Abstract
The following geometrical proximity concepts are discussed: relative closeness and geographic closeness. Consider a setV={v 1,v 2, ...,v n } of distinct points in atwo-dimensional space. The pointv j is said to be arelative neighbour ofv i ifd p (v i ,v j )≤max{d p (v j ,v k ),d p (v j ,v k )} for allv k ∈V, whered p denotes the distance in theL p metric, 1≤p≤∞. After dividing the space around the pointv i into eight sectors (regions) of equal size, a closest point tov i in some region is called anoctant (region, orgeographic) neighbour ofv i. For anyL p metric, a relative neighbour ofv i is always an octant neighbour in some region atv i. This gives a direct method for computing all relative neighbours, i.e. for establishing therelative neighbourhood graph ofV. For every pointv i ofV, first search for the octant neighbours ofv i in each region, and then for each octant neighbourv j found check whether the pointv j is also a relative neighbour ofv i. In theL p metric, 1<p<∞, the total number of octant neighbours is shown to be θ(n) for any set ofn points; hence, even a straightforward implementation of the above method runs in θn 2) time. In theL 1 andL ∞ metrics the method can be refined to a θ(n logn+m) algorithm, wherem is the number of relative neighbours in the output,n-1≤m≤n(n-1). TheL 1 (L ∞) algorithm is optimal within a constant factor.
Zusammenfassung
Folgende Konzepte für geometrische Nähe werden diskutiert: relative Nähe und geographische Nähe. SeiV={v 1, ...,v n } eine Menge von Punkten eines zweidimensionalen Raumes. Ein Punktv j heißt relativer Nachbar vonv i, fallsd p (v i ,v j )≤max {d p (v i ,v k ),d p (v j ,v k )} für allev k∈V, wobeid p den Abstand in derL p-Metrik bezeichnet, 1≤p≤∞. Nach Unterteilung des Raumes umv i in acht Sektoren (Bereiche) gleicher Größe heißt ein nächster Punkt vonv i in einem Bereich ein Oktan-(Bereichs-oder geographischer) Nachbar vonv i. Für jedeL p-Metrik ist ein relativer Nachbar vonv i auch immer ein Oktant-Nachbar in irgendeinem Bereich umv i. Dies leifert eine direkte Methode zur Berechnung aller relativen Nachbarn, d.h. zur Erstellung der relativen Nachbarschaftsgraphen vonV. Für jeden Punktv i vonV werden zuerst die Oktant-Nachbarn vonv j in jedem Bereich gesucht, dann wird für jeden gefundenen Oktant-Nachbarnv j geprüft, obv j auch ein relativer Nachbar vonv i ist. Er wird gezeigt, daß in derL p-Metrik, 1<p<∞, die Gesamtanzahl der Oktant-Nachbarn θ (n) ist für jede Menge mitn Punkten; daher läuft auch eine nicht optimierte Implementierung der oben beschriebenen Methode in θ(n 2) Zeit. In derL 1 undL ∞ Metrik kann die Methode zu einem θ (n logn+m) Algorithmus verfeinert werden, wobeim die Anzahl der relativen Nachbarn des Ergebnisses ist,n-1≤m≤n(n-1). DerL 1 (L ∞)-Algorithmus ist optimal bis auf einen konstanten Faktor.
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Katajainen, J. The region approach for computing relative neighbourhood graphs in the Lp metric. Computing 40, 147–161 (1988). https://doi.org/10.1007/BF02247943
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DOI: https://doi.org/10.1007/BF02247943