XNA学习笔记——线、三角形的绘制

   1: namespace WindowsGame4

   2: {

   3:     public class Game1 : Microsoft.Xna.Framework.Game

   4:     {

   5:         GraphicsDeviceManager graphics;

   6:         SpriteBatch spriteBatch;

   7:  

   8:         public Game1()

   9:         {

  10:             graphics = new GraphicsDeviceManager(this);

  11:             Content.RootDirectory = "Content";

  12:         }

  13:  

  14:         protected override void Initialize()

  15:         {

  16:             // TODO: Add your initialization logic here

  17:  

  18:             base.Initialize();

  19:         }

  20:  

  21:         protected override void LoadContent()

  22:         {

  23:             // Create a new SpriteBatch, which can be used to draw textures.

  24:             spriteBatch = new SpriteBatch(GraphicsDevice);

  25:  

  26:             // TODO: use this.Content to load your game content here

  27:         }

  28:  

  29:         protected override void UnloadContent()

  30:         {

  31:             // TODO: Unload any non ContentManager content here

  32:         }

  33:  

  34:         protected override void Update(GameTime gameTime)

  35:         {

  36:             // Allows the game to exit

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

  38:                 this.Exit();

  39:  

  40:             // TODO: Add your update logic here

  41:  

  42:             base.Update(gameTime);

  43:         }

  44:         protected override void Draw(GameTime gameTime)

  45:         {

  46:             GraphicsDevice.Clear(Color.CornflowerBlue);

  47:  

  48:             // TODO: Add your drawing code here

  49:  

  50:             base.Draw(gameTime);

  51:         }

  52:     }

  53: }

上面就是XNA自动生成的代码，整个类执行的流程如下图所示： http://img.ddvip.com/2009_03_14/1237027284_ddvip_4250.jpeg
按F5，即可运行，弹出如下对话框：  
默认是800x600

1、修改窗口的宽度和高度，还有标题。
在构造函数中添加下面两行代码

   1: Window.Title = "Hello XNA";

   2: graphics.PreferredBackBufferWidth = 640;

   3: graphics.PreferredBackBufferHeight = 480;

注：是在构造函数中，如果添加在Initialize()，没有效果。graphics的类型是GraphicsDeviceManager，也就是图形设备管理器，相当于D3D中的
D3D对象（LPDIRECT3D9   pD3D）。GraphicsDevice device = graphics.GraphicsDevice 通过图形设备管理器得到图形设备，相当于D3D中的设备对象（LPDIRECT3DDEVICE9  pd3dDevice ）。图形设备对象可以用来绘制。

2、设置观察矩阵（Matrix viewMatrix）和投影矩阵（Matrix projectMatrix）

   1: protected override void Initialize()

   2: {

   3:     // TODO: Add your initialization logic here

   4:            

   5:     float viewAngle = MathHelper.PiOver4;

   6:     float aspectRatio = graphics.GraphicsDevice.Viewport.AspectRatio;

   7:     float nearPlane = 0.1f;

   8:     float farPlane = 1000.0f;

   9:     projectMatrix = Matrix.CreatePerspectiveFieldOfView(viewAngle, aspectRatio, nearPlane, farPlane);

  10:  

  11:     Vector3 camPostion = new Vector3(10, 10, -10);

  12:     Vector3 camTarget = new Vector3(0, 0, 0);

  13:     Vector3 camUpVector = new Vector3(0, 1, 0);

  14:     viewMatrix = Matrix.CreateLookAt(camPostion, camTarget, camUpVector);

  15:  

  16:     base.Initialize();

  17:  

  18: }

3、设置顶点

   1: private void InitVertices()

   2: {

   3:     vertices = new VertexPositionColor[30];

   4:  

   5:     vertices[0] = new VertexPositionColor(new Vector3(0, 0, 0), Color.Red);

   6:     vertices[1] = new VertexPositionColor(Vector3.Right * 5, Color.White);

   7:     vertices[2] = new VertexPositionColor(new Vector3(5, 0, 0), Color.White);

   8:     vertices[3] = new VertexPositionColor(new Vector3(4.5f, 0.5f, 0), Color.Plum);

   9:     vertices[4] = new VertexPositionColor(new Vector3(5, 0, 0), Color.White);

  10:     vertices[5] = new VertexPositionColor(new Vector3(4.5f, -0.5f, 0), Color.White);

  11:  

  12:     vertices[6] = new VertexPositionColor(new Vector3(0, 0, 0), Color.White);

  13:     vertices[7] = new VertexPositionColor(Vector3.Up * 5, Color.White);

  14:     vertices[8] = new VertexPositionColor(new Vector3(0, 5, 0), Color.White);

  15:     vertices[9] = new VertexPositionColor(new Vector3(0.5f, 4.5f, 0), Color.White);

  16:     vertices[10] = new VertexPositionColor(new Vector3(0, 5, 0), Color.White);

  17:     vertices[11] = new VertexPositionColor(new Vector3(-0.5f, 4.5f, 0), Color.White);

