Keyframe-based visual–inertial odometry using nonlinear optimization

Combining visual and inertial measurements has become popular in mobile robotics, since the two sensing modalities offer complementary characteristics that make them the ideal choice for accurate visual–inertial odometry or simultaneous localization and mapping (SLAM). While historically the problem has been addressed with filtering, advancements in visual estimation suggest that nonlinear optimization offers superior accuracy, while still tractable in complexity thanks to the sparsity of the underlying problem. Taking inspiration from these findings, we formulate a rigorously probabilistic cost function that combines reprojection errors of landmarks and inertial terms. The problem is kept tractable and thus ensuring real-time operation by limiting the optimization to a bounded window of keyframes through marginalization. Keyframes may be spaced in time by arbitrary intervals, while still related by linearized inertial terms. We present evaluation results on complementary datasets recorded with our custom-built stereo visual–inertial hardware that accurately synchronizes accelerometer and gyroscope measurements with imagery. A comparison of both a stereo and monocular version of our algorithm with and without online extrinsics estimation is shown with respect to ground truth. Furthermore, we compare the performance to an implementation of a state-of-the-art stochastic cloning sliding-window filter. This competitive reference implementation performs tightly coupled filtering-based visual–inertial odometry. While our approach declaredly demands more computation, we show its superior performance in terms of accuracy

Constraints On Porosity And Mass Loss In O-Star Winds From The Modeling Of X-Ray Emission Line Profile Shapes

We fit X-ray emission line profiles in high resolution XMM-Newton and Chandra grating spectra of the early O supergiant zeta Pup with models that include the effects of porosity in the stellar wind. We explore the effects of porosity due to both spherical and flattened clumps. We find that porosity models with flattened clumps oriented parallel to the photosphere provide poor fits to observed line shapes. However, porosity models with isotropic clumps can provide acceptable fits to observed line shapes, but only if the porosity effect is moderate. We quantify the degeneracy between porosity effects from isotropic clumps and the mass-loss rate inferred from the X-ray line shapes, and we show that only modest increases in the mass-loss rate (less than or similar to 40%) are allowed if moderate porosity effects (h(infinity) less than or similar to R-*) are assumed to be important. Large porosity lengths, and thus strong porosity effects, are ruled out regardless of assumptions about clump shape. Thus, X-ray mass-loss rate estimates are relatively insensitive to both optically thin and optically thick clumping. This supports the use of X-ray spectroscopy as a mass-loss rate calibration for bright, nearby O stars

A Generalized Porosity Formalism For Isotropic And Anisotropic Effective Opacity And Its Effects On X-Ray Line Attenuation In Clumped O Star Winds

We present a generalized formalism for treating the porosity-associated reduction in continuum opacity that occurs when individual clumps in a stochastic medium become optically thick. As in previous work, we concentrate on developing bridging laws between the limits of optically thin and thick clumps. We consider geometries resulting in either isotropic or anisotropic effective opacity, and, in addition to an idealized model in which all clumps have the same local overdensity and scale, we also treat an ensemble of clumps with optical depths set by Markovian statistics. This formalism is then applied to the specific case of boundfree absorption of X-rays in hot star winds, a process not directly affected by clumping in the optically thin limit. We find that the Markov model gives surprisingly similar results to those found previously for the single-clump model, suggesting that porous opacity is not very sensitive to details of the assumed clump distribution function. Further, an anisotropic effective opacity favours escape of X-rays emitted in the tangential direction (the venetian blind effect), resulting in a bump of higher flux close to line centre as compared to profiles computed from isotropic porosity models. We demonstrate how this characteristic line shape may be used to diagnose the clump geometry, and we confirm previous results that for optically thick clumping to significantly influence X-ray line profiles, very large porosity lengths, defined as the mean free path between clumps, are required. Moreover, we present the first X-ray line profiles computed directly from line-driven instability simulations using a 3D patch method, and find that porosity effects from such models also are very small. This further supports the view that porosity has, at most, a marginal effect on X-ray line diagnostics in O stars, and therefore that these diagnostics do indeed provide a good clumping insensitive method for deriving O star mass-loss rates

