of material properties other than 3D shapes, textures and colors, in order to solve the rendering equation. A number of material models have been developed, since no model is currently able to reproduce the full range of available materials. Although only few material models have been widely adopted in current rendering systems, the lack of standardisation causes several issues in the 3D modelling workflow, leading to a heavy tool dependency of material appearance. In industry, final decisions about products are often based on a virtual prototype, a crucial step for the production pipeline, usually developed by a collaborations among several departments, which exchange data. Unfortunately, exchanged data often tends to differ from the original, when imported into a different application. As a result, delivering consistent visual results requires time, labour and computational cost. This thesis begins with an examination of the current state of the art in material appearance representation and capture, in order to identify a suitable strategy to tackle material appearance consistency. Automatic solutions to this problem are suggested in this work, accounting for the constraints of real-world scenarios, where the only available information is a reference rendering and the renderer used to obtain it, with no access to the implementation of the shaders. In particular, two image-based frameworks are proposed, working under these constraints. The first one, validated by means of perceptual studies, is aimed to the remapping of BRDF parameters and useful when the parameters used for the reference rendering are available. The second one provides consistent material appearance across different renderers, even when the parameters used for the reference are unknown. It allows the selection of an arbitrary reference rendering tool, and manipulates the output of other renderers in order to be consistent with the reference

Towards the Control of Electrophotographic-based 3-Dimensional Printing: Image-Based Sensing and Modeling of Surface Defects

Electro-Photography (EP) has been used for decades for fast, cheap, and reliable printing in offices and homes around the world. It has been shown that extending the use of EP for 3D printing is feasible; multiple layered prints are already commercially available (color laser printers) but only for a very limited number of layers. Many of the advantages of laser printing make EP 3D printing desirable including: speed, reliability, selective coloring, ability to print a thermoplastic, possibilities for multi-material printing, ability to print materials not amenable to liquid ink formulations. However, many challenges remain before EP-based 3D printing can be commercially viable. A limiting factor in using the same system architecture as a traditional laser printer is that as the thickness of the part increases, material deposition becomes more difficult with each layer since the increased thickness reduces the field strength. Different system configurations have been proposed where the layer is printed on intermediate stations and are subsequently transferred to the work piece. Layer registration and uniform transfer from the intermediate station become crucial factors in this architecture. At the Print Research and Imaging Systems Modeling (PRISM) Lab preliminary tests have confirmed the feasibility of using EP for Additive Manufacturing (AM). However, similar issues were encountered to those reported in literature as the number of layers increased, resulting in non-uniform brittle 3D structures. The defects were present but not obvious at each layer, and as the part built up, the defects add up and became more obvious. The process, as in many printers, did not include a control system for the ultimate system output (print), and the actuation method (electrostatic charge) is not entirely well characterized or sensed to be used in a control system. This research intends to help the development of a model and an image-based sensing system that can be used for control of material deposition defects for an EP 3D printing process. This research leverages from the expertise at RIT and the Rochester area in Printing, Electrophotography, Rapid Prototyping, Control, and Imaging Sciences

A Theory of Refractive Photo-Light-Path Triangulation

International audience3D reconstruction of transparent refractive objects like a plastic bottle is challenging: they lack appearance related visual cues and merely reflect and refract light from the surrounding environment. Amongst several approaches to reconstruct such objects, the seminal work of Light-Path triangulation is highly popular because of its general applicability and analysis of minimal scenarios. A light-path is defined as the piece-wise linear path taken by a ray of light as it passes from source, through the object and into the camera. Transparent refractive objects not only affect the geometric configuration of light-paths but also their radiometric properties. In this paper, we describe a method that combines both geometric and radiometric information to do reconstruction. We show two major consequences of the addition of radiometric cues to the light-path setup. Firstly, we extend the case of scenarios in which reconstruction is plausible while reducing the minimal requirements for a unique reconstruction. This happens as a consequence of the fact that radiometric cues add an additional known variable to the already existing system of equations. Secondly, we present a simple algorithm for reconstruction, owing to the nature of the radiometric cue. We present several synthetic experiments to validate our theories, and show high quality reconstructions in challenging scenarios

BxDF material acquisition, representation, and rendering for VR and design

Photorealistic and physically-based rendering of real-world environments with high fidelity materials is important to a range of applications, including special effects, architectural modelling, cultural heritage, computer games, automotive design, and virtual reality (VR). Our perception of the world depends on lighting and surface material characteristics, which determine how the light is reflected, scattered, and absorbed. In order to reproduce appearance, we must therefore understand all the ways objects interact with light, and the acquisition and representation of materials has thus been an important part of computer graphics from early days. Nevertheless, no material model nor acquisition setup is without limitations in terms of the variety of materials represented, and different approaches vary widely in terms of compatibility and ease of use. In this course, we describe the state of the art in material appearance acquisition and modelling, ranging from mathematical BSDFs to data-driven capture and representation of anisotropic materials, and volumetric/thread models for patterned fabrics. We further address the problem of material appearance constancy across different rendering platforms. We present two case studies in architectural and interior design. The first study demonstrates Yulio, a new platform for the creation, delivery, and visualization of acquired material models and reverse engineered cloth models in immersive VR experiences. The second study shows an end-to-end process of capture and data-driven BSDF representation using the physically-based Radiance system for lighting simulation and rendering

Measuring and simulating haemodynamics due to geometric changes in facial expression

The human brain has evolved to be very adept at recognising imperfections in human skin. In particular, observing someone’s facial skin appearance is important in recognising when someone is ill, or when finding a suitable mate. It is therefore a key goal of computer graphics research to produce highly realistic renderings of skin. However, the optical processes that give rise to skin appearance are complex and subtle. To address this, computer graphics research has incorporated more and more sophisticated models of skin reflectance. These models are generally based on static concentrations of skin chromophores; melanin and haemoglobin. However, haemoglobin concentrations are far from static, as blood flow is directly caused by both changes in facial expression and emotional state. In this thesis, we explore how blood flow changes as a consequence of changing facial expression with the aim of producing more accurate models of skin appearance. To build an accurate model of blood flow, we base it on real-world measurements of blood concentrations over time. We describe, in detail, the steps required to obtain blood concentrations from photographs of a subject. These steps are then used to measure blood concentration maps for a series of expressions that define a wide gamut of human expression. From this, we define a blending algorithm that allows us to interpolate these maps to generate concentrations for other expressions. This technique, however, requires specialist equipment to capture the maps in the first place. We try to rectify this problem by investigating a direct link between changes in facial geometry and haemoglobin concentrations. This requires building a unique capture device that captures both simultaneously. Our analysis hints a direct linear connection between the two, paving the way for further investigatio