创建一个八面体(颠倒金字塔)

我正在使用OpenTK制作一个八面体。我已经创建了一个方形金字塔，需要在顶部的金字塔下面翻译另一个，并将其倒过来创建八面体。

我如何把第二个金字塔倒过来?

下面是我的代码:

#region --- Using Directives ---
using System;
using System.Collections.Generic;
using System.Windows.Forms;
using System.Threading;
using System.Drawing;
using OpenTK;
using OpenTK.Graphics;
using OpenTK.Graphics.OpenGL;
using OpenTK.Platform;
#endregion
namespace Octahedron
{
    public class Octahedron : GameWindow
    {
        #region --- Fields ---
        const float rotation_speed = 180.0f;
        float angle;
        #endregion
        #region --- Constructor ---
        public Octahedron()
            : base(800, 600)
        { }
        #endregion
        #region OnLoad
        protected override void OnLoad(EventArgs e)
        {
            base.OnLoad(e);
            GL.ClearColor(Color.MidnightBlue);
            GL.Enable(EnableCap.DepthTest);
        }
        #endregion
        #region OnResize
        protected override void OnResize(EventArgs e)
        {
            base.OnResize(e);
            GL.Viewport(0, 0, Width, Height);
            double aspect_ratio = Width / (double)Height;
            OpenTK.Matrix4 perspective = OpenTK.Matrix4.CreatePerspectiveFieldOfView(MathHelper.PiOver4, (float)aspect_ratio, 1, 64);
            GL.MatrixMode(MatrixMode.Projection);
            GL.LoadMatrix(ref perspective);
        }
        #endregion
        #region OnUpdateFrame
        protected override void OnUpdateFrame(FrameEventArgs e)
        {
            base.OnUpdateFrame(e);
            var keyboard = OpenTK.Input.Keyboard.GetState();
            if (keyboard[OpenTK.Input.Key.Escape])
            {
                this.Exit();
                return;
            }
        }
        #endregion
        #region OnRenderFrame
        protected override void OnRenderFrame(FrameEventArgs e)
        {
            base.OnRenderFrame(e);
            GL.Clear(ClearBufferMask.ColorBufferBit | ClearBufferMask.DepthBufferBit);
            Matrix4 lookat = Matrix4.LookAt(0, 5, 10, 0, 0, 2, 0, 5, 0);
            GL.MatrixMode(MatrixMode.Modelview);
            GL.LoadMatrix(ref lookat);
            angle += rotation_speed * (float)e.Time;
            GL.Rotate(angle, 0.0f, 1.0f, 0.0f);
            DrawPyramid();
            GL.Translate(0.0f, -2.0f, 0.0f);
            DrawPyramid();
            this.SwapBuffers();
            Thread.Sleep(1);
        }
        #endregion
        #region private void DrawPyramid()
        private void DrawPyramid()
        {
            GL.Begin(PrimitiveType.Triangles);
       //Side0 (red)
                                                  //x,    y,    z
            GL.Color3(1.0f, 0.0f, 0.0f); GL.Vertex3(0.0f, 1.0f, 0.0f);//Top Vertex
                                                   //x,     y,    z
            GL.Color3(1.0f, 0.0f, 0.0f); GL.Vertex3(-1.0f, -1.0f, 1.0f);//Bottom Left Vertex
                                                  //x,     y,    z
            GL.Color3(1.0f, 0.0f, 0.0f); GL.Vertex3(1.0f, -1.0f, 1.0f);//Bottom Right Vertex
      //Side1 (blue)
                                                  //x,    y,    z
            GL.Color3(0.0f, 0.0f, 1.0f); GL.Vertex3(0.0f, 1.0f, 0.0f);//Top Vertex
                                                  //x,    y,    z
            GL.Color3(0.0f, 0.0f, 1.0f); GL.Vertex3(1.0f, -1.0f, 1.0f);//Bottom Left Vertex
                                                  //x,    y,    z
            GL.Color3(0.0f, 0.0f, 1.0f); GL.Vertex3(1.0f, -1.0f, -1.0f);//Bottom Right Vertex
      // Side2 (yellow)
                                                  //x,    y,    z
            GL.Color3(1.0f, 1.0f, 0.0f); GL.Vertex3(0.0f, 1.0f, 0.0f);
                                                  //x,     y,     z
            GL.Color3(1.0f, 1.0f, 0.0f); GL.Vertex3(1.0f, -1.0f, -1.0f);
                                                   //x,     y,     z
            GL.Color3(1.0f, 1.0f, 0.0f); GL.Vertex3(-1.0f, -1.0f, -1.0f);
      // Side3 (pink)
                                                  //x,    y,    z
            GL.Color3(1.0f, 0.0f, 1.0f); GL.Vertex3(0.0f, 1.0f, 0.0f);
                                                   //x,     y,     z
            GL.Color3(1.0f, 0.0f, 1.0f); GL.Vertex3(-1.0f, -1.0f, -1.0f);
                                                   //x,     y,    z
            GL.Color3(1.0f, 0.0f, 1.0f); GL.Vertex3(-1.0f, -1.0f, 1.0f);
     //Side4 (red)
                                                  //x,    y,    z
            GL.Color3(1.0f, 1.0f, 1.0f); GL.Vertex3(0.0f, 1.0f, 0.0f);//Top Vertex
                                                   //x,     y,    z
            GL.Color3(1.0f, 1.0f, 1.0f); GL.Vertex3(-1.0f, -1.0f, 1.0f);//Bottom Left Vertex
                                                  //x,     y,    z
            GL.Color3(1.0f, 1.0f, 1.0f); GL.Vertex3(1.0f, -1.0f, 1.0f);//Bottom Right Vertex
            GL.End();
        }
        #endregion

        #region public static void Main()
        [STAThread]
        public static void Main()
        {
            using (Octahedron oct = new Octahedron())
            {
                oct.Run(60.0, 0.0);
            }
        }
        #endregion
    }
}