// Define the model.
        private void DefineModel()
        {
            // Axes.
            //MainGroup.Children.Add(MeshExtensions.XAxisModel(4, 0.1));
            //MainGroup.Children.Add(MeshExtensions.YAxisModel(4, 0.1));
            //MainGroup.Children.Add(MeshExtensions.ZAxisModel(4, 0.1));
            //MainGroup.Children.Add(MeshExtensions.OriginModel(0.12));

            // Make the ground.
            MeshGeometry3D groundMesh = new MeshGeometry3D();
            const double   dx         = 4;
            const double   dy         = 1;
            const double   dz         = dx;
            const double   groundY    = -2;
            Point3D        corner     = new Point3D(-dx / 2, groundY - dy, -dz / 2);

            Point[] topCoords =
            {
                new Point(0,   0),
                new Point(0,  dz),
                new Point(dx, dz),
                new Point(dx,  0),
            };
            Point[] sideCoords =
            {
                new Point(0,   0),
                new Point(0,  dz),
                new Point(dy, dz),
                new Point(dy,  0),
            };
            groundMesh.AddBox(corner, D3.XVector(dx), D3.YVector(dy), D3.ZVector(dz),
                              sideCoords, sideCoords, sideCoords, sideCoords, topCoords, topCoords);
            MainGroup.Children.Add(groundMesh.MakeModel("metal.jpg"));

            // This group represents the whole robot.
            RobotGroup = new Model3DGroup();
            MainGroup.Children.Add(RobotGroup);

            // Base.
            const double   baseWidth  = 0.4;
            const double   baseLength = 2.5;
            MeshGeometry3D baseMesh   = new MeshGeometry3D();

            baseMesh.AddBox(new Point3D(-baseWidth / 2, 0, -baseWidth / 2),
                            D3.XVector(baseWidth), D3.YVector(baseLength), D3.ZVector(baseWidth));
            GeometryModel3D baseModel = baseMesh.MakeModel(Brushes.Pink);

            const int    numCyl       = 20;
            const double bJointWidth  = 0.5;
            const double bJointRadius = 0.5;

            Point3D[] bPgon = G3.MakePolygonPoints(numCyl, new Point3D(0, 0, 0),
                                                   D3.XVector(bJointRadius), D3.ZVector(-bJointRadius));
            baseMesh.AddCylinder(bPgon, D3.YVector(bJointWidth), true);

            BaseGroup = JoinBones(RobotGroup, new TranslateTransform3D(0, groundY, 0));
            BaseGroup.Children.Add(baseModel);

            // Shoulder.
            const double   shoulderWidth  = 0.4;
            const double   shoulderLength = 2;
            MeshGeometry3D shoulderMesh   = new MeshGeometry3D();

            shoulderMesh.AddBox(new Point3D(-shoulderWidth / 2, 0, -shoulderWidth / 2),
                                D3.XVector(shoulderWidth), D3.YVector(shoulderLength), D3.ZVector(shoulderWidth));

            const double sJointWidth  = 0.5;
            const double sJointRadius = 0.4;

            Point3D[] sPgon = G3.MakePolygonPoints(numCyl, new Point3D(0, 0, -sJointWidth / 2),
                                                   D3.XVector(sJointRadius), D3.YVector(sJointRadius));
            shoulderMesh.AddCylinder(sPgon, D3.ZVector(sJointWidth), true);
            GeometryModel3D shoulderModel = shoulderMesh.MakeModel(Brushes.LightGreen);

            ShoulderGroup = JoinBones(BaseGroup, new TranslateTransform3D(0, baseLength, 0));
            ShoulderGroup.Children.Add(shoulderModel);

            // Elbow.
            const double   elbowWidth  = 0.4;
            const double   elbowLength = 1.5;
            MeshGeometry3D elbowMesh   = new MeshGeometry3D();

            elbowMesh.AddBox(new Point3D(-elbowWidth / 2, 0, -elbowWidth / 2),
                             D3.XVector(elbowWidth), D3.YVector(elbowLength), D3.ZVector(elbowWidth));

            const double eJointWidth  = 0.5;
            const double eJointRadius = 0.4;

            Point3D[] ePgon = G3.MakePolygonPoints(numCyl, new Point3D(0, 0, -eJointWidth / 2),
                                                   D3.XVector(eJointRadius), D3.YVector(eJointRadius));
            elbowMesh.AddCylinder(ePgon, D3.ZVector(eJointWidth), true);
            GeometryModel3D elbowModel = elbowMesh.MakeModel(Brushes.LightBlue);

            ElbowGroup = JoinBones(ShoulderGroup, new TranslateTransform3D(0, shoulderLength, 0));
            ElbowGroup.Children.Add(elbowModel);

            // Wrist.
            const double   wDx       = 1.5;
            const double   wDy       = 0.2;
            const double   wDz       = 0.4;
            MeshGeometry3D wristMesh = new MeshGeometry3D();

            wristMesh.AddBox(new Point3D(-wDx / 2, 0, -wDz / 2),
                             D3.XVector(wDx), D3.YVector(wDy), D3.ZVector(wDz));

            const double wJointRadius = 0.3;

            Point3D[] wPgon = G3.MakePolygonPoints(numCyl, new Point3D(0, -wDy / 2, 0),
                                                   D3.XVector(wJointRadius), D3.ZVector(-wJointRadius));
            wristMesh.AddCylinder(wPgon, D3.YVector(wDy), true);
            GeometryModel3D wristModel = wristMesh.MakeModel(Brushes.Red);

            WristGroup = JoinBones(ElbowGroup, new TranslateTransform3D(0, elbowLength, 0));
            WristGroup.Children.Add(wristModel);

            // Finger 1.
            const double   fDx         = 0.1;
            const double   fDy         = 0.5;
            const double   fDz         = 0.2;
            MeshGeometry3D finger1Mesh = new MeshGeometry3D();

            finger1Mesh.AddBox(new Point3D(-fDx / 2, 0, -fDz / 2),
                               D3.XVector(fDx), D3.YVector(fDy), D3.ZVector(fDz));
            GeometryModel3D finger1Model = finger1Mesh.MakeModel(Brushes.Green);

            Finger1Group = JoinBones(WristGroup, new TranslateTransform3D(-fDx / 2, wDy, 0));
            Finger1Group.Children.Add(finger1Model);

            // Finger 2.
            MeshGeometry3D finger2Mesh = new MeshGeometry3D();

            finger2Mesh.AddBox(new Point3D(-fDx / 2, 0, -fDz / 2),
                               D3.XVector(fDx), D3.YVector(fDy), D3.ZVector(fDz));
            GeometryModel3D finger2Model = finger2Mesh.MakeModel(Brushes.Green);

            Finger2Group = JoinBones(WristGroup, new TranslateTransform3D(fDx / 2, wDy, 0));
            Finger2Group.Children.Add(finger2Model);
        }
 private void elbowSlider_ValueChanged(object sender, RoutedPropertyChangedEventArgs <double> e)
 {
     ElbowGroup.Transform = D3.Rotate(D3.ZVector(), D3.Origin, elbowSlider.Value);
 }