Does anybody know what triangulation algorithm Maya uses? Lacking that, what would be the most probable algoritms to try? I tried a few simple off the top of my head (shortest/longest resulting edges, smallest minimum angle, smallest/biggest area), but they where all wrong. Is Delaunay the most plausible algoritm?
Edit: by the way, pseudo code on how to implement Delaunay for a 2D quad in 3D space to generate two triangles are more than welcome!
Edit 2: Unfortunately, this is not the answer in 3D-space (only applicable in 2D).
I don't like to second-guess people's intentions but if you are simply trying to get out of Maya what is shown in the viewport you can extract Maya's triangulation by starting with MItMeshPolygon::getTriangles.
(The corresponding normals and vertex colours are straightforwardly accessible. UVs require a little more effort -- I don't remember the details (all my Maya code is with my ex employer) but whilst at first glance it may seem like you don't have the data, in fact it's all there, just not conveniently.)
(One further note -- if your artists try hard enough, they can create polygons that crash Maya when getTriangles is called, even though they render OK and can be manipulated with the UI. This used to happen every few months, so it's worth bearing in mind but probably not worth worrying about too much.)
If you don't want to use the API or Python, then running polyTriangulate before exporting, then undo afterwards (to get back the original polygons) would let you examine out the triangulated mesh. (You may want or need to save the scene to a temp file, then reload it afterwards and use file to give it its old name back, if your export process does stuff that is difficult or impossible to undo.)
This is a bit hacky, but you're guaranteed to get the exact triangulation Maya is using. Rather easier than writing your own triangulation code, and almost certainly a LOT easier than trying to work out whatever Maya does internally...
Jonathan Shewchuk has a very popular 2D triangulation tool called Triangle, and a 3D version should appear soon. He also has a number of papers on this topic that might be of use.
You might try looking at Voronoi and Delaunay Techniques by Henrik Zimmer. I don't know if it's what Maya uses, but the paper describes some common techniques.
Here you can find an applet that demonstrates the Incremental, Gift Wrap, Divide and Conquer and QuickHull algorithms computing the Delaunay triangulation in 3D. Pointers to each algorithm are provided.
Related
I am using Poisson surface reconstruction algorithm to reconstruct triangulated mesh surface from points. However, Poisson will always generate a watertight surface, which fills all holes with interpolation.
For some small holes that is the result of data missing, this hole filling is desirable. But for some big holes, I do not want hole filling and just want the surface to remain open.
The figure above shows my idea, the left one is a pointset with normal, the right one is reconstructed surface. I want the top of this surface remains open rather than current watertight result.
Can anyone provide me with some advice, how may I keep these big holes in Poisson surface reconstruction? Or is there any other algorithms that could solve this?
P.S.
Based on the accepted answer to this question, I understand surface reconstruction algorithms could be categorized as explicit ones and implicit ones. Poisson is implicit ones, and explicit ones could naturally handle big hole problem. But since the points data I have are mostly sparse and noisy, I would prefer an implicit one like Poisson.
Your screenshots look like you might be using MeshLab's implementation which is based on an old implementation. This implementation is not capable of trimming the surface.
The latest implementation, however, contains the SurfaceTrimmer that does exactly what you want. Take a look at the examples at the bottom of the page to see how to use it.
To use SurfaceTrimmer program, you have to first use SSDRecon program to reconstruct a mesh surface with --density, then setting a trim value would exactly remove faces under a specific threshold.
Below is a sample usage of that program on the demo eagle data
./SSDRecon --in eagle.points.ply --out eagle.screened.color.ply --depth 10 --density
./SurfaceTrimmer --in eagle.screened.color.ply --out eagle.screened.color.trimmed.ply --trim 7
I have set of binary images, on which i need to find the cross (examples attached). I use findcontours to extract borders from the binary image. But i can't understand how can i determine is this shape (border) cross or not? Maybe opencv has some built-in methods, which could help to solve this problem. I thought to solve this problem using Machine learning, but i think there is a simpler way to do this. Thanks!
Viola-Jones object detection could be a good start. Though the main usage of the algorithm (AFAIK) is face detection, it was actually designed for any object detection, such as your cross.
The algorithm is Machine-Learning based algorithm (so, you will need a set of classified "crosses" and a set of classified "not crosses"), and you will need to identify the significant "features" (patterns) that will help the algorithm recognize crosses.
The algorithm is implemented in OpenCV as cvHaarDetectObjects()
From the original image, lets say you've extracted sets of polygons that could potentially be your cross. Assuming that all of the cross is visible, to the extent that all edges can be distinguished as having a length, you could try the following.
