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Communications of the ACM

121 - 130 of 176 for bentley

Motion planning amidst fat obstacles (extended abstract)

We present an efficient and simple paradigm for motion planning amidst fat obstacles. The paradigm fits in the cell decomposition approach to motion planning and exploits workspace properties that follow from the fatness of the obstacles. These properties allow us to decompose the workspace, subject to some constraints, rather than to decompose the higher-dimensional free space directly. A sequence of uniform steps transforms the workspace decomposition into a free space decomposition of asymptotically the same (expectedly small) size. The approach applies to robots with any fixed number of degrees of freedom and turns out to be successful in many cases: it leads to nearly optimal O(nlogn) algorithms for motion planning in 2D, and for motion planning in 3D amidst obstacles of comparable size. In addition, we obtain algorithms for planning 3D motions among polyhedral obstacles, running in O(n2logn) time, and among arbitrary obstacles, running in time O(n3).


Template-driven interfaces for numerical subroutines

This paper describes a set of interfaces for numerical subroutines. Typing a short (often one-line) description allows one to solve problems in application domains including least-squares data fitting, differential equations, minimization, root finding, and integration. Our approach of “template-driven programming” makes it easy to build such an interface: a simple one takes a few hours to construct, while a few days suffice to build the most complex program we describe.


K-d trees for semidynamic point sets

A K-d tree represents a set of N points in K-dimensional space. Operations on a semidynamic tree may delete and undelete points, but may not insert new points. This paper shows that several operations that require &Ogr;(log N) expected time in general K-d trees may be performed in constant expected time in semidynamic trees. These operations include deletion, undeletion, nearest neighbor searching, and fixed-radius near neighbor searching (the running times of the first two are proved, while the last two are supported by experiments and heuristic arguments). Other new techniques can also be applied to general K-d trees: simple sampling reduces the time to build a tree from &Ogr;(KN log N) to &Ogr;(KN + N log N), and more advanced sampling builds a robust tree in the same time. The methods are straightforward to implement, and lead to a data structure that is significantly faster and less vulnerable to pathological inputs than ordinary K-d trees.


A ray tracing solution for diffuse interreflection

An efficient ray tracing method is presented for calculating interreflections between surfaces with both diffuse and specular components. A Monte Carlo technique computes the indirect contributions to illuminance at locations chosen by the rendering process. The indirect illuminance values are averaged over surfaces and used in place of a constant "ambient" term. Illuminance calculations are made only for those areas participating in the selected view, and the results are stored so that subsequent views can reuse common values. The density of the calculation is adjusted to maintain a constant accuracy, permitting less populated portions of the scene to be computed quickly. Successive reflections use proportionally fewer samples, which speeds the process and provides a natural limit to recursion. The technique can also model diffuse transmission and illumination from large area sources, such as the sky.