PolygonMeshBuilder.cs

using System.Collections.Generic;
using System.Linq;
using System.Numerics;
using SharpGLTF.Geometry;
using SharpGLTF.Geometry.VertexTypes;
using SharpGLTF.Materials;

using VERTEX = SharpGLTF.Geometry.VertexBuilder<
	SharpGLTF.Geometry.VertexTypes.VertexPositionNormal,
	SharpGLTF.Geometry.VertexTypes.VertexTexture1,
	SharpGLTF.Geometry.VertexTypes.VertexEmpty>;

namespace Stylister.Modeling3D.Core.Geometry
{
	public class PolygonMeshBuilder
	{
		private readonly List<Vector2> _points = new List<Vector2>();
		private readonly List<Vector2> _outlinePoints = new List<Vector2>();
		private readonly List<List<Vector2>> _holes = new List<List<Vector2>>();

		private readonly List<double> _epoints = new List<double>();
		private readonly List<int> _eholes = new List<int>();

		public PolygonMeshBuilder(List<Vector2> counters)
		{
			var points = counters;

			AddToEPoints(points);
			_points.AddRange(points);
			_outlinePoints.AddRange(points);
		}

		private void AddToEPoints(List<Vector2> points)
		{
			_epoints.AddRange(points.SelectMany(point => new List<double> { point.X, point.Y }));
		}

		private PolygonMeshBuilder AddHole(List<Vector2> hole)
		{
			_points.AddRange(hole);
			_holes.Add(hole);
			_eholes.Add(_epoints.Count / 2);
			AddToEPoints(hole);

			return this;
		}

		private VertexData BuildVertexData(float depth)
		{
			var result = new VertexData();

			var normals = new List<Vector3>();
			var positions = new List<Vector3>();
			var uvs = new List<Vector2>();

			var bounds = _points.ComputeBounds();

			foreach (var point in _points)
			{
				normals.Add(new Vector3(0, 1.0f, 0));
				positions.Add(new Vector3(point.X, 0, point.Y));
				uvs.Add(new Vector2((point.X - bounds.Min.X) / bounds.Width, (point.Y - bounds.Min.Y) / bounds.Height));
			}

			var res = Earcut.Tessellate(_epoints, _eholes);

			var indices = new List<int>(res);

			if (depth > 0)
			{
				var positionsCount = positions.Count / 3;

				// Add the elements at the depth
				foreach (var point in _points)
				{
					normals.Add(new Vector3(0, -1.0f, 0));
					positions.Add(new Vector3(point.X, -depth, point.Y));
					uvs.Add(new Vector2(1 - (point.X - bounds.Min.X) / bounds.Width, 1 - (point.Y - bounds.Min.Y) / bounds.Height));
				}

				var totalCount = indices.Count;
				for (var i = 0; i < totalCount; i += 3)
				{
					var i0 = indices[i + 0];
					var i1 = indices[i + 1];
					var i2 = indices[i + 2];

					indices.Add(i2 + positionsCount);
					indices.Add(i1 + positionsCount);
					indices.Add(i0 + positionsCount);
				}

				// Add the sides
				AddSide(positions, normals, uvs, indices, bounds, _outlinePoints, depth, false);

				foreach (var hole in _holes)
				{
					AddSide(positions, normals, uvs, indices, bounds, hole, depth, true);
				}
			}

			result.Indices = indices;
			result.Positions = positions;
			result.Normals = normals;
			result.Uvs = uvs;

			return result;
		}

		private void AddSide(List<Vector3> positions, List<Vector3> normals, List<Vector2> uvs, List<int> indices, Bounds bounds, List<Vector2> points, float depth, bool flip)
		{
			var startIndex = positions.Count / 3;
			var uLength = 0.0f;

			for (var i = 0; i < points.Count; i++)
			{
				var p = points[i];
				var p1 = i + 1 > points.Count - 1 ? points[0] : points[i + 1];

				positions.Add(new Vector3(p.X, 0, p.Y));
				positions.Add(new Vector3(p.X, -depth, p.Y));
				positions.Add(new Vector3(p1.X, 0, p1.Y));
				positions.Add(new Vector3(p1.X, -depth, p1.Y));

