Fast Face Detector Training Using Tailored Views

Face detection is an important task in computer vision and often serves as the first step for a variety of applications. State-of-the-art approaches use efficient learning algorithms and train on large amounts of manually labeled imagery. Acquiring appropriate training images, however, is very time-consuming and does not guarantee that the collected training data is representative in terms of data variability. Moreover, available data sets are often acquired under con-trolled settings, restricting, for example, scene illumination or 3D head pose to a narrow range. This paper takes a look into the automated generation of adaptive training samples from a 3D morphable face model. Using statistical insights, the tailored training data guarantees full data variability and is enriched by arbitrary facial attributes such as age or body weight. Moreover, it can automatically adapt to environmental constraints, such as illumination or viewing angle of recorded video footage from surveillance cameras. We use the tailored imagery to train a new many-core imple-mentation of Viola Jones ’ AdaBoost object detection frame-work. The new implementation is not only faster but also enables the use of multiple feature channels such as color features at training time. In our experiments we trained seven view-dependent face detectors and evaluate these on the Face Detection Data Set and Benchmark (FDDB). Our experiments show that the use of tailored training imagery outperforms state-of-the-art approaches on this challenging dataset. 1

Viscosity in spherically symmetric accretion

The influence of viscosity on the flow behaviour in spherically symmetric accretion, has been studied here. The governing equation chosen has been the Navier-Stokes equation. It has been found that at least for the transonic solution, viscosity acts as a mechanism that detracts from the effectiveness of gravity. This has been conjectured to set up a limiting scale of length for gravity to bring about accretion, and the physical interpretation of such a length-scale has been compared with the conventional understanding of the so-called "accretion radius" for spherically symmetric accretion. For a perturbative presence of viscosity, it has also been pointed out that the critical points for inflows and outflows are not identical, which is a consequence of the fact that under the Navier-Stokes prescription, there is a breakdown of the invariance of the stationary inflow and outflow solutions -- an invariance that holds good under inviscid conditions. For inflows, the critical point gets shifted deeper within the gravitational potential well. Finally, a linear stability analysis of the stationary inflow solutions, under the influence of a perturbation that is in the nature of a standing wave, has indicated that the presence of viscosity induces greater stability in the system, than has been seen for the case of inviscid spherically symmetric inflows.Comment: 7 pages. Minor changes made in the version published in MNRA

CaII absorption in the circumstellar disk of Beta Pictoris and other A-type stars

Presented here are the results of observations made at the Mount John Observatory (MJUO) during the spectroscopic campaigns to observe beta Pictoris in 1992, 1993, 1994 and observations conducted in 1995 to characterise the behaviour of the Ca II H and K lines and to test the Falling Evaporating Bodies scenario. Using the method of division by a reference spectrum both narrow and broad variable absorption features in both the redshifted and blue shifted sides of the Ca II H and K lines are clearly detected. The large data set obtained allows the determination of the evolution in terms of velocity, equivalent width, FWHM and timescales of variability of the variable absorption features. These are then compared with the results on Lagrange-Henri et al. (1996) in their paper on the 1992 observing campaign. Lagrange-Henri et al. find that there are 2 velocity regimes and this is confirmed in the MJUO data. The higher the redshift, the smaller the variability timescales and the smaller the absorbing cloud. In contrast the low velocity features tend to be longer lived and to have the deeper absorptions. The correlation between the FWHM and velocity of the features found by Lagrange-Henri et al. is confirmed, but with the larger set of data the correlation is found to be somewhat weaker. Significant activity was seen in each set of observations with long lived absorption features at low velocity almost always being present and it has been found that ¼ of all features observed are most likely due to more than one FEB. The effect of stellar rotation is suggested in the data of some of the strong and more variable absorption features. However conclusive evidence of the changes in equivalent width are not forthcoming. Large numbers of high velocity features are also observed and are seen to vary in timescales no longer than the crossing time for an orbiting body to pass across the stellar disk. This lends further support to the FEB scenario as an explanation for the variable absorption features. The measurement of the filling factors of the clouds of ions indicate that these clouds do in indeed cover large fractions of the stellar disk and some of the lines even exhibit pK/pH less than 1, as predicted. The FEB scenario appears to explain many of the characteristics of the variable absorption features very well, simulations can reproduce many of the absorptions however there are some cases where the FEB scenario fails to adequately explain the observations. The ability for some of the long-lived features to last as long as they are observed would require either large numbers of bodies on similar orbits crossing the line of sight for many weeks, or that there is some other explanation for the origin of the absorptions

Perturbations on steady spherical accretion in Schwarzschild geometry

The stationary background flow in the spherically symmetric infall of a compressible fluid, coupled to the space-time defined by the static Schwarzschild metric, has been subjected to linearized perturbations. The perturbative procedure is based on the continuity condition and it shows that the coupling of the flow with the geometry of space-time brings about greater stability for the flow, to the extent that the amplitude of the perturbation, treated as a standing wave, decays in time, as opposed to the amplitude remaining constant in the Newtonian limit. In qualitative terms this situation simulates the effect of a dissipative mechanism in the classical Bondi accretion flow, defined in the Newtonian construct of space and time. As a result of this approach it becomes impossible to define an acoustic metric for a conserved spherically symmetric flow, described within the framework of Schwarzschild geometry. In keeping with this view, the perturbation, considered separately as a high-frequency travelling wave, also has its amplitude reduced.Comment: 8 pages, no figur

Standing and travelling waves in the shallow-water circular hydraulic jump

A wave equation for a time-dependent perturbation about the steady shallow-water solution emulates the metric an acoustic white hole, even upon the incorporation of nonlinearity in the lowest order. A standing wave in the sub-critical region of the flow is stabilised by viscosity, and the resulting time scale for the amplitude decay helps in providing a scaling argument for the formation of the hydraulic jump. A standing wave in the super-critical region, on the other hand, displays an unstable character, which, although somewhat mitigated by viscosity, needs nonlinear effects to be saturated. A travelling wave moving upstream from the sub-critical region, destabilises the flow in the vicinity of the jump, for which experimental support has been given.Comment: 9 pages, REVTeX, Additional treatment on travelling waves. Extensively revised in the publised version. Contains a full new section on the role of nonlinearit

Implications of nonlinearity for spherically symmetric accretion

We subject the steady solutions of a spherically symmetric accretion flow to a time-dependent radial perturbation. The equation of the perturbation includes nonlinearity up to any arbitrary order, and bears a form that is very similar to the metric equation of an analogue acoustic black hole. Casting the perturbation as a standing wave on subsonic solutions, and maintaining nonlinearity in it up to the second order, we get the time-dependence of the perturbation in the form of a Li\'enard system. A dynamical systems analysis of the Li\'enard system reveals a saddle point in real time, with the implication that instabilities will develop in the accreting system when the perturbation is extended into the nonlinear regime. The instability of initial subsonic states also adversely affects the temporal evolution of the flow towards a final and stable transonic state.Comment: 14 pages, ReVTeX. Substantially revised with respect to the previous version. Three figures and a new section (Sec. VI) adde

Secular instability in quasi-viscous disc accretion

A first-order correction in the $\alpha$-viscosity parameter of Shakura and Sunyaev has been introduced in the standard inviscid and thin accretion disc. A linearised time-dependent perturbative study of the stationary solutions of this "quasi-viscous" disc leads to the development of a secular instability on large spatial scales. This qualitative feature is equally manifest for two different types of perturbative treatment -- a standing wave on subsonic scales, as well as a radially propagating wave. Stability of the flow is restored when viscosity disappears.Comment: 15 pages, 2 figures, AASTeX. Added some new material and upgraded the reference lis