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# Drawing Polygon with CuttingEars algorithm

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**Darkounet789** wrote at 2014-02-05 11:00:

Hi there !

I'm very new to Helix3D (even WPF...) and I've a newby question about a way to draw polygons from scratch. I just can't figure out how the function AddPolygonByCuttingEars should be used ^^" Here is my current code :

Thank you in advance for your answers !

I said convex, of course, they are concave ^^" sorry !

I'm very new to Helix3D (even WPF...) and I've a newby question about a way to draw polygons from scratch. I just can't figure out how the function AddPolygonByCuttingEars should be used ^^" Here is my current code :

```
var visualMesh = new MeshVisual3D();
var geometry = new GeometryModel3D();
var meshbuilder = new MeshBuilder(false,false);
var polygon = new[]
{
new Point(1, 1),
new Point(1, 2),
new Point(4, 3),
new Point(2, 1),
new Point(1.5, 1.5)
};
var polygon3D = new[]
{
new Point3D(1, 1,0),
new Point3D(1, 2,0),
new Point3D(4, 3,0),
new Point3D(2, 1,0),
new Point3D(1.5, 1.5,0)
};
var result = CuttingEarsTriangulator.Triangulate(polygon);
/* 1 */
//meshbuilder.AddPolygon(polygon3D);
/* 2 */
//meshbuilder.AddPolygon(result);
/* 3 */
meshbuilder.AddPolygonByCuttingEars(result);
geometry.Geometry = meshbuilder.ToMesh();
geometry.Material = MaterialHelper.CreateMaterial(Colors.OrangeRed);
visualMesh.Content = geometry;
this.Viewport3D.Children.Add(visualMesh);
```

The polygons are /convex/ -*edited*- -> concave !.- The flatten Polygon3D only appear with false, false as attributes of the MeshBuilder constructor, otherwise I get a 'System.InvalidOperationException' in HelixToolkit.Wpf.dll. However some faces are upside down and the shape is finally not /convex/ -
*edited*- -> concave ! =s

- meshbuilder.AddPolygon(result) line works, but no shapes appears... even with true, true in the MeshBuilder constructor.

- With meshbuilder.AddPolygonByCuttingEars(result); I get a 'System.ArgumentOutOfRangeException' in mscorlib.dll, and I don't really know why...

Thank you in advance for your answers !

**/ Edit /**I said convex, of course, they are concave ^^" sorry !

**Darkounet789** wrote at 2014-02-06 11:21:

Hi !

I think I got it ! I just drawn each triangle by using the indexes given by the cuttingears algorithm. I don't know if it's the right way to do it, but it works ^^.

I think I got it ! I just drawn each triangle by using the indexes given by the cuttingears algorithm. I don't know if it's the right way to do it, but it works ^^.

```
var visualMesh = new MeshVisual3D();
var geometry = new GeometryModel3D();
var meshbuilder = new MeshBuilder(true, true);
var polygon = new PointCollection()
{
new Point(1, 1),
new Point(1, 2),
new Point(2, 3),
new Point(3, 2),
new Point(4, 4),
new Point(4, 2),
new Point(3, 1)
};
var result = CuttingEarsTriangulator.Triangulate(polygon);
List<int> tri = new List<int>();
for (int i = 0; i < result.Count; i++)
{
tri.Add(result[i]);
if (tri.Count == 3)
{
Console.WriteLine("Triangle " + (i / 3).ToString() + " : " + tri[0].ToString() + ", " + tri[1].ToString() + ", " + tri[2].ToString());
meshbuilder.AddTriangle(new Point3D(polygon[tri[0]].X, polygon[tri[0]].Y, 0),
new Point3D(polygon[tri[1]].X, polygon[tri[1]].Y, 0),
new Point3D(polygon[tri[2]].X, polygon[tri[2]].Y, 0));
tri.Clear();
}
}
geometry.Geometry = meshbuilder.ToMesh();
geometry.Material = MaterialHelper.CreateMaterial(Colors.OrangeRed);
visualMesh.Content = geometry;
this.Viewport3D.Children.Add(visualMesh);
```

However, I encounter and new issue... the triangles I'm drawing are one-sided. Only the upper side of the face is rendered. Is there a way to make the triangles double-sided ?**/ FIXED /**: I didn't know I had to set a backmaterial. I discovered that in an another topic ^^**Darkounet789** wrote at 2014-02-13 16:03:

Hey everybody !

You shouldn't use the code I wrote in my previous posts. Instead of this, I now use this couple of functions to fill a polygon with triangles.

Feel free to reuse !

You shouldn't use the code I wrote in my previous posts. Instead of this, I now use this couple of functions to fill a polygon with triangles.

```
//Support for 2D polygons
public static MeshGeometry3D FillPolygon(Polygon p)
{
List<Point3D> pts3D = new List<Point3D>();
foreach (var point in p.Points)
{
pts3D.Add(new Point3D(point.X, point.Y, 0));
}
Polygon3D p3 = new Polygon3D(pts3D);
return FillPolygon(p3);
}
//For 3D polygons
public static MeshGeometry3D FillPolygon(Polygon3D p3)
{
var meshBuilder = new MeshBuilder(false, false);
Polygon polygon = p3.Flatten();
var triangleIndexes = CuttingEarsTriangulator.Triangulate(polygon.Points);
meshBuilder.Append(p3.Points, triangleIndexes);
return meshBuilder.ToMesh();
}
```

The Append method of the MeshBuilder made my day...Feel free to reuse !

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