Shadow edge

If the illuminant is constant, we can remove the influence of the illuminant by computing the first derivative. This works if two neighboring pixels have the same illuminant. If two neighboring pixels have a different illuminant, then we probably have a shadow edge between the two pixels. Weiss suggests simply applying a median filter to the first derivatives to eliminate the influence of the illuminant. 

A method has been developed to identify the nodes which will not be immediately approached by the event and can be turned off to save energy [Liu 02], The method is based on the dual space transformation [O R 98], Figure 2 shows the dual space. Points from the primal space are transformed into lines in the dual space. Lines from the primal space are transformed into points in the dual space. As a result, the dual space is partitioned into cells. The e point, the shadow edge, is contained in the shaded cell. Since the e point can not intersect the n2 line, before it crosses one of the cell boundaries, the N2 node can stay turned off as long as none of Nl, N3 and N4 senses a transition. This method may provide a substantial power reduction for a large sensor field. However, if nodes that line the perimeter around the event misbehave and declare a transition, it will force several other nodes to wake up and waste energy. 

Figure 1. Four Nodes Sense the Moving Light Shadow Edge...

When such features exist, they are penetrated by the electron beam so the material is represented by a three-dimensional point lattice and diffraction only occurs when the Ewald sphere intersects a point. This produces a transmission-type spot pattern. For smooth surfaces, the diffraction pattern appears as a set of streaks normal to the shadow edge on the fluorescent screen, due to the interaction of the Ewald sphere with the rods projecting orthogonally to the plane of the two-dimensional reciprocal lattice of the surface. The reciprocal lattice points are drawn out into rods because of the very small beam penetration into the crystal (2—5 atomic layers). We would emphasize, however, that despite contrary statements in the literature, the appearance of a streaked pattern is a necessary but not sufficient condition by which to define an atomically flat surface. Several other factors, such as the size of the crystal surface region over which the primary wave field is coherent and thermal diffuse scattering effects (electron—phonon interactions) can influence the intensity modulation along the streaks. 

Figure Bl.23.2. (a) Shadow cone of a stationary Pt atom in a 4 keV Ne ion beam, appearing with the overlapping of ion trajectories as a fiinction of the impact parameter. The initial position of the target atom that recoils in the collision is indicated by a solid circle, (b) Plot of the nonnalized ion flux distribution density across the shadow cone in (a). The flux density changes from 0 inside the shadow cone, to much greater than l in the focusing region, converging to 1 away from the shadow cone edge, (c) Blocking cones...

Figures 4.1 la and b, respectively, are examples of dark-field and direct transmission electron micrographs of polyethylene crystals. The ability of dark-field imaging to distinguish between features of the object which differ in orientation is apparent in Fig. 4.11a. The effect of shadowing is evident in Fig. 4.11b, where those edges of the crystal which cast the shadows display sharper contrast.

Fig. 7.1. The direction of the orienting field (dot-dashed line) in a fluctuating cage, when the molecule (shadowed circle) is in its centre (a)-(c), or shifted toward the cell edge (d).

Visible edge Schlieren edge Inner shadow cone... 

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