How to detect border vertices of an open mesh 3d model? - math

There are two kinds of surface mesh models, closed mesh like a sphere or a cube and the second one is the open mesh model which means the surface of the model is not in a closed loop. It is open from somewhere like a hollow pipe.
Sp what I want is I want to detect the border vertices of the open mesh model. there is no border in closed loop mesh but in open mesh we have to detect border vertices for some smoothing, subdivision, etc. operations.
Kindly, suggest me how can I select/detect border vertices ? what is the optimal way to do this ?
by comparing edges of the triangles ? Give me some idea ?
Thanks.


Assuming that you have a manifold mesh, then the border of the mesh are those edges which belong to only one polygon. Edges that are not on the border will belong to two polygons. The border vertices are the vertices that belong to the border edges.
A naive way to find the border vertices is to iterate through all your edges, count how many polygons they belong to, and if they only belong to one polygon, then collect the edge's vertices as border vertices. You will have to remove duplicates vertices from your collection, though.
A second approach is to have your mesh data structure examine each edge as they are added to the mesh, or as polygons are attached to particular edges. In this way, the mesh data structure can keep a list of up-to-date border edges for you, so that when you needed the edges you would not have to find them each time. This will greatly reduce the overhead of determining border edges, although inserting edges and polygons will be slightly more expensive. Your mesh data structure will also take up a bit more memory.


Assuming that your mesh is a 2D (or 2.5D) regular, well-constructed triangulation. You can use some of the properties listed here: http://graphics.stanford.edu/courses/cs468-10-fall/LectureSlides/02_Basics.pdf
Page 9 defines the degree (or valence) of a vertex as the number of incident edges. As shown, all boundary vertices 4 incident edges. "Internal" vertices have 5 incident edges.
Page 17 defines a boundary edge as one that is adjacent to exactly one face.
You might find the discussion on page 22 helpful (closed 2-manifold triangle meshes)

Related

If i already know the Voronoi vertices points, how can i create a polygon from the list?

I'm trying to create this in Gamemaker. I already know the Voronoi vertices but i'm stuck with how to create polygons for each seed object. I need them to be independent so i can split it up later to apply texture mapping to them.
I've tried delaunay already but it doesn't seem as accurate as my voronoi generation. But being that the cicrumradius is the voronoi vertices anyways i feel like i don't need it. The problem with the Delaunay is that it only returns the points near the center of the diagram and doesn't return any points towards the Borders of the Box. The only good thing is that delaunay did skip an extra step and made it easier to return if the the seeds x and y are within the circumradius then just add them to the list of vertices
Is there any way to make a polygon from a plot of points from a data structure?

Pick the midpoint of each edge and the distance to each site then sort the result and pick the first and second (when they are equal) and save them into polygons. For the borders there is of course only 1 edge.
Duplicate:Getting polygons from voronoi edges

Get Edges between Vertices (outer polygon)

I'm using THREE.JS and I have a this mesh with different surfaces. Of each surface I got its vertices. Now I want to create edges ( connect the vertices with lines). The vertices are in an arbitrary order, so I can't simply connect v1 with v2, v2 with v3 and so on. I think I have to walk through them with a ray clockwise or counter clockwise and put them in the right order somehow and I have to somehow check their distances, so that the horizontal line between the inner vertices doesn't appear, rather it should go right along the real edges,but I don't know how... Any idea?
(the spheres are the vertices that are the real corners of the surface, the orange lines are the wrong edges that need to be corrected and the blue lines are the edges of each single face)
I need to create these 8 edges (red)

Assuming that you are able to assign a unique number to the vertices and to uniquely associate every triangle to a face, and that the triangulation is watertight, the edges of a face are those edges that are common to a triangle of that face and a triangle of another. When you have all edges of a face, it is a trivial matter to chain them in a polygon.

