Questions tagged [computational-geometry]
The study of efficient algorithms and data structures to solve various problems involving point sets, line segments, polygons, polyhedra, simplices, etc.
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questions
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The implicit form of a NURBS curve
I am trying to evaluate and analyse a NURBS curve to generate a mechanism. I understand that the general form of a NURBS curve is commonly written as a parametric equation in the form of $f_{par}(t)$.
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4
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1answer
105 views
Computation of the tensor of curvature on surface mesh
Is there a formula which enables the computation the tensor of curvature knowing the following at each vertex and cell of a triangulated mesh:
Normal vector
Two arbitrary vectors in the tangent space
...
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2answers
59 views
Calculating versors of a plane from the normal versor
I'm trying to calculate the 2 perpendicular versors (unit vectors), $\vec{n_1}$ and $\vec{n_2}$, that define a plane whose normal versor (unit vector) is $\vec{n_n}$.
For example, assuming that the ...
2
votes
1answer
70 views
Dividing a continuous domain into small squares; how to perform storage and querying?
I recently had a software engineering interview and was asked a series of questions that was a bit outside of knowledge realm, and I feel like there's some scientific computing principles here (I took ...
2
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0answers
38 views
Extracting a mid-plane for thick shell analysis
I have a complex part that contains features of the form shown in the figure below.
Because of the cost of 3D finite element simulation of the part, I want to try an analysis with 2D thick shells.
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0answers
122 views
Getting euclidean distance between vector A and C without anyway of retrieving them when their distances with a common vector B is known
Motivation:
My plan is to get the overall euclidean distance matrix for all the vectors in N number of dataset. Each dataset is basically an array of n-dimensional points. For e.g: A dataset can be ...
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0answers
52 views
How to determine the orientation of convex/concave hexahedra?
I am writing a code that checks the orientation of a list of vertices (along with face connectivity) describing both convex and concave hexahedra. The face connectivity table stores the list of vertex ...
0
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1answer
112 views
Algorithm to convert STL files to STEP files
My goal is to learn the algorithms that allow to convert STL files to STEP files.
I am struggling to find learning materials.
Can you suggest here research papers, books, open source code about this ...
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vote
2answers
162 views
Dyadic operations, fourth order tensors and Tensor algebra
I am trying to understand the dyadic operation for a while since I am interested in Elasticity problems. I believe an intuitive understanding (rather than assuming) will give me good problem solving ...
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0answers
73 views
How to distinguish primary hosts (stars) and orbiting satellites (planets) and tertiary bodies (moons) by their mass and trajectory?
I posted this question in the astronomy stackexchange. There are no responses, and it was suggested that I pose the question here. The "too long, didn't read" was taken from a comment, and ...
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0answers
85 views
Jacobian Matrix of 2D element mapped to 3D
Note: I previously posted this question to MathStackExchange, but got no attention there. So I'm rewritting and trying over here.
Problem summary
Given a common¹ set of shape functions defined at ...
0
votes
1answer
55 views
Parametric surface in 3D
I want to create a mesh for a 3D surface with coordinates defined by a parametric function. Is it possible to define this mesh using Gmsh? If it is not possible, what free software do you recommend me?...
4
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1answer
112 views
Solving geodesics on triangular meshes gives negative distances
I have implemented the heat method for geodesics:
https://www.cs.cmu.edu/~kmcrane/Projects/HeatMethod/paperCACM.pdf
When I run it I am getting a solution that, visually, seems correct:
In this image, ...
2
votes
0answers
64 views
Why is bounding a surface with a capsule is better than with a cylinder to detect intersections?
In this article:
https://www.geometrictools.com/Documentation/IntersectionOfCylinders.pdf
the writer says: "If you plan on using cylinders
for bounding volumes in a real-time graphics engine—...
1
vote
1answer
91 views
Calculate the arc length of a Steinmetz curve numerically
I'd like to know the length made by the intersection curve of two orthogonal cylinders of different radii a and b where a > b >0.
I came across this post that provides a solution with an ...
3
votes
1answer
77 views
Good rectangular covering of an SDF
I have a 2D SDF describing my shape, but it's fine to think of it as a black/white image (black="inside" white="outside").
I want to generate a small set of rectangles (say, 8 of ...
4
votes
1answer
92 views
Algorithm to merge two polygons (using connectivities)?
I am struggling with implementing an algorithm that does one simple thing:
Consider two polygons (one can just draw any two polygons and number their vertices), whose connectivities in a node list are:...
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vote
0answers
36 views
Largest triangle that contains a point
Given the location of $n$ points on a 2D plane ($P_1, P_2, \ldots, P_n$); and the location of a special point $X$.
