/// <summary>
        /// Fills the given vertex structure using information from the given AC-File-Objects.
        /// </summary>
        /// <param name="objInfo">The object information from the AC file.</param>
        /// <param name="acMaterials">A list containing all materials from the AC file.</param>
        /// <param name="structure">The VertexStructure to be filled.</param>
        /// <param name="transformStack">Current matrix stack (for stacked objects).</param>
        private static void FillVertexStructure(VertexStructure structure, List <ACMaterialInfo> acMaterials, ACObjectInfo objInfo, Matrix4Stack transformStack)
        {
            List <Tuple <int, int> > standardShadedVertices = new List <Tuple <int, int> >();

            transformStack.Push();
            try
            {
                // Perform local transformation for the current AC object
                transformStack.TransformLocal(objInfo.Rotation);
                transformStack.TranslateLocal(objInfo.Translation);

                // Build structures material by material
                for (int actMaterialIndex = 0; actMaterialIndex < acMaterials.Count; actMaterialIndex++)
                {
                    ACMaterialInfo actMaterial = acMaterials[actMaterialIndex];

                    VertexStructureSurface actStructSurface = structure.CreateOrGetExistingSurface(actMaterial.CreateMaterialProperties());
                    bool isNewSurface = actStructSurface.CountTriangles == 0;

                    // Create and configure vertex structure
                    actStructSurface.Material   = NamedOrGenericKey.Empty;
                    actStructSurface.TextureKey = !string.IsNullOrEmpty(objInfo.Texture) ? new NamedOrGenericKey(objInfo.Texture) : NamedOrGenericKey.Empty;
                    actStructSurface.MaterialProperties.DiffuseColor  = actMaterial.Diffuse;
                    actStructSurface.MaterialProperties.AmbientColor  = actMaterial.Ambient;
                    actStructSurface.MaterialProperties.EmissiveColor = actMaterial.Emissive;
                    actStructSurface.MaterialProperties.Shininess     = actMaterial.Shininess;
                    actStructSurface.MaterialProperties.SpecularColor = actMaterial.Specular;

                    // Initialize local index table (needed for vertex reuse)
                    int   oneSideVertexCount = objInfo.Vertices.Count;
                    int[] localIndices       = new int[oneSideVertexCount * 2];
                    for (int loop = 0; loop < localIndices.Length; loop++)
                    {
                        localIndices[loop] = int.MaxValue;
                    }

                    // Process all surfaces
                    foreach (ACSurface actSurface in objInfo.Surfaces)
                    {
                        // Get the vertex index on which to start
                        int startVertexIndex   = structure.CountVertices;
                        int startTriangleIndex = actStructSurface.CountTriangles;

                        // Only handle surfaces of the current material
                        if (actSurface.Material != actMaterialIndex)
                        {
                            continue;
                        }

                        // Sort out unsupported surfaces
                        if (actSurface.VertexReferences.Count < 3)
                        {
                            continue;
                        }
                        if (actSurface.IsLine)
                        {
                            continue;
                        }
                        if (actSurface.IsClosedLine)
                        {
                            continue;
                        }

                        // Preprocess referenced vertices
                        int        oneSideSurfaceVertexCount     = actSurface.VertexReferences.Count;
                        int        countSurfaceSides             = actSurface.IsTwoSided ? 2 : 1;
                        int[]      onStructureReferencedVertices = new int[oneSideSurfaceVertexCount * countSurfaceSides];
                        List <int> surfaceVertexReferences       = actSurface.VertexReferences;
                        for (int loop = 0; loop < surfaceVertexReferences.Count; loop++)
                        {
                            Vector2 actTexCoord = actSurface.TextureCoordinates[loop];

                            if (!actSurface.IsFlatShaded)
                            {
                                // Try to reuse vertices on standard shading
                                if (localIndices[surfaceVertexReferences[loop]] == int.MaxValue)
                                {
                                    Vector3 position = Vector3.Transform(
                                        objInfo.Vertices[surfaceVertexReferences[loop]].Position,
                                        transformStack.Top);
                                    localIndices[surfaceVertexReferences[loop]] = structure.AddVertex(new Vertex(
                                                                                                          position, Color4.White, actTexCoord, Vector3.Zero));
                                    if (actSurface.IsTwoSided)
                                    {
                                        localIndices[surfaceVertexReferences[loop] + oneSideVertexCount] = structure.AddVertex(new Vertex(
                                                                                                                                   position, Color4.White, actTexCoord, Vector3.Zero));
                                    }
                                }

                                // Store vertex reference for this surface's index
                                onStructureReferencedVertices[loop] = localIndices[surfaceVertexReferences[loop]];
                                if (actSurface.IsTwoSided)
                                {
                                    onStructureReferencedVertices[loop + oneSideSurfaceVertexCount] =
                                        localIndices[surfaceVertexReferences[loop] + oneSideVertexCount];
                                }
                            }
                            else
                            {
                                // Create one vertex for one reference for flat shading
                                Vector3 position = Vector3.Transform(
                                    objInfo.Vertices[surfaceVertexReferences[loop]].Position,
                                    transformStack.Top);
                                onStructureReferencedVertices[loop] = structure.AddVertex(new Vertex(
                                                                                              position, Color4.White, actTexCoord, Vector3.Zero));
                                if (actSurface.IsTwoSided)
                                {
                                    onStructureReferencedVertices[loop + oneSideSurfaceVertexCount] = structure.AddVertex(new Vertex(
                                                                                                                              position, Color4.White, actTexCoord, Vector3.Zero));
                                }
                            }
                        }

