internal void PushModelTransform(Transform3D transform) 
 {
     if (transform != null && transform != Transform3D.Identity) 
     {
         _modelTransformStack.Push(transform.Value);
     }
 } 

        private void Expand(GeometryModel3D model, Transform3D transformation)
        {
            Transform3D ot;
            if (originalTransforms.ContainsKey(model))
                ot = originalTransforms[model];
            else
            {
                ot = model.Transform;
                originalTransforms.Add(model, ot);
            }

            Transform3D totalTransform = Transform3DHelper.CombineTransform(transformation, ot);

            var mesh = model.Geometry as MeshGeometry3D;
            if (mesh == null)
                return;
            var bounds = new Rect3D();
            foreach (int i in mesh.TriangleIndices)
                bounds.Union(totalTransform.Transform(mesh.Positions[i]));

            Point3D p = bounds.Location;
            Vector3D d = p - actualExpandOrigin;
            d *= Expansion;
            Point3D p2 = actualExpandOrigin + d;
            var t = new TranslateTransform3D(p2 - p);

            model.Transform = Transform3DHelper.CombineTransform(ot, t);
        }

        public static Vector3D Transform3DVector(Transform3D transform, Vector3D vector)
        {
            Point3D input = new Point3D(vector.X, vector.Y, vector.Z);
            Point3D output;

            return transform != null && transform.TryTransform(input, out output) ? new Vector3D(output.X, output.Y, output.Z) : vector;
        }

		public static MatrixTransform3D Rotate3D(Transform3D transform, double x, double y, double z, Point3D center, Vector3D up, Vector3D look, RotationType type)
		{
			if (type != RotationType.LockAxisY)
			{
				up = transform.Transform(up);
			}
			if (type != RotationType.LockAxisZ)
			{
				look = transform.Transform(look);
			}
			center = transform.Transform(center);
			Vector3D axisX = Vector3D.CrossProduct(up, look);
			Matrix3D matrix = new Matrix3D();
			matrix.RotateAt(new Quaternion(axisX, x), center);
			matrix.RotateAt(new Quaternion(up, y), center);
			matrix.RotateAt(new Quaternion(look, z), center);
			MatrixTransform3D mOriginTransform = transform as MatrixTransform3D;
			//mOriginTransform.Matrix.RotateAt(
			try
			{
				return new MatrixTransform3D(Matrix3D.Multiply(mOriginTransform.Matrix, matrix));
			}
			catch (Exception err)
			{
				Exceptions.LogOnly(err);
				return null;
			}
		}

 //Given a 3d object, it will apply the transform to it and return the new object. Note that it will not
 //overwrite the objects current transformation:
 public static Model3DGroup applyTransform(Model3DGroup obj, Transform3D transform)
 {
     Model3DGroup newObj = new Model3DGroup();
     newObj.Children.Add(obj);
     newObj.Transform = transform;
     return newObj;
 }

		public static Matrix3D CalcRotationMatrix(double x, double y, double z, Point3D center, Vector3D up, Vector3D look, Transform3D transform, RotationType type)
		{
			//Transform3DGroup trm = new Transform3DGroup();
			//trm.Children.Add(transform);
			Vector3D realup = transform.Transform(up);
			if (type != RotationType.LockAxisY)
			{
				up = realup;
			}
			if (type != RotationType.LockAxisZ)
			{
				look = transform.Transform(look);
			}
			center = transform.Transform(center);
			Vector3D axisX = Vector3D.CrossProduct(up, look);
			Matrix3D matrix = new Matrix3D();
			//Quaternion q = new Quaternion();
			//q.
			double ang = AngleBetween(realup, YAxis) + x;
			if (ang >= 90)
			{
				x = 90 - ang;
			}
			matrix.RotateAt(new Quaternion(axisX, x), center);
			matrix.RotateAt(new Quaternion(up, y), center);
			matrix.RotateAt(new Quaternion(look, z), center);
			return matrix;
		}

 internal static void PrependInverseTransform(Transform3D transform, ref Matrix3D viewMatrix)
 {
     if (transform != null && transform != Transform3D.Identity) 
     {
         PrependInverseTransform(transform.Value, ref viewMatrix); 
     } 
 }

 /// <summary>
 /// Combines two transforms.
 /// </summary>
 /// <param name="t1">
 /// The first transform.
 /// </param>
 /// <param name="t2">
 /// The second transform.
 /// </param>
 /// <returns>
 /// The combined transform group.
 /// </returns>
 public static Transform3D CombineTransform(Transform3D t1, Transform3D t2)
 {
     var g = new Transform3DGroup();
     g.Children.Add(t1);
     g.Children.Add(t2);
     return g;
 }

        /// <summary>
        /// Exports the light.
        /// </summary>
        /// <param name="light">
        /// The light.
        /// </param>
        /// <param name="inheritedTransform">
        /// The inherited transform.
        /// </param>
        protected override void ExportLight(Light light, Transform3D inheritedTransform)
        {
            base.ExportLight(light, inheritedTransform);