On The Importance Of The Interclump Medium For Superionization: O VI Formation In The Wind Of Zeta Puppis

We have studied superionization and X-ray line formation in the spectra of zeta Pup using our new stellar atmosphere code (XCMFGEN) that can be used to simultaneously analyze optical, UV, and X-ray observations. Here, we present results on the formation of the O VI lambda lambda 1032, 1038 doublet. Our simulations, supported by simple theoretical calculations, show that clumped wind models that assume void in the interclump space cannot reproduce the observed O vi profiles. However, enough O vi can be produced if the voids are filled by a low-density gas. The recombination of O vi is very efficient in the dense material, but in the tenuous interclump region an observable amount of O vi can be maintained. We also find that different UV resonance lines are sensitive to different density regimes in z Pup: C IV is almost exclusively formed within the densest regions, while the majority of O vi resides between clumps. N v is an intermediate case, with contributions from both the tenuous gas and clumps

Feline infectious peritonitis: role of the feline coronavirus 3c gene in intestinal tropism and pathogenicity based upon isolates from resident and adopted shelter cats.

Feline infectious peritonitis virus (FIPV) was presumed to arise from mutations in the 3c of a ubiquitous and largely nonpathogenic feline enteric coronavirus (FECV). However, a recent study found that one-third of FIPV isolates have an intact 3c and suggested that it is not solely involved in FIP but is essential for intestinal replication. In order to confirm these assumptions, 27 fecal and 32 FIP coronavirus isolates were obtained from resident or adopted cats from a large metropolitan shelter during 2008-2009 and their 3a-c, E, and M genes sequenced. Forty percent of coronavirus isolates from FIP tissues had an intact 3c gene, while 60% had mutations that truncated the gene product. The 3c genes of fecal isolates from healthy cats were always intact. Coronavirus from FIP diseased tissues consistently induced FIP when given either oronasally or intraperitoneally (i.p.), regardless of the functional status of their 3c genes, thus confirming them to be FIPVs. In contrast, fecal isolates from healthy cats were infectious following oronasal infection and shed at high levels in feces without causing disease, as expected for FECVs. Only one in three cats shed FECV in the feces following i.p. infection, indicating that FECVs can replicate systemically, but with difficulty. FIPVs having a mutated 3c were not shed in the feces following either oronasal or i.p. inoculation, while FIPVs with intact 3c genes were shed in the feces following oronasal but not i.p. inoculation. Therefore, an intact 3c appears to be essential for intestinal replication. Although FIPVs with an intact 3c were shed in the feces following oronasal inoculation, fecal virus from these cats was not infectious for other cats. Attempts to identify potential FIP mutations in the 3a, 3b, E, and M were negative. However, the 3c gene of FIPVs, even though appearing intact, contained many more non-synonymous amino acid changes in the 3' one-third of the 3c protein than FECVs. An attempt to trace FIPV isolates back to enteric strains existing in the shelter was only partially successful due to the large region over which shelter cats and kittens originated, housing conditions prior to acquisition, and rapid movement through the shelter. No evidence could be found to support a recent theory that FIPVs and FECVs are genetically distinct

On the Importance of the Interclump Medium for Superionization: O VI Formation in the Wind of Zeta Pup

We have studied superionization and X-ray line formation in the spectra of Zeta Pup using our new stellar atmosphere code (XCMFGEN) that can be used to simultaneously analyze optical, UV, and X-ray observations. Here, we present results on the formation of the O VI ll1032, 1038 doublet. Our simulations, supported by simple theoretical calculations, show that clumped wind models that assume void in the interclump space cannot reproduce the observed O VI profiles. However, enough O VI can be produced if the voids are filled by a low density gas. The recombination of O VI is very efficient in the dense material but in the tenuous interclump region an observable amount of O VI can be maintained. We also find that different UV resonance lines are sensitive to different density regimes in Zeta Pup : C IV is almost exclusively formed within the densest regions, while the majority of O VI resides between clumps. N V is an intermediate case, with contributions from both the tenuous gas and clumps.Comment: Accepted for publication in ApJL, 4 pages with 3 figure