  18:  

  19:     vertices[12] = new VertexPositionColor(new Vector3(0, 0, 0), Color.White);

  20:     vertices[13] = new VertexPositionColor(Vector3.Forward * 5, Color.White);

  21:     vertices[14] = new VertexPositionColor(new Vector3(0, 0, -5), Color.White);

  22:     vertices[15] = new VertexPositionColor(new Vector3(0, 0.5f, -4.5f), Color.White);

  23:     vertices[16] = new VertexPositionColor(new Vector3(0, 0, -5), Color.White);

  24:     vertices[17] = new VertexPositionColor(new Vector3(0, -0.5f, -4.5f), Color.White);

  25:  

  26:     vertices[18].Position = new Vector3(3, 0, 0);

  27:     vertices[18].Color = Color.Blue;

  28:  

  29:     vertices[19].Position = new Vector3(0, 3, 0);

  30:     vertices[19].Color = Color.Red;

  31:  

  32:     vertices[20].Position = new Vector3(0, 0, -3);

  33:     vertices[20].Color = Color.Green;

  34: }

private VertexPositionColor[] vertices; //VertexPositionColor表示该Vertex中有位置(Position)和颜色(Color)信息
private VertexDeclaration vertDeclaration;//VertexDeclaration是用来声明该Vertex中有哪些信息（位置、颜色、纹理、发现）
看下面的代码

   1: public void DrawUsingPresetEffect()

   2: {

   3:     device.VertexDeclaration = new VertexDeclaration(device, VertexPositionColor.VertexElements);

   4:     device.DrawUserPrimitives<VertexPositionColor>(PrimitiveType.LineList, vertices, 0, 12);

   5:     device.DrawUserPrimitives<VertexPositionColor>(PrimitiveType.TriangleList, vertices, 18, 1);

   6: }

device.vertDeclaration = new VertexDeclaration(device, VertexPositionColor.VertexElements);
DrawUserPrimitives中的第三个参数表示从哪个位置开始，第四个参数表示绘制多少个图元
0，12 表示vertices数组中从下标为0开始，绘制12条直线。18，1表示vertices数组中从下标18开始，绘制一个三角形
注：XNA的坐标系是右手坐标系，和D3D不同。

4、BasicEffect

一开始接触的是2D，绘制用的是SpriteBatch的Draw方法。到了3D，就迷糊了。

   1: protected override void Draw(GameTime gameTime)

   2: {

   3:     GraphicsDevice.Clear(ClearOptions.Target | ClearOptions.DepthBuffer, Color.CornflowerBlue, 1, 0);

   4:  

   5:     // TODO: Add your drawing code here

   6:     basicEffect.World = Matrix.Identity;

   7:     basicEffect.View = viewMatrix;

   8:     basicEffect.Projection = projectMatrix;

   9:     basicEffect.VertexColorEnabled = true;//如果没有这句，颜色的效果体现不出来

  10:     basicEffect.Begin();

  11:     foreach (EffectPass pass in basicEffect.CurrentTechnique.Passes)

  12:     {

  13:         pass.Begin();

  14:         cCross.DrawUsingPresetEffect();

  15:         pass.End();

  16:     }

  17:     basicEffect.End();

  18:     base.Draw(gameTime);

  19: }

以前用的FFP（Fixed Function Pipeline 固定渲染管道）XNA已不再支持。XNA默认的是可编程的渲染管道，进行顶点渲染（Vertex Shader ）和像素渲染（Pixel Shader），用HLSL编写，也就是说在3D中绘制物体都要用到HLSL。但由于HLSL的语法接近于C，所以在XNA中提供了一个BasicEffect这样的类，所有绘制在basicEffect.Begin()和basicEffect.End()中。像代码中的pass，Technique都是HLSL中的东西。需要对HLSL有一个基本的了解。

注:ClearOptions.Target是颜色缓存的刷新。上面漏了一个非常重要的知识点：在Draw函数的开始添加device.RenderState.CullMode = CullMode.None; 不然绘制的三角形有50%是看不见的。这就是背面剔除。

5、程序逻辑

   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:         || Keyboard.GetState().IsKeyDown(Keys.Escape))

   6:         this.Exit();

   7:  

   8:     // TODO: Add your update logic here          

   9:     base.Update(gameTime);

  10: }

键盘状态的判断：Keyboard.GetState().IsKeyDown(Keys.Escape)
注：关于Update的参数GameTime。因为XNA默认的刷新频率是60Hz。也就是1秒钟，调用Update方法60次。但是游戏的帧数并不一定是60fps，所以提供了GameTime，可以根据相关的属性进行设置。下面是其中一种方式。

   1: if(timeElapse >= ((double)1/fps)*1000)

   2: {

   3:     hero.UpdateHero();

   4:     timeElapse = 0;  

   5: }

   6: else

   7: {

   8:     timeElapse += gameTime.ElapsedGameTime.Milliseconds;

   9: }