            // todo...
            // http://www.povray.org/documentation/view/3.6.1/34/
        }

 public Plant()
 {
     var x = new Vector3D(1, 0, 0);
     var r1 = new RotateTransform3D(new AxisAngleRotation3D(x, 80));
     var r2 = new RotateTransform3D(new AxisAngleRotation3D(x, -70));
     var r3 = new RotateTransform3D(new AxisAngleRotation3D(x, -10));
     var t1 = new TranslateTransform3D(0, 0, 0.5);
     var t2 = new TranslateTransform3D(0, 0, 0.7);
     var t3 = new TranslateTransform3D(0, 0, 1.0);
     var s1 = new ScaleTransform3D(0.5, 0.5, 0.5);
     var s2 = new ScaleTransform3D(0.3, 0.3, 0.3);
     var s3 = new ScaleTransform3D(0.8, 0.8, 0.8);
     var m1 = new Transform3DGroup();
     m1.Children.Add(r1);
     m1.Children.Add(s1);
     m1.Children.Add(t1);
     var m2 = new Transform3DGroup();
     m2.Children.Add(r2);
     m2.Children.Add(s2);
     m2.Children.Add(t2);
     var m3 = new Transform3DGroup();
     m3.Children.Add(r3);
     m3.Children.Add(s3);
     m3.Children.Add(t3);
     T1 = m1;
     T2 = m2;
     T3 = m3;
 }

        // Transfrom that moves the world to a camera coordinate system
        // where the camera is at the origin looking down the negative z
        // axis and y is up.
        //
        // NOTE: We consider camera.Transform to be part of the view matrix.
        //
        internal static Matrix3D CreateViewMatrix(Transform3D transform, ref Point3D position, ref Vector3D lookDirection, ref Vector3D upDirection)
        {
            Vector3D zaxis = -lookDirection;
            zaxis.Normalize();

            Vector3D xaxis = Vector3D.CrossProduct(upDirection, zaxis);
            xaxis.Normalize();

            Vector3D yaxis = Vector3D.CrossProduct(zaxis, xaxis);

            Vector3D positionVec = (Vector3D) position;
            double cx = -Vector3D.DotProduct(xaxis, positionVec);
            double cy = -Vector3D.DotProduct(yaxis, positionVec);
            double cz = -Vector3D.DotProduct(zaxis, positionVec);
            
            Matrix3D viewMatrix = new Matrix3D(
                xaxis.X, yaxis.X, zaxis.X, 0,
                xaxis.Y, yaxis.Y, zaxis.Y, 0,
                xaxis.Z, yaxis.Z, zaxis.Z, 0,
                cx, cy, cz, 1);

            PrependInverseTransform(transform, ref viewMatrix);

            return viewMatrix;
        }

        // http://blogs.msdn.com/cfs-file.ashx/__key/CommunityServer-Components-PostAttachments/00-04-01-86-12/SceneSortingHelper_2E00_cs

        /// <summary>
        /// Sort Modelgroups in Farthest to Closest order, to enable transparency
        /// Should be applied whenever the scene is significantly re-oriented
        /// </summary>
        public static void AlphaSort(Point3D cameraPosition, Model3DCollection models, Transform3D worldTransform)
        {
            var sortedList = models.OrderBy(model => Point3D.Subtract(cameraPosition, worldTransform.Transform(model.Bounds.Location)).Length);
            models.Clear();
            foreach (var model in sortedList)
            {
                models.Add(model);
            }
        }

        /// <summary>
        /// Exports the model.
        /// </summary>
        /// <param name="model">
        /// The model.
        /// </param>
        /// <param name="inheritedTransform">
        /// The inherited transform.
        /// </param>
        protected override void ExportModel(GeometryModel3D model, Transform3D inheritedTransform)
        {
            var mesh = model.Geometry as MeshGeometry3D;
            if (mesh == null)
            {
                return;
            }

            // todo
        }

 protected override void ExportModel(GeometryModel3D model, Transform3D transform)
 {
     writer.WriteLine(String.Format("o object{0}", objectNo++));
     writer.WriteLine(String.Format("g group{0}", groupNo++));
     string matName = String.Format("mat{0}", matNo++);
     writer.WriteLine(String.Format("usemtl {0}", matName));
     ExportMaterial(matName, model.Material, model.BackMaterial);
     var mesh = model.Geometry as MeshGeometry3D;
     ExportMesh(mesh, Transform3DHelper.CombineTransform(transform, model.Transform));
 }

        //------------------------------------------------------ 
        //
        //  Public Methods 
        //
        //-----------------------------------------------------

        //------------------------------------------------------ 
        //
        //  Public Properties 
        // 
        //------------------------------------------------------
 
        //-----------------------------------------------------
        //
        //  Public Events
        // 
        //------------------------------------------------------
 
