Questions tagged [computational-geometry]
Using computers to solve geometric problems. Questions with this tag should typically have at least one other tag indicating what sort of geometry is involved, such as ag.algebraic-geometry or mg.metric-geometry.
449
questions
0
votes
0
answers
34
views
Cutting convex polygons into affine-equivalent quadrilaterals
We continue from Tiling the plane with quadrilaterals that are mutually non-congruent and affine equivalent
Question: Can any convex polygon $C$ be partitioned into some finite number $m$ of ...
1
vote
0
answers
53
views
Covering a unit square with odd number of equal area triangles - optimally
We add a bit to this post: Cutting off odd numbers of equal area triangles from a unit square
Question: Given an odd integer n, how does one cover the unit square completely with n equal area ...
0
votes
0
answers
18
views
Covering triangles with mutually congruent planar regions - optimally
We continue from this old post: From a given triangle, to cut 2 mutually congruent convex pieces that together 'use' maximum area of the triangle and go from partitioning to covering.
Question:...
3
votes
2
answers
192
views
Writing a smooth plane quartic as the vanishing of $Q_0Q_2 - Q_1^2$ for quadratic $Q_0,Q_1,Q_2$
It seems well-known that any smooth plane quartic can be written as the vanishing of $Q_0Q_2 -Q_1^2$. Is there a good way to work out these quadratic factors $Q_0,Q_1,Q_2$? For example, given the ...
3
votes
0
answers
71
views
Fast numerical integration of $\int_{[0,\:1)^d}\left|f_x(y)-g(y)\right|^p\:{\rm d}y$ for varying $x\in[0,1)^d$
Let $k\in\mathbb N$ and $y_1,\ldots,y_k\in[0,1)^d$ with $$\frac1k\sum_{i=1}^kh(y_i)\approx\int_{[0,\:1)^d}h(y)\:{\rm d}y\tag1$$ for every nice enough function $h:[0,1)^d\to\mathbb R$.
Now let $p\ge1$, ...
1
vote
0
answers
61
views
Counting Voronoi cells generated by lattice points
I am working on a problem in dynamical systems where I need to count Voronoi cells arising from nearest neighbours to a subset of the lattice. (See the picture below for an example: the shaded region ...
3
votes
1
answer
163
views
Resultants and elimination theory
Consider an ideal $I = \langle f_1,\dotsc,f_n\rangle$ in the ring $k[x_1,\dotsc,x_m]$.
Define the $i$-th elimination ideal to be $I_i = I \cap k[x_{i+1},\dotsc,x_m]$.
For any two polynomials $f$ and $...
2
votes
0
answers
95
views
Checking existence of a non-crossing Hamiltonian path in geometric graphs
I am interested in the following computational problem. Given a geometric graph (i.e, a graph drawn in the plane so that its vertices are represented by points in general position and its edges are ...
2
votes
0
answers
54
views
Biconvex Lens - an 'oriented' convex container for planar point sets
We continue On some optimal containers of a set of points on the 2D plane.
Let us define a biconvex lens as the intersection of two circular disks - not necessarily of the same radii. Such a figure ...
0
votes
1
answer
96
views
How many samples do you need to get constant dispersion?
Let $C_n$ be the hypercube $[-1,1]^n$. For $a_1,\cdots,a_s \in C_n$, define its dispersion $D(a_1,\cdots,a_s)$ as $\max_{x \in C_n}\min_{i \in [s]} \|x-a_i\|_{2}$. Let $0< \lambda < 1$ be a ...
16
votes
1
answer
525
views
Aperiodic monotile in $\mathbb{R}$
Motivation. Recently a group of researchers found an aperiodic monotile in $\mathbb{R}^2$, answering a long-standing question. There are many results in higher dimensions, so let's explore the lower ...
1
vote
0
answers
57
views
Inside-out dissections of polygons - a generalization
Definitions (Inside-out polygonal dissections): a polygon P has an inside out dissection into P' if P′ is congruent to P and the perimeter of P becomes interior to P' and so the perimeter of P' is ...
2
votes
1
answer
158
views
Dispersion of a "random" subset of $[-1,1]^2$
Let $C$ be the square $[-1,1]^2$. Let $a_1,\dots,a_m$ be points chosen independently and uniformly at random from $C$. Let $d_m$ (dispersion) be the random variable $\max_{x \in C}{\min_{j \in [m]}{\|...
2
votes
1
answer
43
views
Example of worst case distributions for 4D convex hull
My understanding is that convex hull of n points in 4D could have O(n²) edges in the worst case. Source: https://sites.cs.ucsb.edu/~suri/cs235/ConvexHull.pdf
This same source writes
In 4D, there are ...
5
votes
3
answers
506
views
If a polyhedron in $\mathbb{R}^3$ has local intersections, does it also have more global intersections?
Consider a simplicial complex $K$. A piecewise linear map $f: K \to \mathbb{R}^n$ is an almost-embedding if $f(\sigma) \cap f(\tau) = \emptyset$ for any two disjoint simplices $\sigma,\tau$ in $K$.
...
