# How to triangulate polygon with holes?

**URL:** https://discourse.vtk.org/t/how-to-triangulate-polygon-with-holes/11497
**Category:** Support
**Created:** [May 20, 2023, 5:02pm UTC](https://discourse.vtk.org/t/how-to-triangulate-polygon-with-holes/11497 "2023-05-20T17:02:41Z")
**Posts on this page:** 4
**Page:** 1

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### Author: ![gongwaner](https://discourse.vtk.org/user_avatar/discourse.vtk.org/gongwaner/32/6996_2.png) [@gongwaner](https://discourse.vtk.org/u/gongwaner)
#### Post date: [May 20, 2023, 5:02pm UTC](https://discourse.vtk.org/t/how-to-triangulate-polygon-with-holes/11497/1 "2023-05-20T17:02:41Z")

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I have a set of 3d points defining a polygon A, and a set of points defining polygon B. All these points are planar, and A is actually a loop within B.  
My question is: how can i generate a triangulated polygon with hole? ie. the green region on the right.

![image](https://discourse.vtk.org/uploads/default/original/2X/4/492074821f5819bbbaa5be74aa8e53c20dc6ac13.png)

I searched relevant documentation and found that vtkContourTriangulator::TriangulateContours([https://vtk.org/doc/nightly/html/classvtkContourTriangulator.html#a69c679d25ea3ccad36ad9fcd45864a3a](https://vtk.org/doc/nightly/html/classvtkContourTriangulator.html#a69c679d25ea3ccad36ad9fcd45864a3a)) can do this. However, I don’t know how to use this function. Is there any example code for this?

Thanks.

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### Author: ![dgobbi](https://discourse.vtk.org/user_avatar/discourse.vtk.org/dgobbi/32/18_2.png) [@dgobbi](https://discourse.vtk.org/u/dgobbi)
#### Post date: [May 20, 2023, 8:28pm UTC](https://discourse.vtk.org/t/how-to-triangulate-polygon-with-holes/11497/2 "2023-05-20T20:28:49Z")

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An example is here:

[https://examples.vtk.org/site/Cxx/Modelling/ContourTriangulator/](https://examples.vtk.org/site/Cxx/Modelling/ContourTriangulator/)

The typical usage of this class is

```c++
vtkNew<vtkContourTriangulator> triangulator;
triangulator->SetInputData(contourData);
triangulator->Update();
vtkPolyData *polygons = triangulator->GetOutput();

```

If you have concentric contours, you can append them before triangulating them. For good results, the contours should be in opposite directions. If B is counterclockwise, then A should be clockwise.

```c++
vtkNew<vtkAppendPolyData> appendData;
appendData->AddInputData(contourB);
appendData->AddInputData(contourA);
appendData->Update();

vtkNew<vtkContourTriangulator> triangulator;
triangulator->SetInputData(appendData->GetOutput());
triangulator->Update();
vtkPolyData *polygons = triangulator->GetOutput();

```

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<div class="post-metadata">

### Author: ![gongwaner](https://discourse.vtk.org/user_avatar/discourse.vtk.org/gongwaner/32/6996_2.png) [@gongwaner](https://discourse.vtk.org/u/gongwaner)
#### Post date: [May 21, 2023, 2:06am UTC](https://discourse.vtk.org/t/how-to-triangulate-polygon-with-holes/11497/3 "2023-05-21T02:06:48Z")

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thanks! that’s exatcly what I’m looking for.

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<div class="post-metadata">

### Author: ![gongwaner](https://discourse.vtk.org/user_avatar/discourse.vtk.org/gongwaner/32/6996_2.png) [@gongwaner](https://discourse.vtk.org/u/gongwaner)
#### Post date: [May 23, 2023, 8:38am UTC](https://discourse.vtk.org/t/how-to-triangulate-polygon-with-holes/11497/6 "2023-05-23T08:38:47Z")

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I tried the code above and encountered an empty result. After a bit of toggle I found that the root cause is I forgot to set the lines of polygon’s polydata. For anyone who needs to do the same, remember to set the lines before using vtkContourTriangulator.