Range data processing: representation of surfaces by edges

Journal ArticleRepresentation of surfaces by edges is an important and integral part of a robust 3-D model based recognition scheme. Edges in a range image describe the intrinsic characteristics about the shape of the objects. In this paper we present three approaches for detecting edges in 3-D range data. The approaches are based on computing the gradient, thresholding, thinning and fitting straight lines or curves; fitting 3-D lines to a set of points, and detecting changes in the direction of unit normal vectors on the surface. These approaches are applied locally in a small neighborhood of a point. The neighbors of a 3-D point are found by using the k-d tree algorithm. As compared to previous work on range processing, the approaches presented here are applicable not only to sensor range data corresponding to any one view of the scene, but also to 3-D model data obtained using the Computer-Aided Geometric Design (CAGD) techniques, and to 3-D model built using the sensor data such as the data obtained by combining several views of an object. We present several examples where the data is synthetically genetrated, obtained from CAGD methods or obtained from a laser scanner. A comparison of the techniques is presnted

Long-Lived Spin Coherence States

We study evolution of electron spin coherence having non-homogeneous direction of spin polarization vector in semiconductor heterostructures. It is found that the electron spin relaxation time due to the D'yakonov-Perel' relaxation mechanism essentially depends on the initial spin polarization distribution. This effect has its origin in the coherent spin precession of electrons diffusing in the same direction. We predict a long spin relaxation time of a novel structure: a spin coherence standing wave and discuss its experimental realization

Hole spin relaxation in intrinsic and pp-type bulk GaAs

We investigate hole spin relaxation in intrinsic and $p$-type bulk GaAs from a fully microscopic kinetic spin Bloch equation approach. In contrast to the previous study on hole spin dynamics, we explicitly include the intraband coherence and the nonpolar hole-optical-phonon interaction, both of which are demonstrated to be of great importance to the hole spin relaxation. The relative contributions of the D'yakonov-Perel' and Elliott-Yafet mechanisms on hole spin relaxation are also analyzed. In our calculation, the screening constant, playing an important role in the hole spin relaxation, is treated with the random phase approximation. In intrinsic GaAs, our result shows good agreement with the experiment data at room temperature, where the hole spin relaxation is demonstrated to be dominated by the Elliott-Yafet mechanism. We also find that the hole spin relaxation strongly depends on the temperature and predict a valley in the density dependence of the hole spin relaxation time at low temperature due to the hole-electron scattering. In $p$-type GaAs, we predict a peak in the spin relaxation time against the hole density at low temperature, which originates from the distinct behaviors of the screening in the degenerate and nondegenerate regimes. The competition between the screening and the momentum exchange during scattering events can also lead to a valley in the density dependence of the hole spin relaxation time in the low density regime. At high temperature, the effect of the screening is suppressed due to the small screening constant. Moreover, we predict a nonmonotonic dependence of the hole spin relaxation time on temperature associated with the screening together with the hole-phonon scattering. Finally, we find that the D'yakonov-Perel' mechanism can markedly contribute to the .... (omitted due to the limit of space)Comment: 11 pages, 7 figures, Phys. Rev. B, in pres

Bound hole states in a ferromagnetic (Ga,Mn)As environment

A numerical technique is developed to solve the Luttinger-Kohn equation for impurity states directly in k-space and is applied to calculate bound hole wave functions in a ferromagnetic (Ga,Mn)As host. The rich properties of the band structure of an arbitrarily strained, ferromagnetic zinc-blende semiconductor yields various features which have direct impact on the detailed shape of a valence band hole bound to an active impurity. The role of strain is discussed on the basis of explicit calculations of bound hole states.Comment: 9 pages, 10 figure

Influence of canal water distribution system on water productivity of selected kharif crops in distributaries of Eastern Yamuna Canal (EYC) command area

Water productivity of selected kharif season crops in the three distributaries of the Eastern Yamuna Canal Command depended on the conjunctive use of canal and ground water. Present investigations were carried out to assess the effect of water distribution on the productivity of the selected crops as well as the conjunctive use practices being followed in absence of the water in Kharif Season. It was found that in the kharif season water availability was insufficient for transplanting and irrigating paddy in the middle and tail reaches of EYC. As a result, farmers switched over from paddy to sorghum crop, thereby loosing Rs. 3075 and Rs. 3715 per ha in the middle and tail distributaries of EYC, respectively. However, the high conveyance losses resulted into lowering of the conveyance efficiencies of the three distributaries in the range of 43-44 per cent. This indicated that there lies ample scope of increasing the water availability by properly checking the seepage and evaporation losses. The balance amount of water required by crops was replenished by the ground water sources resulting into increased pressure on stored water. The water use behavior was similar in three distributaries with varying magnitude. There is a great potential of increasing food production with assured irrigation, which is currently covers about 30% of the total arable land, contributing about 56% of the total food production of the country. Present investigation established the need for increasing the conveyance efficiencies to a great extent which will result into increased culturable commands as well as adoption of better cropping patterns in the EYC command area

Participatory Ranking of Fodders in the Western Hills of Nepal

Fodder is an important source of feed of the ruminants in Nepal. In the mid hills of Nepal, farmers generally practice integrated farming system that combines crop cultivation with livestock husbandry and agroforestry. Tree fodders are good sources of protein during the forage and green grass scarcity periods especially in dry season. Local communities possess indigenous knowledge for the selection of grasses and tree fodders at different seasons in mid hills of western Nepal. A study was conducted on the perception of farmers with respect to selection of fodder species in eight clusters in Kaski and Lumjung districts that range 900-2000 meter above sea level and receive average precipitation of 2000- 4500mm per annum. During the fodder preference ranking, farmers prepared the inventory of fodders found around the villages and nearby forests and selected top ten most important fodders in terms of their availability, palatability, fodder yield, milk yield and milk fat yield. In total, 23 top ranking fodders species were selected from the eight clusters. These fodder species were also ranked using pairwise ranking and weighted scoring methods and ranking was done on the basis of merit numbers obtained from weighted scores. The analysis revealed Artocarpus lakoocha as best tree fodder followed by Ficus semicordata, Thysanolena maxima and Ficus calvata. Similarly, the calendar of fodders trees for lopping season and the best feeding time was prepared on the basis of farmers\u27 local knowledge. This study suggests strategies for promotion of locally preferred tree fodder species and supplementing tree fodder with feed in different seasons depending on their availability and local preferences

Quantitative assessment of reentrainment in the electrocyclone

The paper was devoted to the investigation of the reentrainment which was a parasitic effect incipient at the gas-cleaning systems – cyclones. It was demonstrated that the reentrainment arises at the speed of the aerosol from 14 to 27 m/sec. The quantitative characteristics of the reentrainment were determined.The research project has been supported by Russian Foundation for Basic Research (grant 14–08–00046а)

Spin relaxation in nn-type ZnO quantum wells

We perform an investigation on the spin relaxation for $n$-type ZnO (0001) quantum wells by numerically solving the kinetic spin Bloch equations with all the relevant scattering explicitly included. We show the temperature and electron density dependence of the spin relaxation time under various conditions such as impurity density, well width, and external electric field. We find a peak in the temperature dependence of the spin relaxation time at low impurity density. This peak can survive even at 100 K, much higher than the prediction and measurement value in GaAs. There also exhibits a peak in the electron density dependence at low temperature. These two peaks originate from the nonmonotonic temperature and electron density dependence of the Coulomb scattering. The spin relaxation time can reach the order of nanosecond at low temperature and high impurity density.Comment: 6 pages, 4 figure