1
vote
1
answer
106
views
Smallest trapeziums containing a given convex n-gon
Question: Given a planar convex $n$-gon $C$, to find the smallest area / smallest perimeter trapezium (trapezoid) - a convex quadrilateral with at least one pair of mutually parallel edges - that ...
3
votes
0
answers
62
views
Cutting triangles into triangles with equal longest side
This post elaborates on a specific instance of Cutting convex polygons into triangles of same diameter .
Question: For any integer n, can any triangle be cut into n non-degenerate triangles all of ...
2
votes
1
answer
107
views
Do there exist smaller simplicial models of barycentric subdivisions?
Let $S$ be a simplicial complex and let $Bary(S)$ denote its barycentric subdivision.
Of course, the geometric realizations of $S$ and $Bary(S)$ are homeomorphic.
However, one issue that arises in ...
0
votes
0
answers
25
views
What is the most dense sample for which the Crust algorithm returns an incorrect polygonal reconstruction?
The Crust algorithm by Amenta, Bern, and Eppstein computes a polygonal reconstruction of a smooth curve $C$ without boundary from a discrete set of sample points $S$. It is known that if $S$ is an a $\...
-2
votes
1
answer
190
views
Are there any non-elementary functions that are computable?
Does a function $\mathit{f}:\mathbb{R}→\mathbb{R}$ being non-elementary (not expressible as a combination of finitely many elementary operations), imply that it is not computable?
The particular case ...
5
votes
1
answer
240
views
Counting points above lines
Consider a set $P$ of $N$ points in the unit square and a set $L$ of $N$ non-vertical lines. Can we count the number of pairs $$\{(p,\ell)\in P\times L: p\; \text{lies above}\; \ell\}$$ in time $\...
5
votes
0
answers
156
views
Computing sums with linear conditions quickly
Let $f:\{1,\dotsc,N\}\to \mathbb{C}$, $\beta:\{1,\dotsc,N\}\to [0,1]$ be given by tables (or, what is basically the same, assume their values can be computed in constant time). For $0\leq \gamma_0\leq ...
1
vote
0
answers
57
views
Explicit form of boundary operators of topological cones
Let $\Omega$ be a regular, finite, $n$-dimensional CW complex with chain modules $\mathscr{C}_k$ and boundary operators $\partial_k$.
For many problems in computational geometry, a key operation is to ...
3
votes
1
answer
97
views
Constrained morphing of polygons
This post continues 'Constrained morphing' of planar convex regions
If an $m$-gon $P_m$ is to be morphed (altered continuously) into an $n$-gon $P_n$ with same area and perimeter, can one ...
2
votes
1
answer
77
views
'Constrained morphing' of planar convex regions
Morphing may be defined as a continuous transition of one shape to another. This post is about modifying planar regions continuously from one form to another under some constraints.
Qn: If $C_1$ and $...
1
vote
0
answers
31
views
Computational hardness of a discrete generalized rectangle packing problem
I have a decision problem that is clearly in NP, but I cannot seem to prove that it is in P, nor can I prove its NP-hardness. I attribute this more to my inexperience than to the problem's difficulty (...
11
votes
1
answer
324
views
Tarski-Seidenberg for strict inequalities and bounded quantification
This theorem says that quantifiers over real variables can be eliminated from classical first order formulae built from equations and inequalities between polynomials with rational coefficients, ie in ...
3
votes
0
answers
104
views
Hemisphere containing the maximum number of points scattered on a sphere
Consider a set of points $x_1, \ldots,x_n$ on $\mathbb{S}^{k-1}$ (the unit sphere in $\mathbb{R}^k$). The goal is finding the hemisphere which contains the maximum number of $x_i$'s. Basically, we ...
11
votes
2
answers
575
views
A quadratic $O(N)$ invariant equation for 4-index tensors
Consider an $O(N)$ invariant quadratic equation
$$
T_{ijkl}= T_{ijmn}T_{klmn}+ T_{ikmn}T_{jlmn}+ T_{ilmn}T_{jkmn},
$$
where $T_{ijkl}$ is a real, totally symmetric 4-tensor, and the indices run from 1 ...
3
votes
1
answer
94
views
Results in computational geometry utilizing doubling dimension of a metric space
According to Wikipedia,
However, many results from classical harmonic analysis and computational geometry extend to the setting of metric spaces with doubling measures.
My question is: what are some ...
0
votes
0
answers
81
views
Upper bound on the diameter of a convex lattice n-gon with a given area
Given the area $A$ of a strictly convex polygon with $n$ vertices with integer Cartesian coordinates, there are usually several non-equivalent polygons. The relationship between the area, the number ...
1
vote
0
answers
44
views
Triangulation of polygons with all triangles having a common angle
Following Partition of polygons into 'strongly acute' and 'strongly obtuse' triangles, we record another triangulation question.
Question: Given an n-vertex polygonal region ("n-...
6
votes
2
answers
188
views
Partition of polygons into 'strongly acute' and 'strongly obtuse' triangles
Definition: Let us refer to obtuse triangles with the largest angle strictly above a given cutoff value as 'strongly obtuse' - the definition is parametrized by the cutoff value. Likewise, strongly ...