Find three points $P_i,P_j,P_k$ ($i \neq j \neq k$) such that point $X$ is inside the ...
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0answers
45 views
Storing and retrieving two-dimensional and three-dimensional data
I work on computational geometry.
A huge number of two-dimensional and three-dimensional data are found in my project. Coordinates of polygon and polyhedrons vertices consisted of two-dimensional and ...
0
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2answers
99 views
Problem of half-planes intersection
Consider the half-planes $\{x \leqslant 2\}$ and $\{x+y \leqslant 3\}$. These two half-planes are coded with the R package 'rcdd' as follows:
...
3
votes
3answers
272 views
How to determine if 2 rays intersect?
We are given the 2D coordinates of 2 points: the first point is where the ray starts and it goes through the second point. We are given another ray in the same way. How do we determine if they have a ...
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0answers
17 views
Minimal covering of rectangle with fixed, overlapping rectangles
A finite set $R$ of fixed, axis aligned 2-D rectangles $r_i=\left\{x_{0i},y_{0i},W_i,H_i \right\}$ is given. These rectangles are potentially overlapping. Given a new axis aligned rectangle $t$, I ...
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37 views
Hi I am trying to model a 2D Lug angle using Gmsh 4.6. How can I combine transfinite quad and regular full quad meshes in the following geo file?
I need transfinite mesh a small section of the bolt hole to insert a crack. However, The transfinite mesh and regular full quad mesh seem being incompatible and throwing errors. How can I combine ...
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0answers
48 views
Producing Voronoi diagram in three dimensional
A Voronoi diagram is a kind of tesselation that divided the medium into polygons in 2D and polyhedrons in 3D.
Although there are many algorithms to construct a Voronoi diagram, some of them are faster ...
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0answers
123 views
Fortune algorithm for voronoi diagram
Although there are many algorithms to construct Voronoi diagram, some of them are faster than others. Based on my knowledge Fortune algorithm is fastest for construct Voronoi diagram either in two ...
3
votes
3answers
235 views
What are some algorithms to calculate the width of an arbitrary polygon when a bounding box approximation is inaccurate
What are some alternative algorithms to creating a bounding box for finding the max width of a concave, simple winding polygon, like the one in the below image? I prefer solutions that are more ...
2
votes
3answers
209 views
For traditional FEM and FVM, why can't we use mesh to represent geometry and use the mesh which represent the geometry to do the computation directly?
Isogeometric analysis [1] has the advantage of integrating geometric and mesh models using NURBS or Spline. At the same time, I would like to ask a question to my friends: for traditional FEM and FVM, ...
4
votes
4answers
204 views
Can the mesh generation methods in FVM and FEM be totally based on the knowledge of the mesh generation theory in computer graphics?
The main references of mesh generation methods in computer graphics (CG) I found are discrete Differential Geometry [1] and a famous book "Polygon Mesh Processing" [2], while the "...
2
votes
1answer
116 views
Project to nearest point on convex polyhedron
I have a point $y \in \mathbb{R}^d$ and a convex polyhedron $\mathcal{P}$ given as the intersection of half-spaces:
$$\mathcal{P} = \{x \in \mathbb{R}^d \mid a_1 \cdot x \le b_1, \dots, a_n \cdot x \...
0
votes
0answers
62 views
Surface mesh from labeled 3D points
I'm trying to figure out how to create a surface mesh from a set of labeled 3D points. The 3D object could be something like part of a cave system or asteroid where there would be parts of the surface ...
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0answers
268 views
Finding points inside cells of power (generalized Voronoi) diagram
Suppose we have a set of points $p_1,\ldots,p_n\in\mathbb R^d$ as well as a set of weights $w_1,\ldots,w_n\in\mathbb R$. Recall that the power cell associated to the pair $(p_k,w_k)$ is given by:
$$\...
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2answers
103 views
How to minimize $(x-a)^2+(y-b)^2$ subject to $ \sqrt{a}+\sqrt{b}=\sqrt{2}$?
I am not sure if this is on-topic here, but I am trying.
Let $x,y$ be positive real numbers. I am trying to find
$$ \min_{\sqrt{a}+\sqrt{b}=\sqrt{2}}(x-a)^2+(y-b)^2$$
I tried using Mathematica for ...
0
votes
1answer
98 views
Applying weak form
I have two dimensional equation and I want to solve it using Finite Element Methods.
$$ \nabla . (\alpha(x,y)\nabla u(x,y)) + \dfrac{\partial u(x,y)}{\partial x}+\dfrac{\partial u(x,y)}{\partial y}+u(...