				var v1 = new Vector3(p.X, 0, p.Y);
				var v2 = new Vector3(p1.X, 0, p1.Y);
				var v3 = v2 - v1;
				var v4 = new Vector3(0, 1, 0);
				var vn = Vector3.Cross(v3, v4);
				vn = Vector3.Normalize(vn);

				uvs.Add(new Vector2(uLength / bounds.Width, 0));
				uvs.Add(new Vector2(uLength / bounds.Width, 1));
				uLength += v3.Length();
				uvs.Add(new Vector2(uLength / bounds.Width, 0));
				uvs.Add(new Vector2(uLength / bounds.Width, 1));

				if (!flip)
				{
					normals.Add(new Vector3(-vn.X, -vn.Y, -vn.Z));
					normals.Add(new Vector3(-vn.X, -vn.Y, -vn.Z));
					normals.Add(new Vector3(-vn.X, -vn.Y, -vn.Z));
					normals.Add(new Vector3(-vn.X, -vn.Y, -vn.Z));

					indices.Add(startIndex);
					indices.Add(startIndex + 1);
					indices.Add(startIndex + 2);

					indices.Add(startIndex + 1);
					indices.Add(startIndex + 3);
					indices.Add(startIndex + 2);
				}
				else
				{
					normals.Add(new Vector3(vn.X, vn.Y, vn.Z));
					normals.Add(new Vector3(vn.X, vn.Y, vn.Z));
					normals.Add(new Vector3(vn.X, vn.Y, vn.Z));
					normals.Add(new Vector3(vn.X, vn.Y, vn.Z));

					indices.Add(startIndex);
					indices.Add(startIndex + 2);
					indices.Add(startIndex + 1);

					indices.Add(startIndex + 1);
					indices.Add(startIndex + 2);
					indices.Add(startIndex + 3);
				}

				startIndex += 4;
			}
		}

		public MeshBuilder<VertexPositionNormal, VertexTexture1, VertexEmpty> Build(float depth = 0)
		{
			VERTEX CreateVertexFromIndex(VertexData vertexData1, int index)
			{
				return new VERTEX(
					new VertexPositionNormal(vertexData1.Positions[index],vertexData1.Normals[index]),
					new VertexTexture1(vertexData1.Uvs[index]),
					new VertexEmpty());
			}

			var result = VERTEX.CreateCompatibleMesh();

			var vertexData = BuildVertexData(depth);

			var material = new MaterialBuilder("material");

			var primitive = result.UsePrimitive(material);

			for (var i = 2; i < vertexData.Indices.Count; i += 3)
			{
				var idx0 = vertexData.Indices[i - 2];
				var idx1 = vertexData.Indices[i - 1];
				var idx2 = vertexData.Indices[i - 0];

				var a = CreateVertexFromIndex(vertexData, idx0);
				var b = CreateVertexFromIndex(vertexData, idx1);
				var c = CreateVertexFromIndex(vertexData, idx2);

				primitive.AddTriangle(a, b, c);
			}

			return result;
		}
	}

	public class VertexData
	{
		public List<int> Indices { get; set; }
		public List<Vector3> Positions { get; set; }
		public List<Vector3> Normals { get; set; }
		public List<Vector2> Uvs { get; set; }
	}
}

yes, it seems it's correct now... dunno if it's working for you, thought.

I would improve the VertexData code a bit further:

public class VertexData
{
		public List<int> Indices { get; set; }
		public List<Vector3> Positions { get; set; }
		public List<Vector3> Normals { get; set; }
		public List<Vector2> Uvs { get; set; }

              VERTEX GetVertex(int index)
			{
                                index = Indices[ index ];

				return new VERTEX(
					new VertexPositionNormal( Positions[index], Normals[index] ),
					new VertexTexture1( Uvs[index] ),
					new VertexEmpty( ) );
                        }
}


{

for (var i = 2; i < vertexData.Indices.Count; i += 3)
			{
                                var a = vertexData.GetVertex(i-2);
				var b = vertexData.GetVertex(i-1);
				var c = vertexData.GetVertex(i-0);

				primitive.AddTriangle(a, b, c);
                         }
}

you could even move the whole "Build" method inside VertexData class...