XNA学习笔记——Quake式相机

通常，在FPS游戏中，玩家可以自由地上下左右旋转观察，这些运动对应绕Up（y轴）和Right（x轴）的旋转，或更具体地说，绕(0,1,0)和(1,0,0)方向的旋转。但是绕z轴的旋转是不被允许的，这会导致玩家向左或向右弯曲脖子，在第一人称游戏中不使用，除非玩家被击中倒在地板上。矩阵的乘法顺序是很重要的，因为当你组合两个旋转时，第二个旋转轴会被第一个旋转所改变。

简单地说，需要存储两个变量：绕Up方向的旋转量和绕Right方向的旋转量。然后，当需要计算总的旋转量时，使用“在绕Up旋转之后绕Right旋转”组合两个旋转。输入方面，使用鼠标旋转相机，使用键盘移动相机，再将输入与旋转量联系起来。有了前面的基础，这里就容易多了。

先看Initialize()

   1: protected override void Initialize()

   2: {

   3:     // TODO: Add your initialization logic her

   4:     updownRot = 0.0f;//上下旋转角度

   5:     rightleftRot = 0.0f;//左右旋转角度

   6:     rotSpeed = 0.05f;//旋转速率

   7:     moveSpeed = 0.05f;//移动速率

   8:     Mouse.SetPosition(Window.ClientBounds.Width / 2, Window.ClientBounds.Height / 2);

   9:     originalMState = Mouse.GetState();

  10:    

  11:  

  12:     float fov = MathHelper.PiOver4;

  13:     float aspect = GraphicsDevice.DisplayMode.AspectRatio;

  14:     float nearClip = 0.5f;

  15:     float farClip = 100.0f;

  16:     projMat = Matrix.CreatePerspectiveFieldOfView(fov, aspect, nearClip, farClip);

  17:  

  18:     camPosition = new Vector3(0, 2, 10);//相机初始位置

  19:     originalForward = new Vector3(0, 0, -1);//初始方向

  20:     originalUp = new Vector3(0, 1, 0);

  21:    

  22:     UpadteViewMatrix();

  23:     base.Initialize();

  24: }

然后是UpdateViewMatrix()

   1: private void UpadteViewMatrix()

   2: {

   3:     cameraRotation = Matrix.CreateRotationX(updownRot) * Matrix.CreateRotationY(rightleftRot);

   4:   

   5:     rotatedForward = Vector3.Transform(originalForward, cameraRotation);

   6:     rotatedUp = Vector3.Transform(originalUp, cameraRotation);

   7:  

   8:     finalLook = camPosition + rotatedForward;

   9:     //finalUp = camPosition + rotatedUp;

  10:     finalUp = rotatedUp;

  11:  

  12:     viewMat = Matrix.CreateLookAt(camPosition, finalLook, finalUp);

  13: }

旋转矩阵设在UpdateViewMatrix中是因为updownRot和rightleftRot是经常变化的。对于finalLook，上一次记录中已经讲过了。但有没有注意到，
finalUp = rotatedUp。这是因为FPS相机中，不是任意旋转的。finalUp是一个方向，而finalLook是一个视点，rotatedForward才是方向。也就是说
finalUp和rotatedUp都是原始方向进过一次旋转变换而来，而finalLook是需要Position和rotatedForward叠加而成。有点啰嗦了。

再是Update()

   1: protected override void Update(GameTime gameTime)

   2: {

   3:     // Allows the game to exit

   4:     if (GamePad.GetState(PlayerIndex.One).Buttons.Back == ButtonState.Pressed)

   5:         this.Exit();

   6:  

   7:     // TODO: Add your update logic here

   8:     currentMState = Mouse.GetState();

   9:     if (currentMState != originalMState)

  10:     {

  11:         float xdiffence = currentMState.X - originalMState.X;

  12:         float ydiffence = currentMState.Y - originalMState.Y;

  13:  

  14:         rightleftRot -= rotSpeed * xdiffence;

  15:         updownRot -= rotSpeed * ydiffence;

  16:  

  17:         Mouse.SetPosition(Window.ClientBounds.Width/2, Window.ClientBounds.Height/2);

  18:         UpadteViewMatrix();

  19:     }

  20:  

  21:     kState = Keyboard.GetState();

  22:     if (kState.IsKeyDown(Keys.W))

  23:         MoveCameraPosition(Vector3.Forward);

  24:     else if (kState.IsKeyDown(Keys.S))

  25:         MoveCameraPosition(Vector3.Backward);

  26:     else if (kState.IsKeyDown(Keys.A))

  27:         MoveCameraPosition(Vector3.Left);

  28:     else if (kState.IsKeyDown(Keys.D))

  29:         MoveCameraPosition(Vector3.Right);

  30:  

  31:     base.Update(gameTime);

  32: }

因为旋转量由鼠标控制。鼠标往右，即人往右看，原本在屏幕中央的物体就往左移动（逆时针）。鼠标往左，即人往左看，原本在屏幕中央的物体就往右移动（顺时针）。而到底旋转多少，由偏离屏幕中央的距离决定。旋转量（updownRot和rightleftRot）变化之后，就是更新观察矩阵（调用UpdateViewMatrix）。旋转矩阵会影响观察矩阵的视点和Up方向，从而影响平移的方向。所以关于键盘的操作在调用UpdateViewMatrix之后。

接下来是MoveCameraPosition()

   1: private void MoveCameraPosition(Vector3 direction)

   2: {

   3:     //Vector3 originalForward = new Vector3(-1.7f, -1, -1);

   4:     Vector3 rotatedForward = Vector3.Transform(direction, cameraRotation);

   5:     camPosition += moveSpeed * rotatedForward;

   6:     UpadteViewMatrix();

   7:  

   8: }

和上一个记录一样。