Reject all polygons that did not have exactly 12 vertices required to
form your polygon.
Re-order the vertices such that the shortest edge length is first.
Create a best fit perspective transformation that maps your vertices onto a cross of uniform size
Examine the residuals generated by using this transformation to project your cross back onto the uniform cross, where the residual for any given point is the distance between the projected point and the corresponding uniform point.
If all the residuals are within your defined tolerance, you've found a cross.
Note that this works primarily due to the simplicity of the geometric shape you're searching for. Your contours will also need to have noise removed for this to work, e.g. each line within the cross needs to be converted to a single simple line.
Depending on your requirements, you could try some local feature detector like SIFT or SURF. Check OpenSURF which is an interesting implementation of the latter.
after some days of struggle, i came to a conclusion that the only robust way here is to use SVM + HOG. That's all.
You could erode each blob and analyze their number of pixels is going down. No mater the rotation scaling of the crosses they should always go down with the same ratio, excepted when you're closing down on the remaining center. Again, when the blob is small enough you should expect it to be in the center of the original blob. You won't need any machine learning algorithm or training data to resolve this.
I have a lot of polygons. Ideally, all the polygons must not overlap one other, but they can be located adjacent to one another.
But practically, I would have to allow for slight polygon overlap ( defined by a certain tolerance) because all these polygons are obtained from user hand drawing input, which is not as machine-precised as I want them to be.
My question is, is there any software library components that:
Allows one to input a range of polygons
Check if the polygons are overlapped more than a prespecified tolerance
If yes, then stop, or else, continue
Create mesh in terms of coordinates and elements for the polygons by grouping common vertex and edges together?
More importantly, link back the mesh edges to the original polygon(s)'s edge?
Or is there anyone tackle this issue before?
This issue is a daily "bread" of GIS applications - this is what is exactly done there. We also learned that at a GIS course. Look into GIS systems how they address this issue. E.g. ArcGIS define so called topology rules and has some functions to check if the edited features are topologically correct. See http://webhelp.esri.com/arcgisdesktop/9.2/index.cfm?TopicName=Topology_rules
This is pretty long, only because the question is so big. I've tried to group my comments based on your bullet points.
Components to draw polygons
My guess is that you'll have limited success without providing more information - a component to draw polygons will be very much coupled to the language and UI paradigm you are using for the rest of your project, ie. code for a web component will look very different to a native component.
Perhaps an alternative is to separate this element of the process out from the rest of what you're trying to do. There are some absolutely fantastic pre-existing editors that you can use to create 2d and 3d polygons.
Inkscape is an example of a vector graphics editor that makes it easy to enter 2d polygons, and has the advantage of producing output SVG, which is reasonably easy to parse.
In three dimensions Blender is an open source editor that can be used to produce arbitrary geometries that can be exported to a number of formats.
If you can use a google-maps API (possibly in an native HTML rendering control), and you are interested in adding spatial points on a map overlay, you may be interested in related click-to-draw polygon question on stackoverflow. From past experience, other map APIs like OpenLayers support similar approaches.
Check whether polygons are overlapped
Thomas T made the point in his answer, that there are families of related predicates that can be used to address this and related queries. If you are literally just looking for overlaps and other set theoretic operations (union, intersection, set difference) in two dimensions you can use the General Polygon Clipper
You may also need to consider the slightly more generic problem when two polygons that don't overlap or share a vertex when they should. You can use a Minkowski sum to dilate (enlarge) two and three dimensional polygons to avoid such problems. The Computational Geometry Algorithms Library has robust implementations of these algorithms.
I think that it's more likely that you are really looking for a piece of software that can perform vertex welding, Christer Ericson's book Real-time Collision Detection includes extensive and very readable description of the basics in this field, and also on related issues of edge snapping, crack detection, T-junctions and more. However, even though code snippets are included for that book, I know of no ready made library that addresses these problems, in particular, no complete implementation is given for anything beyond basic vertex welding.
Obviously all 3D packages (blender, maya, max, rhino) all include built in software and tools to solve this problem.
Group polygons based on vertices
From past experience, this turned out to be one of the most time consuming parts of developing software to solve problems in this area. It requires reasonable understanding of graph theory and algorithms to traverse boundaries. It is worth relying upon a solid geometry or graph library to do the heavy lifting for you. In the past I've had success with igraph.
Link the updated polygons back to the originals.
