# Tag Info

## Hot answers tagged polygons

5

There is quite a bit of work on this important problem. Some of the most insightful work is by Helmut Alt and collaborators. He wrote a survey in 2009: Helmut Alt. "The computational geometry of comparing shapes." Efficient Algorithms. Springer Berlin Heidelberg, 2009. 235-248. (Springer link.)           Image from Helmut Alt ...

3

The problem you describe is known as the red-blue intersection problem. Here, we have a red set of $n_1$ segments and a blue set of $n_2$ segments and we know that there are only intersections between segments of different colour. Although this special case has been extensively studied, the $O(n\log n + k)$ algorithm, with $n=n_1+n_2$ is still optimal1 when ...

3

Call the center point $(c_x, c_y)$. The function $d(p_x,p_y) = \sqrt{(p_x-c_x)^2+(p_y-c_y)^2}$ computes the distance from the point $(p_x,p_y)$ to the center of the circle. For each edge of the polygon create its line equation from its two endpoints. If the two endpoints are $(a,b)$ and $(c,d)$ then this line is $y = (\frac{d-b}{c-a})\cdot(x-a) + b$. Solve ...

3

You have a 2D convex polygon $G$, and a 3D polyhedron $H$. Let $P_G$ denote the plane that the polygon is contained in. The following should work: Their intersection is a 2D polygon. You can find the edges of the intersection as follows: For each triangle of $H$: If the triangle intersects $G$, output this intersection. This procedure outputs the ...

3

After conducting more research I did find a solution, but first I will examine solutions suggested by posters and considered by myself and review why they didn't work. This problem reduces to finding all chordless cycles in a planar graph. This was one of my first thoughts early on, but this doesn't work when you consider the following: 5-------f----...

2

Run Flood Fill from any point and the farthest points in the both directions are your result. If you find that the distance in one of the directions is zero it means that it was one of them. Exploiting the very same idea, if you try finding pixels with the least number of surrounding pixels and apply limited BFS Flood Fill to find out where it can go - at ...

2

One reasonable approach is to use RANSAC to find a homography that causes many points to be aligned (or approximately aligned). You'd apply this procedure to align the set of vertices of the first polygon with the set of vertices of the second polygon. The homography captures translation, rotation, and scaling, so it allows correcting for all of them. ...

2

In the paper you reference, Theorem 5 is a claim about simple polygons, where Theorem 6 deals with convex polygons, which are a special kind of simple polygons. The 'As we have seen before, ...' refers to this line: 'As we have seen before, each such test can be done in $O(k)$ time.$^8$', where $8$ references another paper. However, even though I do not ...

2

Yes, there are more algorithms to do so in $\mathcal O(n)$. The first is dated back to ElGindy and Avis in 1981, Lee 1983 and Joe & Simpson in 1985. The visibility algorithms use stack (the first one three stacks, further only one) and process vertices in order they appear at boundary. The visibility algorithms described in book Art Gallery Theorems and ...

2

Suppose $P = \lbrace x: Ax \leq b \rbrace$ is the polyhedra, where $x \in \mathbb{R}^n$. Further assume $P$ is full dimensional, then let $x_0$ be some interior point in P. Let $c \in \mathbb{R}^n$ (and $c \neq 0$) be the objective function you are interested to minimize (say). Then since $x_0 \in int(P)$, there exists a ball $B$ centered at $x_0$ with ...

2

I would use a winding number algorithm. There are a few, but the fastest goes like this:Imagine a line from your point along the positive x-axis. Now, for every edge of your polygon, determine if it crosses this line. if it crosses the line from below to above, then increment the winding number (which is initially zero), if it crosses going from above to ...

1

Here is one approach to identify all rotational symmetries of the graph: Pick any three vertices, $v_1,\dots,v_3$. Loop over all possible combinations of three vertices $w_1,\dots,w_3$. For each such combination, find the unique rotation that maps $v_1 \mapsto w_1,\dots,v_3 \mapsto w_3$, then check whether this is a rotational symmetry of the entire ...

1

It is not too hard. connect the point to the end-point of an edge. If none of these two segments has an intersection with convex-hull, it means you can see that edge completely (as you consider a convex polygon). To find the intersection, you can use a binary search and find that is there any intersection or not.

1

First, assume we already have a list $L$ with all segments in $P$ that are at least partially visible from $p$, ordered by the smallest angle $\theta$ such that a segment is visible from $p$ at angle $\theta$. (This can be done with a (angular) sweep line algorithm in $O(n\log n)$) Now, observe that if a line from $p$ to some point on the segment $s_i$ in $... 1 Graham scan for a convex hull works if you have an ordering of points$a_1,a_2,...a_N$such that you have a sequence$p_1 < p_2 <...< p_k$where your convex hull is$a_{p_1}, a_{p_2},...,a_{p_k}$of which it is not possible to have a winding number greater than$1$. In general for any polygon$P$this might not work, but for a star shaped polygon, ... 1 Build a graph, with one vertex per black pixel, and an edge between two pixels if they are adjacent. Compute all-pairs shortest-path distances$d(x,y)$. For each vertex$x$, compute $$f(x) = \max \{d(x,y) : y \in V\}.$$ Find the vertex (pixel)$x$with the maximum score for$f(x)$. This will be one endpoint of the shape. Find the other vertex$y$that ... 1 Compute the topological skeleton of the black pixels. This will form a curved line one pixel wide (i.e., it will be isomorphic to a line). Use the two endpoints of this line as your answer. You can find the endpoints by using depth-first search, or (for a more robust solution) by computing the tangent line at each point$p$and checking for a neighbor ... 1 Your problem is a special case of planar point location. The algorithm/data structure you found by Snarak and Tarjan can answer point location (and thus point in polygon) queries in$O(\log n)$time using$O(n \log n)$preprocessing and$O(n)\$ space. That is optimal. There are two other data structures that achieve this: a data structure by Kirkpatrick and a ...

1

One possible approach is to try RANSAC. RANSAC is applicable where we expect: (a) some fraction of the observations (the rays) are erroneous (outliers) and should be ignored, and (b) the remaining ones are all approximately consistent with some low-dimensional probabilistic model. The RANSAC method then tries to find the best model that (a) minimizes the ...

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