        //----------------------------------------------------- 
        //
        //  Internal Methods 
        //
        //-----------------------------------------------------

 
        internal void PushVisualTransform(Transform3D transform)
        { 
            Debug.Assert(!HasModelTransformMatrix, 
                "ModelTransform stack should be empty when pusing a visual transform");
 
            if (transform != null && transform != Transform3D.Identity)
            {
                _visualTransformStack.Push(transform.Value);
            } 
        }

 private void AddContours(GeometryModel3D model, Transform3D transform)
 {
     var p = ContourPlane.Position;
     var n = ContourPlane.Normal;
     var segments = MeshGeometryHelper.GetContourSegments(model.Geometry as MeshGeometry3D, p, n).ToList();
     foreach (var contour in MeshGeometryHelper.CombineSegments(segments, 1e-6).ToList())
     {
         if (contour.Count == 0)
             continue;
         view2.Children.Add(new TubeVisual3D { Diameter = 0.03, Path = new Point3DCollection(contour), Fill = Brushes.Green });
     }
 }

 public static PerspectiveCamera GetTransformedPerspectiveCamera(PerspectiveCamera camera, Transform3D transform)
 {
     return new PerspectiveCamera
     {
         LookDirection = transform.Transform(camera.LookDirection),
         UpDirection = transform.Transform(camera.UpDirection),
         FieldOfView = camera.FieldOfView,
         FarPlaneDistance = camera.FarPlaneDistance,
         NearPlaneDistance = camera.NearPlaneDistance,
         Position = transform.Transform(camera.Position)
     };
 }

        /// <summary>
        /// Exports the model.
        /// </summary>
        /// <param name="model">
        /// The model.
        /// </param>
        /// <param name="inheritedTransform">
        /// The inherited transform.
        /// </param>
        protected override void ExportModel(GeometryModel3D model, Transform3D inheritedTransform)
        {
            var mesh = model.Geometry as MeshGeometry3D;
            if (mesh == null)
            {
                return;
            }

            // writer.WriteLine("Transform {");
            // todo: add transform from model.Transform and inheritedTransform
            this.writer.WriteLine("Shape {");

            this.writer.WriteLine("  appearance Appearance {");

            // todo: set material properties from model.Material
            this.writer.WriteLine("    material Material {");
            this.writer.WriteLine("      diffuseColor 0.8 0.8 0.2");
            this.writer.WriteLine("      specularColor 0.5 0.5 0.5");
            this.writer.WriteLine("    }");
            this.writer.WriteLine("  }"); // Appearance

            this.writer.WriteLine("  geometry IndexedFaceSet {");
            this.writer.WriteLine("    coord Coordinate {");
            this.writer.WriteLine("      point [");

            foreach (var pt in mesh.Positions)
            {
                this.writer.WriteLine(string.Format(CultureInfo.InvariantCulture, "{0} {1} {2},", pt.X, pt.Y, pt.Z));
            }

            this.writer.WriteLine("      ]");
            this.writer.WriteLine("    }");

            this.writer.WriteLine("    coordIndex [");
            for (int i = 0; i < mesh.TriangleIndices.Count; i += 3)
            {
                this.writer.WriteLine(
                    string.Format(
                        CultureInfo.InvariantCulture,
                        "{0} {1} {2},",
                        mesh.TriangleIndices[i],
                        mesh.TriangleIndices[i + 1],
                        mesh.TriangleIndices[i + 2]));
            }

            this.writer.WriteLine("    ]");
            this.writer.WriteLine("  }"); // IndexedFaceSet

            this.writer.WriteLine("}"); // Shape

            // writer.WriteLine("}"); // Transform
        }

 /// <summary>
 /// Combines two transforms.
 /// Null Values are treated like the Identity transform.
 /// </summary>
 /// <param name="t1">
 /// The first transform.
 /// </param>
 /// <param name="t2">
 /// The second transform.
 /// </param>
 /// <returns>
 /// The combined transform group.
 /// </returns>
 public static Transform3D CombineTransform(Transform3D t1, Transform3D t2)
 {
     if (t1 == null && t2 == null)
         return Transform3D.Identity;
     if (t1 == null && t2 != null)
         return t2;
     if (t1 != null && t2 == null)
         return t1;
     var g = new Transform3DGroup();
     g.Children.Add(t1);
     g.Children.Add(t2);
     return g;
 }

		public static Matrix3D CalculateRotationMatrix(double x, double y, double z, Point3D center, Vector3D up, Vector3D look, Transform3D transform)
		{
			//Transform3DGroup trm = new Transform3DGroup();
			//trm.Children.Add(transform);
			up = transform.Transform(up);
			look = transform.Transform(look);
			center = transform.Transform(center);
			Vector3D axisZ = Vector3D.CrossProduct(up, look);
			Matrix3D matrix = new Matrix3D();
			matrix.RotateAt(new Quaternion(axisZ, x), center);
			matrix.RotateAt(new Quaternion(up, y), center);
			matrix.RotateAt(new Quaternion(look, z), center);
			return matrix;
		}