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votes
0answers
61 views
Why does the naive barycentric hodgestar fail?
The discrete exterior calculus is defined first using circumcentric dual cells, because the primal and dual edges are orthogonal and thus the dual cells are convex. This leads to a diagonal hodge star ...
0
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1answer
783 views
Desmos saying there are too many variables [closed]
I wasn't sure if this is a Computational Science SE question or a Stack Overflow question but I think it's more of this one.
I wanted to make a graph where I can rotate a hyperbola (not parabola or ...
1
vote
1answer
123 views
How do I find the portion of a cell/voxel lying within a defined surface?
We have a 3-dimensional grid of voxels (or cells), with individual voxels being of volume $dx\,dy\,dz$ where $dx=dy=dz=1$.
A cone-like surface is defined by some function, $z = f(x, y)$, which in ...
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votes
1answer
114 views
Detecting degenerate triangles with very thin structures
Between the two ears in the following bunny images, there are some degenerate triangles I want to detect. It looks like a volume-less thin slits.
If the question is not clear, please let me know.
0
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1answer
58 views
Systematically outputting sign vectors of restricted hyperplane arrangement
(I previously asked this question on Sage's dedicated Q&A site, but got no response, so I figured it would be worth trying here.)
I have a way of constructing hyperplane arrangements in Sage, ...
2
votes
2answers
1k views
How to determine the Jacobian Ratio for triangle element?
I am trying to implement an algorithm to find the Jacobian ratio for each triangle in mesh as a part of mesh quality check.
Let's say that I have vertices of the triangle: $P_1(x_1, y_1, z_1)$, $P_2(...
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vote
0answers
27 views
Simplification of vertices and dihedral angle relations of a polygonal chain
I am trying to understand the generation of Cartesian coordinates of polygonal system or poly line with fixed bond angles and fixed link lengths. I assumed the bond angle to be the same and link ...
3
votes
2answers
92 views
Packing spheres inside a geometry
I am looking for packing spheres (can be monodisperse or polydisperse with known radii distribtuions) inside a geometry. I am sure this is a well explored scientific problem with applications in ...
2
votes
2answers
115 views
Is the similar subdivision of a delaunay mesh still delaunay?
I have a delaunay triangulation for a 2d box with say an airfoil inside. If I uniformly refine this mesh by subdividing each triangle in the mesh into 4 triangles by halving each edge, is the ...
2
votes
1answer
88 views
How to divide points on a 3D complex surface into two regions based on a closed curve defined on this surface?
My problem seems simple but I can't find an algorithm that will do that for me for any 3D complex surface.
I have a really complex shape 3D surface and a closed curve on it defined by some points (...
0
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1answer
40 views
Produce vertex displacements from volumetric shrinkage data on unstructured meshes
I was wondering what would be an efficient way to produce compatible displacements for mesh nodes/vertices if the computed data is volume shrinkage of each element/cell in the unstructured mesh?
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0answers
72 views
How to approach geographic data interpolation by distance?
let's say I have a set of geographic locations (lat, lng) resulting from a query. Those locations have some kind of internal ranking, my set is sorted by this number in a descending order.
Now I'm ...
0
votes
1answer
46 views
How to find the nearest point inside a list in a given direction
Being $\bar{\mathbf{x}} \in \mathbb{R}^3$ a point and $S =\{\mathbf{x}\}_{i=1}^N \in \mathbb{R}^3$ a sample of N points. I am looking for a simple algorithm to determine the nearest point in $S$ in ...
5
votes
1answer
164 views
Maximum and Minimum distance from query point within bounding box
I'm reading an article regarding approximating sums using KD-trees (similar to FMM).
As part of the effort I'm trying to make sense of this article , which is cited.
I'm having trouble understanding ...
3
votes
2answers
138 views
Find shortest path around a cylinder represented by 3d triangular mesh
Suppose I have a 3d triangular mesh with the topology of a finite cylinder. Let $C$ be a vertex on that mesh.
How can I find the shortest path from $C$ to itself that goes around the cylinder? By ...
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vote
0answers
84 views
Derivatives over a Finite Element mesh
I have a data extracted from Comsol on some node points and I know the coordinates of each node.
Does anyone know how Comsol calculate the partial derivative from the values at each node and also ...
0
votes
0answers
81 views
Dividing Point Cloud into voxels
I am reading a paper on implementing convolution neural network for a 3D point cloud. In this paper, they are dividing the point cloud into voxels. Is there any easy way to do it using point cloud ...