我的相机怎么了?

我使用VS2010与c#和XNA 4.0。我正在阅读Aaron Reed的《Learning XNA 4.0》一书。我将相机游戏组件添加到我的项目中，但是这个相机的行为真的很奇怪。移动鼠标使它"跳跃"，然后崩溃。我不知道是什么引起的。代码看起来和书上的几乎一模一样。它以前工作，但现在它不是，虽然我不记得我是否改变了什么(我想没有)。请注意，我只是在游戏中注册了这个组件，仅此而已。它应该起作用，但它没有。任何帮助都太好了。下面是代码:

using Microsoft.Xna.Framework;
using Microsoft.Xna.Framework.Input;
namespace MyGame
{
    public class Camera : Microsoft.Xna.Framework.GameComponent
    {
        public Matrix View { get; protected set; }
        public Matrix Projection { get; protected set; }
        public Vector3 CameraPosition { get; protected set; }
        public Vector3 CameraDirection { get; protected set; }
        public Vector3 CameraUp { get; protected set; }
        private MouseState prevMouseState;
        public Camera(Game game, Vector3 pos, Vector3 target, Vector3 up)
            : base(game)
        {
            // Some initialization of camera.
            CameraPosition = pos;
            CameraDirection = target - pos;
            CameraDirection.Normalize();
            CameraUp = up;
            CreateLookAt();
            Projection = Matrix.CreatePerspectiveFieldOfView(
                MathHelper.PiOver4,                
                (float)Game.Window.ClientBounds.Width /
                (float)Game.Window.ClientBounds.Height,
                1f, 3000f);
        }
        public override void Initialize()
        {
            // Set mouse position and do initial get state.
            Mouse.SetPosition(Game.Window.ClientBounds.Width / 2,
                Game.Window.ClientBounds.Height / 2);
            prevMouseState = Mouse.GetState();
            base.Initialize();
        }

        public override void Update(GameTime gameTime)
        {
            // Standard WSAD keyboard movement.
            if (Keyboard.GetState().IsKeyDown(Keys.W))
                CameraPosition += CameraDirection;
            if (Keyboard.GetState().IsKeyDown(Keys.S))
                CameraPosition -= CameraDirection;
            if (Keyboard.GetState( ).IsKeyDown(Keys.A))
                CameraPosition += Vector3.Cross(CameraUp, CameraDirection);
            if (Keyboard.GetState().IsKeyDown(Keys.D))
                CameraPosition -= Vector3.Cross(CameraUp, CameraDirection);
            // Yaw rotation (via mouse).
            CameraDirection = Vector3.Transform(CameraDirection,
                Matrix.CreateFromAxisAngle(CameraUp, (-MathHelper.PiOver4 / 150) *
                (Mouse.GetState().X - prevMouseState.X) 
                ));
            // Pitch rotation (via mouse).
            CameraDirection = Vector3.Transform(CameraDirection,
                Matrix.CreateFromAxisAngle(Vector3.Cross(CameraUp, CameraDirection),
                (MathHelper.PiOver4 / 100) *
                (Mouse.GetState().Y - prevMouseState.Y) 
                ));
            CameraUp = Vector3.Transform(CameraUp,
                Matrix.CreateFromAxisAngle(Vector3.Cross(CameraUp, CameraDirection),
                (MathHelper.PiOver4 / 100) *
                (Mouse.GetState().Y - prevMouseState.Y)
                ));
            prevMouseState = Mouse.GetState();
            CreateLookAt();
            base.Update(gameTime);
        }
        private void CreateLookAt()
        {
            View = Matrix.CreateLookAt(CameraPosition,
                CameraPosition + CameraDirection, CameraUp);
        }
    }
}

谁能检查一下这个代码是否适用于他?任何想法都很棒!

//编辑1:在游戏初始化中，我使用以下代码:

Camera cam = new Camera(this, new Vector3(0, 0, 100), Vector3.Zero, Vector3.Up);
Components.Add(cam);

，然后绘制模型。我使用cam的视图和投影矩阵在模型的BasicEffect。

//编辑2:出了严重的问题。我做了一些调试，似乎在某些时候那些Vector3对象的X,Y,Z属性设置为NaN。我不知道怎么回事，什么时候。而且(也许这没什么)在运行了一段时间后，gameTime显示经过的游戏时间只有16毫秒(就像它跳过帧一样?)并且游戏运行缓慢。无论我什么时候调试，这16毫秒总是一样的。

是否有可能我搞砸了XNA/c#设置?如果是这样，有人能给我点提示吗?

    // Pitch rotation (via mouse).
    CameraDirection = Vector3.Transform(CameraDirection,
        Matrix.CreateFromAxisAngle(Vector3.Cross(CameraUp, CameraDirection),
        (MathHelper.PiOver4 / 100) *
        (Mouse.GetState().Y - prevMouseState.Y) 
        ));
    CameraUp = Vector3.Transform(CameraUp,
        Matrix.CreateFromAxisAngle(Vector3.Cross(CameraUp, CameraDirection),
        (MathHelper.PiOver4 / 100) *
        (Mouse.GetState().Y - prevMouseState.Y)
        ));

Vector3.Transform(CameraDirection, Matrix.CreateFromAxisAngle(Vector3.Normalize(Vector3.Cross(CameraUp, CameraDirection)), ...