Figure/ground organization is a step of perceptual organization that assigns a contour to one of the two abutting regions. Peterson et al showed that familiar configurations of contours, such as outlines of recognizable objects, provide a powerful cue that can dominate traditional f/g cues such as symmetry. In this work we: (1) provide an operationalization of “familiar configuration” in terms of prototypical local shapes, without requiring global object recognition; (2) show that a classifier based on this cue works well on images of natural scenes. A dataset of 200 natural images was hand segmented into disjoint regions by human subjects. Subjects then provided a f/g label for each contour associated with a pair of abutting segments [Fowlkes, Martin & Malik ECVP03]. Our goal is to correctly predict these f/g labels from image measurements. We use “shape context” to represent local shape configuration at each point. Shape context [Belongie, Malik & Puzicha ICCV01] is a shape descriptor which summarizes local arrangement of edges, relative to the center point, in a log-polar fashion. In order to work with grayscale images, we use a variant of shape context, geometric blur [Berg & Malik CVPR01], aligned to local tangent direction. We cluster a large set of these descriptors to construct a small list of prototypical shape configurations, or “shapemes” (analogous to phonemes). Shapemes capture important local structures such as convexity and parallelism. For each point along a contour, we measure the similarity of its local shape descriptor to each shapeme. These measurements are combined using a logistic regression classifier to predict the f/g label. By averaging the classifier outputs over all points on each contour, we obtain an error rate of 30% (chance is 50%). This compares favorably to the traditional f/g cues used in [Fowlkes et al 03]. Enforcing consistency constraints at junctions reduces the error rate further to 22%, making it a promising model of figure/ground organization.
Fatigue crack growth is examined in P/M 2124 aluminum alloys reinforced with SiC particles (SiCp) and whiskers (SiCw) over a wide spectrum of growth rates from 10−12 to 10−4 m per cycle. Effects of aging treatment, orientation of crack growth direction with respect to the rolling direction, mean stress (or stress ratio), and reinforcement volume percent on the fatigue crack growth threshold are investigated in terms of crack tip shielding mechanisms. Comparison to fatigue crack growth in the unreinforced alloy indicates that crack growth resistance in the composites is superior to the monolithic alloy at low stress intensity ranges, ΔK. Specifically, at the lower growth rates the superior crack growth resistance of the composites is due to the formation of tortuous crack paths and a consequently enhanced roughness induce crack closure. Fatigue response near threshold is found to be relatively insensitive to changes in aging treatment with observed variations reflecting the associated changes in plastic behavior. In whisker reinforced composites, the orientation of the crack with respect to the rolling direction had a significant effect on fatigue crack growth rate while in the particulate reinforced composites orientational effects were not significant. Increasing the volume fraction resulted in higher crack growth resistance at low growth rates in the particulate reinforced materials. At high stress ratios, resistance in both whisker and particulate reinforced composites was lowered, and the measured fatigue crack propagation threshold, ΔK th, was found to be independent of the reinforcement morphology and volume percent.
My forthcoming book Bourdieu in the City: Challenging Urban Theory (Cambridge, Polity Press, 2023) is not intended as an eclectic combination of the structuralist and the phenomenological takes on the city rehearsing Pierre Bourdieu’s influential critique of the deadly antinomy of objectivism and subjectivism. Nor does it aim just to make room for the author of Distinction in the pantheon of theorists before which students of the city are expected to genuflect. I intend the book, not as an addition, but a challenge to the urban canon and a springboard for a possible reconstruction of urban theory and inquiry around what I christen the Bourdieusian trialectic of symbolic space, social space, and physical space. In this paper, I provide a compact characterization of the trialectic and then draw out its implications for the theory and comparative study of the city.
Phosphines are commonly employed as ligands for late-transition-metal-catalyzed reactions. While a plethora of ligands with chirality α to the phosphine are used, the P-stereogenic counterparts are less common. This paper is a very nice report on the synthesis of these compounds, which are much more difficult to obtain. The use of (R)-i-Pr-PHOX as the ligand on the ruthenium center gave the best results. Sodium tert-amyloxide was the optimal base, while a variety of benzylic chlorides were shown to be appropriate electrophiles. The use of ethyl bromide gave lower yield and enantioselectivity (70% yield, 57% ee). The products were isolated as the air-stable borane adducts.
While astonishingly successful, Internet is still less reliable than the phone system and supports very limited user choice and control. As many researchers observed, multipath routing is a promising paradigm to address these issues. In this thesis, we argue that multipath routing can indeed go a long way towards these goals as well as lead to a more scalable, extensible, and evolvable Internet. We begin by describing Yet Another Multipath Routing (YAMR) protocol that provably constructs a set of paths resilient to any one interdomain link failure. YAMR uses an efficient scheme to construct the paths and a novel failure hiding technique to further reduce the control plane overhead. Next, we describe Pathlet Routing, a protocol that departs from the path-vector paradigm. Pathlet routing allows ASes to advertise policy-compliant path segments called pathlets, and allows users to stitch them together, thus forming a complete path suitable for the user's particular needs. Pathlet routing greatly reduces the forwarding table size, can efficiently express a wide class of routing policies, and provide an exponential number of paths to the users. Finally, we investigate how pathlet routing can be a basis for an evolvable Internet architecture.
A two-dimensional array of diffusively-coupled nonlinear electronic circuits, Chua's cells, operating in a bistable regime, is used to find the shortest path connecting two points of a given image. Images are previously stored in the system by modulating the diffusion coefficients. Different types of images were considered, from black and white pictures (where 0 means no propagation and 1 propagation), to analog figures (with many intermediate states) and, in all cases, the algorithm used for such calculations succeeded to find the shortest path. The full description of the algorithm is here described and applied to nontrivial cases where the shortest path strongly differs from the straight line.
