375 research outputs found

    Surface Denoising based on Normal Filtering in a Robust Statistics Framework

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    During a surface acquisition process using 3D scanners, noise is inevitable and an important step in geometry processing is to remove these noise components from these surfaces (given as points-set or triangulated mesh). The noise-removal process (denoising) can be performed by filtering the surface normals first and by adjusting the vertex positions according to filtered normals afterwards. Therefore, in many available denoising algorithms, the computation of noise-free normals is a key factor. A variety of filters have been introduced for noise-removal from normals, with different focus points like robustness against outliers or large amplitude of noise. Although these filters are performing well in different aspects, a unified framework is missing to establish the relation between them and to provide a theoretical analysis beyond the performance of each method. In this paper, we introduce such a framework to establish relations between a number of widely-used nonlinear filters for face normals in mesh denoising and vertex normals in point set denoising. We cover robust statistical estimation with M-smoothers and their application to linear and non-linear normal filtering. Although these methods originate in different mathematical theories - which include diffusion-, bilateral-, and directional curvature-based algorithms - we demonstrate that all of them can be cast into a unified framework of robust statistics using robust error norms and their corresponding influence functions. This unification contributes to a better understanding of the individual methods and their relations with each other. Furthermore, the presented framework provides a platform for new techniques to combine the advantages of known filters and to compare them with available methods

    Cosmology at Low Frequencies: The 21 cm Transition and the High-Redshift Universe

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    Observations of the high-redshift Universe with the 21 cm hyperfine line of neutral hydrogen promise to open an entirely new window onto the early phases of cosmic structure formation. Here we review the physics of the 21 cm transition, focusing on processes relevant at high redshifts, and describe the insights to be gained from such observations. These include measuring the matter power spectrum at z~50, observing the formation of the cosmic web and the first luminous sources, and mapping the reionization of the intergalactic medium. The epoch of reionization is of particular interest, because large HII regions will seed substantial fluctuations in the 21 cm background. We also discuss the experimental challenges involved in detecting this signal, with an emphasis on the Galactic and extragalactic foregrounds. These increase rapidly toward low frequencies and are especially severe for the highest redshift applications. Assuming that these difficulties can be overcome, the redshifted 21 cm line will offer unique insight into the high-redshift Universe, complementing other probes but providing the only direct, three-dimensional view of structure formation from z~200 to z~6.Comment: extended review accepted by Physics Reports, 207 pages, 44 figures (some low resolution); version with high resolution figures available at http://pantheon.yale.edu/~srf28/21cm/index.htm; minor changes to match published versio

    Extending the Reach of Directional Dark Matter Experiments Through Novel Detector Technologies

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    Dark matter is believed to comprise over 80% of the matter in the Universe. Its composition could be in the form of weakly interacting massive particles (WIMPs), which are predicted by extensions of the Standard Model, namely supersymmetric theories. Even though hints of its existence were detected in astronomical observations over eighty years ago, its detection through means other than the gravitational influence on observable luminous matter still eludes us. Currently, there are many ongoing direct detection experiments, that aim to measure the signals left by the elastic scattering of WIMPs with nuclei in the detector target material. The detection and identification of dark matter is made difficult, however, by the small interaction cross-section with ordinary matter and the large parameter space that it could inhabit. As such large detectors are needed to probe this parameter space, but potential detections can appear ambiguous in origin due to the presence of backgrounds and a lack of a strong fingerprint in the energy spectrum of detected events. Fortunately, there are two signatures that could point to the Galactic origin of the signal. These are the annual modulation and directional signatures, but of the two, the latter can provide the strongest evidence. This thesis discusses the many challenges of directional detection utilizing the low pressure time projection chamber (TPC) technology and describes the experimental efforts to overcome them. A study of low-energy recoils to explore the achievable discrimination threshold and directional sensitivity in a real detector is described. Next, I discuss progress towards a path for detector scale-up while retaining sensitivity by employing a newly identified electronegative TPC gas. The development of a novel readout technology for large detectors is discussed. Finally, the last chapter is devoted to a new idea on a method to detect directionality in a high pressure detector

    Visualising the Synthetic Universe

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    Star formation cannot truly be understood from observational data alone; only with simulations is it possible to assemble the complete picture. Observations guide the physics we build into our simulations, yet the impact of different star formation and feedback models can only be investigated with simulations. Synthetic observations allow us to make a realistic comparison to true observations as well as teach us about the emission tracers we depend upon. Through coupling the stellar population synthesis code SLUG2 to galaxy simulations, we can generate synthetic star formation rate tracer maps. These maps assume different stellar metallicities, star formation rate surface densities, and suffer from varied amounts of extinction. This allows us to explore and constrain the environmental effects on the characteristic emission lifetimes — the duration for which a tracer is visible. With these emission lifetimes and in conjunction with a new statistical method, the ‘uncertainty principle for star formation’, constraints can be placed upon the durations of different evolutionary phases of the star formation process, allowing us to better understand the physics of star formation and feedback on sub-galactic scales. Studying the interstellar medium can also reveal information about stellar feedback: the gas density structure is altered as a result of the injected energy, momentum, and matter. Surveys of the CO emission in galaxies can tell us how the properties of this medium have evolved over cosmic time. Using DESPOTIC to model CO line emission of gas found within the IllustrisTNG50 cosmological simulation, we produce an equivalent synthetic survey. This synthetic survey can be used as a basis for comparison and predictor of observational trends

    Chip-scale optical frequency comb sources for terabit communications

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    To keep up with the ever-increasing data transmission speed needs, data center interconnects are scaling up to provide multi-Tbit/s connectivity. These links require a high number of WDM channels, while the associated transceivers should be compact and energy efficient. Scaling the number of channels, however, is still limited by the lack of adequate optical sources. In this book, we investigate novel chip-scale frequency comb generators as multi-wavelength light sources for Tbit/s WDM links

    Chip-scale optical frequency comb sources for terabit communications

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    The number of devices connected to the internet and the required data transmission speeds are increasing exponentially. To keep up with this trend, data center interconnects should scale up to provide multi-Tbit/s connectivity. With typical distances from a few kilometers to 100 km, these links require the use of a high number of WDM channels. The associated transceivers should have low cost and footprint. The scalability of the number of channels, however, is still limited by the lack of adequate optical sources. In this book, we investigate novel chip-scale frequency comb generators as multi-wavelength light sources in WDM links. With a holistic model, we estimate the performance of comb-based WDM links, and we compare the transmission performance of different comb generator types, namely a quantum-dash mode-locked laser diode and a microresonator-based Kerr comb generator. We characterize their potential for massively-parallel WDM transmission with various transmission experiments. Combined with photonic integrated circuits, these comb sources offer a path towards highly scalable, compact, and energy-efficient Tbit/s transceivers
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