0
votes
0
answers
69
views
Boolean operation on n dimensional polyhedron
A polyhedron in $R^n$ is defined by a set of half-planes: $P = \{x \in R^n \mid Ax - b \le 0\}$.
Given a set of polyhedra in $R^n$, $ P_1, P_2, \dotsc, P_k$, is there an algorithm/implementation that ...
1
vote
0
answers
68
views
Translate of a geodesic that goes through a fixed point on $\mathbb{H}$
Consider the complex upper half plane $\mathbb{H}$ with the hyperbolic geometry. Fix a point $z \in \mathbb{H}$ and also a geodesic $c$. I want to find a hyperbolic translation $\gamma c$ passes that ...
1
vote
0
answers
41
views
Dissection of polygons that preserve boundaries
Ref:
Inside-out polygonal dissections
Further queries on inside-out polygonal dissections
Question: Consider any two polygons P1 and P2 with equal area and equal perimeter. Is it always possible to ...
2
votes
0
answers
150
views
Is orthogonal polygon with crossings count NP-complete?
The are several NP-complete problems related to the construction of orthogonal simple polygons. Rapport showed that it is NP-complete to decide the existence of orthogonal simple polygon that passes ...
1
vote
0
answers
29
views
Partitioning convex n-gons into least number of equal area convex quadrilaterals
This post adds a bit to Partitioning convex polygons into quadrilaterals of equal area and perimeter
Question: How does one achieve the partition of any given convex n-gon into the least number of ...
2
votes
0
answers
56
views
The fastest way to sample points on an implicit manifold, or projecting points on a manifold
Given a compact manifold $M$ in $R^n$, $M = f(x)$, f(x) is infinitely differentiable. $x$ $\in$ $R^n$, I want to find a bunch of samples on the manifold.
Currently, I'm setting up an SQP optimization ...
2
votes
0
answers
80
views
Ellipse of least perimeter that contains a given triangle
This post is related to Smallest 3-ellipses that contain triangles and tries to clarify a basic issue.
Question: Given a general triangle T, How does one find and characterize the ellipse of least ...
11
votes
1
answer
317
views
Complexity of counting regions in hyperplane arrangements
Let $H_1,\ldots,H_n$ be hyperplanes in $\Bbb R^d$. Denote $\mathcal{H} :=\{H_1,\ldots,H_n\}$ and let $c(\mathcal{H})$ be the number of regions in the complement: $\Bbb R^d\setminus \bigcup H_i$.
...
1
vote
0
answers
36
views
What are some other methods for partitioning an n-dimensional space based on a set of points in that space?
So this is a very general question, but I'm curious if there are any other methods for partitioning an n-dimensional space based on the location of a set of points, either randomly chosen or specified,...
1
vote
0
answers
32
views
Efficient solution to linear matrix equations
A general form for a linear matrix equation can be written as
$$AX + XB + \sum C_iXD_i$$
If $C_i$ and $D_i$ are all 0, then this simplifies into a well known and studied matrix equation that has an ...
1
vote
0
answers
89
views
Computer algebra tools for finding real dimension of an algebraic variety
I have a system of polynomial equations with the unknowns being real numbers. The set of solutions is infinite. What software can I use to compute the real dimension of the solution set?
The CAD-based ...
2
votes
1
answer
71
views
Determining if a polygon is convex using relations on orientation of each ordered triple of points
I am reading a paper by Szekeres and Peters on computing the 17-point case of the Erdős–Szekeres conjecture. The conjecture states that the minimum number of points in the plane (in general position, ...
0
votes
1
answer
116
views
Reference request: How to construct a diffeomorphism between point clouds
I'm interested in the following question:
Given two sets $S = \{x_1, ..., x_N\}$ and $T = \{y_1, ..., y_N\}$ each consisting of $N$ distinct points in $\mathbb{R}^n$, how can we construct a ...
2
votes
0
answers
44
views
Cylinder orientation representation
I'm trying to find an efficient computation and representation for the following problem.
Given a cylinder with height $h$ and radius $r$ with a given position $\mathbf{x} = [x, y, z]$ and $N$ number ...
1
vote
0
answers
33
views
Fermat point amidst polygonal obstacles
Consider $k$ distinct points in 2D-plane with $n$ convex polygonal obstacles. Is there a poly-time algorithm (poly in $k$ and the total number of obstacle vertices) to find a point outside of all ...
3
votes
0
answers
207
views
Explicit computations of finite covers of genus one curves with two points of ramification
I have an explicit genus one curve $E$ with two points $p_1$ and $p_2$ on it and am looking for an explicit degree seven cover $X\to E$ with ramification precisely over $p_i$, with a single preimage ...
1
vote
0
answers
51
views
Are cells of 4-polytopes a convex polyhedron by definition?
I'm going by the Wikipedia definition for a 4-polytope.
Do by definition, cells of 4-polytopes have to be a convex polyhedra?
If not, then are there polyhedra with non-convex faces?
If yes, is it the ...