更新VertexBuffer中顶点的位置

我正尝试着在XNA 4.0中创造一个粒子系统。粒子在初始化发射器的世界中被渲染。问题是，我想在update()上更新粒子的位置，但它不起作用，粒子被渲染，但只在世界的中心位置0,0,0。

每个粒子由一组带有纹理的顶点组成。

有谁能帮我吗?下面是粒子发射器的类:

public class ParticleEmitter : RenderableGameObject
{
    VertexBuffer vertexBuffer;
    VertexDeclaration vertexDeclaration;
    Effect bbEffect;
    VertexPositionTexture[] vertices;
    Texture2D texture;
    int noVertices = 0;
    struct Particle
    {
        public Vector3 position;
    }
    Particle[] particles = new Particle[5];
    public ParticleEmitter(Game1 game)
        : base(game)
    {
        SetupParticles();
        SetupVertices();
        bbEffect = game.Content.Load<Effect>("Effects/Particle");
        texture = game.Content.Load<Texture2D>("Textures/fire");
    }
    private void SetupVertices()
    {
        int vertexIndex = 0;
        vertices = new VertexPositionTexture[particles.Length * 6];
        for (int i = 0; i < particles.Length; ++i)
        {
            vertices[vertexIndex++] = new VertexPositionTexture(particles[i].position, new Vector2(0, 0));
            vertices[vertexIndex++] = new VertexPositionTexture(particles[i].position, new Vector2(1, 0));
            vertices[vertexIndex++] = new VertexPositionTexture(particles[i].position, new Vector2(1, 1));
            vertices[vertexIndex++] = new VertexPositionTexture(particles[i].position, new Vector2(0, 0));
            vertices[vertexIndex++] = new VertexPositionTexture(particles[i].position, new Vector2(1, 1));
            vertices[vertexIndex++] = new VertexPositionTexture(particles[i].position, new Vector2(0, 1));
        }
        noVertices = vertexIndex;
        vertexBuffer = new VertexBuffer(game.GraphicsDevice, typeof(VertexPositionTexture), vertices.Length, BufferUsage.WriteOnly);
        vertexBuffer.SetData(vertices);
        vertexDeclaration = new VertexDeclaration(VertexPositionTexture.VertexDeclaration.GetVertexElements());
    }
    private void SetupParticles()
    {
        for (int i = 0; i < particles.Length; ++i)
        {
            particles[i] = new Particle();
            particles[i].position = Position * i;
        }
    }
    private void UpdateVertices()
    {
        for (int i = 0; i < particles.Length; ++i)
        {
            vertices[i] = new VertexPositionTexture(particles[i].position, new Vector2(0, 0));
            vertices[i] = new VertexPositionTexture(particles[i].position, new Vector2(1, 0));
            vertices[i] = new VertexPositionTexture(particles[i].position, new Vector2(1, 1));
            vertices[i] = new VertexPositionTexture(particles[i].position, new Vector2(0, 0));
            vertices[i] = new VertexPositionTexture(particles[i].position, new Vector2(1, 1));
            vertices[i] = new VertexPositionTexture(particles[i].position, new Vector2(0, 1));
        }
        game.GraphicsDevice.SetVertexBuffer(null);
        vertexBuffer.SetData(vertices);
    }
    private void UpdateParticles()
    {
        for (int i = 0; i < particles.Length; ++i)
        {
            particles[i].position = Position;
        }
    }
    public override void Update(GameTime gameTime)
    {
        UpdateParticles();
        UpdateVertices();
        base.Update(gameTime);
    }
    public override void Draw()
    {
        DrawParticles(game.camera.viewMatrix);
    }
    private void DrawParticles(Matrix currentViewMatrix)
    {
        bbEffect.CurrentTechnique = bbEffect.Techniques["CylBillboard"];
        bbEffect.Parameters["xWorld"].SetValue(Matrix.Identity);
        bbEffect.Parameters["xView"].SetValue(currentViewMatrix);
        bbEffect.Parameters["xProjection"].SetValue(game.camera.projectionMatrix);
        bbEffect.Parameters["xCamPos"].SetValue(game.camera.Position);
        bbEffect.Parameters["xAllowedRotDir"].SetValue(new Vector3(0, 1, 0));
        bbEffect.Parameters["xBillboardTexture"].SetValue(texture);
        foreach (EffectPass pass in bbEffect.CurrentTechnique.Passes)
        {
            game.GraphicsDevice.BlendState = BlendState.AlphaBlend;
            pass.Apply();
            game.GraphicsDevice.SetVertexBuffer(vertexBuffer);
            int noTriangles = noVertices / 3;
            game.GraphicsDevice.DrawPrimitives(PrimitiveType.TriangleList, 0, noTriangles);
        }
    }
}

private void UpdateParticles()
{
    for (int i = 0; i < particles.Length; ++i)
    {
        particles[i].position = Position;
    }
}

struct Particle
{
    public Vector3 position;
    public Vector3 velocity;
}

// Pass a shared, per-thread instance of Random into this method
private void SetupParticles(Random random)
{
    for (int i = 0; i < particles.Length; ++i)
    {
        particles[i] = new Particle() {
            position = Vector.Zero,
            velocity = new Vector3((float)(random.NextDouble() * 2 - 1),
                                   (float)(random.NextDouble() * 2 - 1),
                                   (float)(random.NextDouble() * 2 - 1))
        };
    }
}

Vector3 gravity = new Vector3(0, -9.8f, 0);
private void UpdateParticles(float time)
{
    for (int i = 0; i < particles.Length; ++i)
    {
        particles[i].velocity += gravity * time;
        particles[i].position += particles[i].velocity * time;
    }
}