在Windows Phone中進行3D開發之六旋轉

       上節內容中，我們讓物體擁有了最基本的平移和縮放的運動。現在我們來看三大運動之一的旋轉運動。

 

       同上節的知識一樣，要讓物體發生旋轉，只需要在對應的座標軸上與一個矩陣相乘。這個矩陣的構造方法有三種，分別是：

              Matrix  CreateRotationX(float radians);

              Matrix  CreateRotationY(floatradians);

              Matrix  CreateRotationZ(float radians);

代表在x、y、z三個座標軸上的旋轉角度，其中radians是弧度表示，如果想使用我們熟悉的角度的話，可以使用MathHelper.ToRadians(10)進行轉換。

       除了沿著座標軸旋轉以外，還有一種是採用飛行器的偏航、翻滾、俯仰表示方式進行的旋轉，使用的矩陣構造方法是：

              Matrix  CreateFromYawPitchRoll(float yaw, float pitch, float roll);

有興趣的朋友可以自行學習。

 

       還是沿用上節課我們建好的XNA項目，在VS2010中開啟該項目。開啟Game1.cs檔案，我們來修改Game1類。為其新增成員變數，代表旋轉矩陣：

               Matrix  rotateMatrix=Matrix.Identity;

同時為了便於觀察，我們將上節使用的scaleMatrix取值為0.5倍的縮放：

              Matrixs  caleMatrix = Matrix.CreateScale(0.5f);

然後在Draw()方法中修改basicEffect對象的World屬性：

              basicEffect.World= scaleMatrix * translateMatrix * rotateMatrix;

在Update()方法中去掉對scaleMatrix的變化並增加對rotateMatrix的處理：

              //scaleMatrix= Matrix.CreateScale(0.9f);

              rotateMatrix*= Matrix.CreateRotationZ(MathHelper.ToRadians(10));

也即每次點擊螢幕產生10度的旋轉。

      

       運行程式，觀察旋轉的結果。然後嘗試將Draw()中的

              basicEffect.World= scaleMatrix * translateMatrix * rotateMatrix;

改成

              basicEffect.World= scaleMatrix * rotateMatrix * translateMatrix;

來觀察對旋轉的影響。從中可以看出，前者是先沿著物體的x轉產生了平移，然後再按平移後所在的位置的z軸進行旋轉，產生了像螺線一樣的運動。而後者是先沿z軸進行了旋轉，再沿x軸進行平移，產生了一邊向螢幕右側平移同時繞自身z軸旋轉的效果。因此，在實際項目中要小心處理矩陣運算。

 

       另外，也可以修改代碼，觀察沿x、y軸旋轉的情況。

 

       當我們讓三角形繞y軸旋轉的時候，會發現一個奇怪的現象，隨著物體的旋轉，當物體的背面朝向我們時，我們就看不到它了。其實這是渲染引擎的最佳化設定，因為對於物體的背面，實際上是不進行渲染的，也就是常說的背面消隱，有需要關注的朋友可以尋找相關的資料，在這裡要想取消背面消隱選項，只需要修改一下GraphicsDevice的柵格化參數，即在Draw()方法中加入下面的代碼：

           GraphicsDevice.Clear(Color.CornflowerBlue);

           RasterizerState rasterizerState = new RasterizerState();

           rasterizerState.CullMode = CullMode.None;

           GraphicsDevice.RasterizerState = rasterizerState;

 

       附本節Game1類的完整源碼：

    public class Game1 : Microsoft.Xna.Framework.Game    {        GraphicsDeviceManager graphics;        Camera camera;        Matrix world = Matrix.Identity;        BasicEffect basicEffect;        VertexPositionColor[] triangle;        Matrix translateMatrix=Matrix.Identity;        Matrix scaleMatrix = Matrix.CreateScale(0.5f);        Matrix rotateMatrix = Matrix.Identity;        public Game1()        {            graphics = new GraphicsDeviceManager(this);            Content.RootDirectory = "Content";            // Frame rate is 30 fps by default for Windows Phone.            TargetElapsedTime = TimeSpan.FromTicks(333333);            // Extend battery life under lock.            InactiveSleepTime = TimeSpan.FromSeconds(1);            graphics.IsFullScreen = true;        }        /// <summary>        /// Allows the game to perform any initialization it needs to before starting to run.        /// This is where it can query for any required services and load any non-graphic        /// related content.  Calling base.Initialize will enumerate through any components        /// and initialize them as well.        /// </summary>        protected override void Initialize()        {            // TODO: Add your initialization logic here            base.Initialize();        }        /// <summary>        /// LoadContent will be called once per game and is the place to load        /// all of your content.        /// </summary>        protected override void LoadContent()        {            camera = new Camera(this, new Vector3(0, 0, 5), Vector3.Zero, Vector3.Up, MathHelper.PiOver4, GraphicsDevice.Viewport.AspectRatio, 1.0f, 50.0f);            Components.Add(camera);            basicEffect = new BasicEffect(GraphicsDevice);            triangle = new VertexPositionColor[]{                new VertexPositionColor(new Vector3(0, 1, 0), Color.Red),                new VertexPositionColor(new Vector3(1, -1, 0), Color.Green),                new VertexPositionColor(new Vector3(-1,-1, 0), Color.Blue)            };        }        /// <summary>        /// UnloadContent will be called once per game and is the place to unload        /// all content.        /// </summary>        protected override void UnloadContent()        {            // TODO: Unload any non ContentManager content here        }        /// <summary>        /// Allows the game to run logic such as updating the world,        /// checking for collisions, gathering input, and playing audio.        /// </summary>        /// <param name="gameTime">Provides a snapshot of timing values.</param>        protected override void Update(GameTime gameTime)        {            // Allows the game to exit            if (GamePad.GetState(PlayerIndex.One).Buttons.Back == ButtonState.Pressed)                this.Exit();            TouchPanel.EnabledGestures = GestureType.Tap;            if (TouchPanel.IsGestureAvailable)            {                GestureSample gestureSample = TouchPanel.ReadGesture();                if (gestureSample.GestureType == GestureType.Tap)                {                    translateMatrix *= Matrix.CreateTranslation(0.3f, 0, 0);                     //scaleMatrix = Matrix.CreateScale(0.9f);                    rotateMatrix *= Matrix.CreateRotationY(MathHelper.ToRadians(10));                }            }            base.Update(gameTime);        }        /// <summary>        /// This is called when the game should draw itself.        /// </summary>        /// <param name="gameTime">Provides a snapshot of timing values.</param>        protected override void Draw(GameTime gameTime)        {            GraphicsDevice.Clear(Color.CornflowerBlue);            RasterizerState rasterizerState = new RasterizerState();            rasterizerState.CullMode = CullMode.None;            GraphicsDevice.RasterizerState = rasterizerState;            basicEffect.World = scaleMatrix * translateMatrix * rotateMatrix;            basicEffect.View = camera.view;            basicEffect.Projection = camera.projection;                        foreach (EffectPass pass in basicEffect.CurrentTechnique.Passes)            {                pass.Apply();                GraphicsDevice.DrawUserPrimitives<VertexPositionColor>(PrimitiveType.TriangleStrip, triangle, 0, 1);            }            base.Draw(gameTime);        }    }

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