Again, from past experience, this is just a case of careful bookkeeping, and some very careful design of your mesh classes up-front. I'd like to give more advice, but even after spending a big chunk of the last six months on this, I'm still struggling to find a "nice" way to do this.
Other Comments
If you're interacting with users, I would strongly recommend avoiding this issue where possible by using an editor that "snaps", rounding all user entered points onto a grid. This will hopefully significantly reduce the amount of work that you have to do.
Yes, you can use OGR. It has python bindings. Specifically, the Geometry class has an Intersects method. I don't fully understand what you want in points 4 and 5.
My requirements:
A user should be able to draw something by hand. Then after he takes off his pen (or finger) an algorithm smooths and transforms it into some basic shapes.
To get started I want to transform a drawing into a rectangle which resembles the original as much as possible. (Naturally this won't work if the user intentionally draws something else.) Right now I'm calculating an average x and y position, and I'm distinguishing between horizontal and vertical lines. But it's not yet a rectangle but some kind of orthogonal lines.
I wondered if there is some well-known algorithm for that, because I saw it a few times at some touchscreen applications. Do you have some reading tip?
Update: Maybe a pattern recognition algorithm would help me. There are some phones which request the user to draw a pattern to unlock it's keys.
P.S.: I think this question is not related to a particular programming language, but if you're interested, I will build a web application with RaphaelGWT.
The Douglas-Peucker algorithm is used in geography (to simplify a GPS track for instance) I guess it could be used here as well.
Based on your description I guess you're looking for a vectorization algorithm. Here are some pointers that might help you:
https://en.wikipedia.org/wiki/Image_tracing
http://outliner.codeplex.com/ - open source vectorizer of the edges in the raster pictures.
http://code.google.com/p/shapelogic/wiki/vectorization - describes different vectorization algorithm implementations
http://cardhouse.com/computer/vector.htm
There are a lot of resources on vectorization algorithms, I'm sure you'll be able to find something that fits your needs. I don't know how complex these algorithms are to implement them, though,
I am interested to read and understand the 2D mesh algorithms. A search on Google reveals a lot of papers and sources, however most are too academic and not much on beginner's side.
So, would anyone here recommend any reading sources ( suitable for the beginners), or open source implementation that I can learn from the start? Thanks.
Also, compared to triangular mesh generation, I have more interest in quadrilateral mesh and mix mesh( quad and tri combined).
I second David's answer regarding Jonathan Shewchuk's site as a good starting point.
In terms of open source software, it depends on what you are looking for exactly.
If you are interested in mesh generation, you can have a look at CGAL's code. Understanding the low level parts of CGAL's code is too much for a beginner. However, having a look at the higher level algorithms can be quite interesting even for a beginner. Also note that the documentation of CGAL is very detailed.
You can also have a look at TetGen, but its source code is monolithic and is not documented (it is more of an end user software rather than a library, even if it can also be called simply from other programs). Still, it is fairly readable, and the user manual contains a short presentation of mesh generation, with some references.
If you are also interested in mesh processing, you can have a look at OpenMesh.
More informations about your goals would definitely help providing more relevant pointers.
The first link on your Google search takes you to Jonathan Shewchuk's site. This is not actually a bad place to start. He has a program called triangle which you can download for 2D triangulation. On that page there is a link to references used in creating triangle, including a link to a description of the triangluation algorithm.
There are several approaches to mesh generation. One of the most common is to create a Delaunay triangulation. Triangulating a set of points is fairly simple and there are several algorithms which do that, including Watson's and Rupert's as used in triangle
When you want to create a constrained triangulation, where the edges of the triangulation match the edges of your input shape it is a bit harder, because you need to recover certain edges.
I would start by understanding Delaunay triangulation. Then maybe look at some of the other meshing algorithms.
Some of the common topics that you will find in mesh generation papers are
Robustness - that is how to deal with floating point round off errors.
Mesh quality - ensuring the shapes of the triangles/tetrahedrons are close to equilateral. Whether this is important depends on why you are creating the mesh. For analysis work it is very important,
How to choose where to insert the nodes in the mesh to give a good mesh distribution.
Meshing speed
Quadrilateral/Hexahedral mesh generation. This is harder than using triangles/tetrahedra.
3D mesh generation is much harder than 2D so a lot of the papers are on 3D generation
Mesh generation is a large topic. It would be helpful if you could give some more information on what aspects (eg 2D or 3D) that you are interested in. If you can give some idea of what you ant to do then maybe I can find some better sources of information.