Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Planar Visibility: Testing and Counting   In this paper we consider query versions of visibility testing and visibility
counting. Let $S$ be a set of $n$ disjoint line segments in $\R^2$ and let $s$
be an element of $S$. Visibility testing is to preprocess $S$ so that we can
quickly determine if $s$ is visible from a query point $q$. Visibility counting
involves preprocessing $S$ so that one can quickly estimate the number of
segments in $S$ visible from a query point $q$.
  We present several data structures for the two query problems. The structures
build upon a result by O'Rourke and Suri (1984) who showed that the subset,
$V_S(s)$, of $\R^2$ that is weakly visible from a segment $s$ can be
represented as the union of a set, $C_S(s)$, of $O(n^2)$ triangles, even though
the complexity of $V_S(s)$ can be $\Omega(n^4)$. We define a variant of their
covering, give efficient output-sensitive algorithms for computing it, and
prove additional properties needed to obtain approximation bounds. Some of our
bounds rely on a new combinatorial result that relates the number of segments
of $S$ visible from a point $p$ to the number of triangles in $\bigcup_{s\in S}
C_S(s)$ that contain $p$.

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The categories of this paper are: ao-sci
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The categories of this paper are: mtrl-th
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The categories of this paper are: quant-ph
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The categories of this paper are: hep-lat
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The categories of this paper are: nucl-ex
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The categories of this paper are: acc-phys
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The categories of this paper are: hep-th
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The categories of this paper are: patt-sol
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The categories of this paper are: solv-int
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The categories of this paper are: nucl-th
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The categories of this paper are: physics
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The categories of this paper are: supr-con
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The categories of this paper are: dg-ga
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The categories of this paper are: funct-an
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The categories of this paper are: adap-org
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The categories of this paper are: comp-gas