Find the distance between two points on any 3d surface

I am making a game in Unity3d and I need a pathfinding algorithm that can guide enemy's towards the player on a 3d surface. The problem is that the 3d surface can take any shape, so it can be a 3d sphere, cube, torus and many more shapes.
I tried using A* but for that formula I need the distance between the two points, and since the object is curved I cannot get that so easily. I found that you can use the Haversine formula if its a sphere, but that won't work on a torus or a random 3d shape.
I want this kind of result except with every kind of object:
https://www.youtube.com/watch?v=hvunNq7yVcU
Is there a way/algorithm that I can use to get that result. I know there is something called nav mesh but I need to program it myself. Also I cannot find how nav mesh approaches this dilema. I am going to use the triangles of my object as nodes.
So my question boils down to:
Does anyone know a algorithm for pathfinding that works on any 3d surface?
Thanks in advance.

I think your problem is that you are not using a graph, I would suggest that you look into a tutorial on how to create a graph, for the language you are using if you can, (this may also help here they are using edges to connect their node which is needed if you have more then one weight). If you do make a graph you will need a node class. Each node must contain pointers to any nodes that it is connected to and an ID of some kind. In your case that is probably all you need but it is also possible to assign a weight to each move if you also have an edge class (connectors between nodes) which would be used to connect the nodes. If you do have an edge class your nodes will have pointers to edges instead of other nodes and each edge will have a weight and a pointer to 1 or 2 nodes (depending on if it is a directed path or not). You can also make a graph class to contain all of your nodes and edges.
Summary:
make a node class and determine if you need the edge class (if everything has a weight of 1 you can get away with out it). Use the node class to create a graph to represent your map with each tile being a node with pointers to connected tiles. Use A* or dijkstra's algorithm to search your graph to find the shortest path.
note: most examples you will find will be for 2d graphs, yours is no different except that there are no bounds on yours, you just need to connect the nodes to their adjacent tiles.

Building a graph from a point cloud

I have a PCL Point Cloud. Basically, I need to write some code that does the following:
Example
Basically, I need to build a graph/edge map of the point cloud. Where each node represents a point, and those points have pointers/edges to neighbouring points. And preferably, it cannot form a corner edge as seen in the picture. (This could be enforced by saying a point cannot have a large change in l1 norm too (taxicab distance. add all axis), not just l2 norm).
I need to do this because, it's useful for all my other algorithms. Normal computation etc.
I'm currently at a loss of how to implement this. My point cloud is unorganized. I could sort it into a KD Tree but I'm not sure if that is related to this or how I might use this.

The graph/edge map is the same as a triangulation between the vertices.
In your case, as you only want to connect vertices which are close together, Delaunay Triangulation will work.
The edges are the connections between vertices in your graph.
PCL has ConcaveHull, which will triangulate the surface of your vertices, given an alpha value. This alpha value is the maximum radius for each triangle, in your case, half the known distance between diagonal vertices.

How to identified face from line drawing?

A line drawing is like a graph but its vertices have x,y position. There are no crossing edges. For example, a line drawing like this is a line drawing with 13 vertices numbered by 0-12. A face is a cycle that doesn't have a path that 'inside' it. Faces in the example would be
(0,1,3,2,0), (2,3,5,4,2), (4,5,8,7,4), (7,8,12,11,7) and (0,2,4,7,11,10,9,6,0)
The cycle (0,1,3,5,4,2,0) is NOT a face because there is a path that located inside it, named (2,3). Cycle (0,1,3,5,8,12,11,10,9,6,0) is also NOT a face because there is a path (0,2,4,7,11), located inside it. What algorithm can I use to identify faces like the ones in the example?

Assume all your edges are line segments; every planar graph can be drawn using only line segments. Also assume the graph is connected. Now the algorithm is pretty simple:
Construct a directed graph, such that the vertices are same as in the original graph and there's two directed edges for every original edge, one in each direction
Start with a random (directed) edge that's not been used yet. At its end, choose the next outgoing edge clockwise (or counterclockwise will do as well, just always the same). To decide which edge that is, you'll have to compute from the coordinates of vertices in the planar embedding. You'd better precompute this edge order for each vertex beforehand.
Keep doing that with the end of the selected edge, until you reach the starting vertex. At that point, you've completed a face.
When there's no unused edges, you've found all faces in the graph
Or, use a library like Boost, that has an efficient implementation of such task

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