Intersection Queries for Flat Semi-Algebraic Objects in Three Dimensions and Related Problems

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Let \(\mathcal{T}\) be a set of \(n\) flat (planar) semi-algebraic regions in \(\mathbb{R}^{3}\) of constant complexity (e.g., triangles, disks), which we call plates . We wish to preprocess \(\mathcal{T}\) into a data structure so that for a query object \(\gamma\) , which is also a plate, we can quickly answer various intersection queries , such as detecting whether \(\gamma\) intersects any plate of \(\mathcal{T}\) , reporting all the plates intersected by \(\gamma\) , or counting them. We also consider two simpler cases of this general setting: (i) the input objects are plates and the query objects are constant-degree parametrized algebraic arcs in \(\mathbb{R}^{3}\) ( arcs , for short), or (ii) the input objects are arcs and the query objects are plates in \(\mathbb{R}^{3}\) . Besides being interesting in their own right, the data structures for these two special cases form the building blocks for handling the general case. By combining the polynomial-partitioning technique with additional tools from real algebraic geometry, we present many different data structures for intersection queries, which also provide trade-offs between their size and query time. For example, if \(\mathcal{T}\) is a set of plates and the query objects are algebraic arcs, we obtain a data structure that uses \(O^{*}(n^{4/3})\) storage (where the \(O^{*}(\cdot)\) notation hides factors of the form \(n^{\varepsilon}\) , for an arbitrarily small \(\varepsilon>0\) ) and answers an arc-intersection query in \(O^{*}(n^{2/3})\) time. This result is significant since the exponents do not depend on the specific shape of the input and query objects. We generalize and slightly improve this result: for a parameter \(s\in[n^{4/3},n^{t_{q}}]\) , where \({t_{q}}\geq 3\) is the number of real parameters needed to specify a query arc, the query time can be decreased to \(O^{*}((n/s^{1/{t_{q}}})^{\tfrac{2/3}{1-1/{t_{q}}}})\) by increasing the storage to \(O^{*}(s)\) . Our approach can be extended to many additional intersection-searching problems in three dimensions, even when the input or query objects are not flat.