                        // Build object geometry
                        switch (actSurface.VertexReferences.Count)
                        {
                        case 3:
                            // Front side
                            actStructSurface.AddTriangle(
                                onStructureReferencedVertices[0],
                                onStructureReferencedVertices[1],
                                onStructureReferencedVertices[2]);

                            // Back side
                            if (actSurface.IsTwoSided)
                            {
                                actStructSurface.AddTriangle(
                                    onStructureReferencedVertices[5],
                                    onStructureReferencedVertices[4],
                                    onStructureReferencedVertices[3]);
                            }
                            break;

                        case 4:
                            // Front side
                            actStructSurface.AddTriangle(
                                onStructureReferencedVertices[0],
                                onStructureReferencedVertices[1],
                                onStructureReferencedVertices[2]);
                            actStructSurface.AddTriangle(
                                onStructureReferencedVertices[2],
                                onStructureReferencedVertices[3],
                                onStructureReferencedVertices[0]);

                            // Back side
                            if (actSurface.IsTwoSided)
                            {
                                actStructSurface.AddTriangle(
                                    onStructureReferencedVertices[6],
                                    onStructureReferencedVertices[5],
                                    onStructureReferencedVertices[4]);
                                actStructSurface.AddTriangle(
                                    onStructureReferencedVertices[4],
                                    onStructureReferencedVertices[7],
                                    onStructureReferencedVertices[6]);
                            }
                            break;

                        default:
                            if (!actSurface.IsTwoSided)
                            {
                                // Front side
                                actStructSurface.AddPolygonByCuttingEars(onStructureReferencedVertices);
                            }
                            else
                            {
                                // Front and back side
                                actStructSurface.AddPolygonByCuttingEars(onStructureReferencedVertices.Subset(0, oneSideSurfaceVertexCount));
                                actStructSurface.AddPolygonByCuttingEars(onStructureReferencedVertices.Subset(oneSideSurfaceVertexCount, oneSideSurfaceVertexCount));
                            }
                            break;
                        }

                        // Perform shading
                        if (actSurface.IsFlatShaded)
                        {
                            actStructSurface.CalculateNormalsFlat(
                                startTriangleIndex, actStructSurface.CountTriangles - startTriangleIndex);
                        }
                        else
                        {
                            // Nothing to be done for now..
                            int vertexCount = structure.CountVertices - startVertexIndex;
                            if (vertexCount > 0)
                            {
                                standardShadedVertices.Add(
                                    Tuple.Create((int)startVertexIndex, vertexCount));
                            }
                        }
                    }

                    // Calculate default shading finally (if any)
                    foreach (var actStandardShadedPair in standardShadedVertices)
                    {
                        structure.CalculateNormals(
                            actStandardShadedPair.Item1,
                            actStandardShadedPair.Item2);
                    }
                    standardShadedVertices.Clear();

                    // Append generated VertexStructure to the output collection
                    if ((actStructSurface.CountTriangles <= 0) &&
                        (isNewSurface))
                    {
                        structure.RemoveSurface(actStructSurface);
                    }
                }

                //Fill in all child object data
                foreach (ACObjectInfo actObjInfo in objInfo.Childs)
                {
                    FillVertexStructure(structure, acMaterials, actObjInfo, transformStack);
                }
            }
            finally
            {
                transformStack.Pop();
            }
        }
        /// <summary>
        /// Reads a facet.
        /// </summary>
        private void ReadFacet(StreamReader reader, string normalString, VertexStructure newStructure, StlImportOptions importOptions)
        {
            m_cachedPoints.Clear();

            // Read all geometry
            Vector3 normal = ParseNormal(normalString);

            ReadLine(reader, "outer");
            while (true)
            {
                string  line = reader.ReadLine();
                Vector3 point;
                if (TryParseVertex(line, out point))
                {
                    m_cachedPoints.Add(point);
                    continue;
                }

                string id, values;
                ParseLine(line, out id, out values);

                if (id == "endloop")
                {
                    break;
                }
            }

            // Read end
            ReadLine(reader, "endfacet");

            // Overtake geometry data
            VertexStructureSurface targetSurfae = newStructure.FirstSurface;
            int pointCount = m_cachedPoints.Count;

            switch (m_cachedPoints.Count)
            {
            case 0:
            case 1:
            case 2:
                break;

            case 3:
                if (importOptions.IsChangeTriangleOrderNeeded())
                {
                    targetSurfae.AddTriangle(
                        new Vertex(m_cachedPoints[2], Color4.Transparent, Vector2.Zero, normal),
                        new Vertex(m_cachedPoints[1], Color4.Transparent, Vector2.Zero, normal),
                        new Vertex(m_cachedPoints[0], Color4.Transparent, Vector2.Zero, normal));
                }
                else
                {
                    targetSurfae.AddTriangle(
                        new Vertex(m_cachedPoints[0], Color4.Transparent, Vector2.Zero, normal),
                        new Vertex(m_cachedPoints[1], Color4.Transparent, Vector2.Zero, normal),
                        new Vertex(m_cachedPoints[2], Color4.Transparent, Vector2.Zero, normal));
                }
                break;

            default:
                int[] indices = new int[pointCount];
                if (importOptions.IsChangeTriangleOrderNeeded())
                {
                    for (int loop = pointCount - 1; loop > -1; loop--)
                    {
                        indices[loop] = newStructure.AddVertex(
                            new Vertex(m_cachedPoints[loop], Color4.Transparent, Vector2.Zero, normal));
                    }
                }
                else
                {
                    for (int loop = 0; loop < pointCount; loop++)
                    {
                        indices[loop] = newStructure.AddVertex(
                            new Vertex(m_cachedPoints[loop], Color4.Transparent, Vector2.Zero, normal));
                    }
                }

                targetSurfae.AddPolygonByCuttingEars(indices);
                break;
            }
        }