XNA 4.0 - Alpha和多个纹理..搞砸了

我做到了…方法如下:

using System;
using System.Collections.Generic;
using System.Linq;
using Microsoft.Xna.Framework;
using Microsoft.Xna.Framework.Audio;
using Microsoft.Xna.Framework.Content;
using Microsoft.Xna.Framework.GamerServices;
using Microsoft.Xna.Framework.Graphics;
using Microsoft.Xna.Framework.Input;
using Microsoft.Xna.Framework.Media;
namespace StencilTest
{
    public class Game1 : Microsoft.Xna.Framework.Game
    {
        GraphicsDeviceManager graphics;
        SpriteBatch spriteBatch;
        Texture2D star, cloud, shape;
        AlphaTestEffect alphaTestEffect;
        DepthStencilState stencilAlways;
        DepthStencilState stencilKeep;
        RenderTarget2D rt;
        public Game1()
        {
            graphics = new GraphicsDeviceManager(this);
            Content.RootDirectory = "Content";
            graphics.PreferredDepthStencilFormat = DepthFormat.Depth24Stencil8;
            graphics.PreferredBackBufferWidth = 800;
            graphics.PreferredBackBufferHeight = 600;
            IsMouseVisible = true;
        }
        protected override void Initialize()
        {
            base.Initialize();
        }
        Texture2D grass;
        protected override void LoadContent()
        {
            // Create a new SpriteBatch, which can be used to draw textures.
            spriteBatch = new SpriteBatch(GraphicsDevice);
            star = Content.Load<Texture2D>("star1");
            cloud = Content.Load<Texture2D>("cloud1");
            shape = Content.Load<Texture2D>("shape");
            back = Content.Load<Texture2D>("back");
            grass = Content.Load<Texture2D>("grass1");
            Matrix projection = Matrix.CreateOrthographicOffCenter(0, shape.Width, shape.Height, 0, 0, 1);
            Matrix halfPixelOffset = Matrix.CreateTranslation(-0.5f, -0.5f, 0);
            alphaTestEffect = new AlphaTestEffect(GraphicsDevice);
            alphaTestEffect.VertexColorEnabled = true;
            alphaTestEffect.DiffuseColor = Color.White.ToVector3();
            alphaTestEffect.AlphaFunction = CompareFunction.Equal;
            alphaTestEffect.ReferenceAlpha = 0;
            alphaTestEffect.World = Matrix.Identity;
            alphaTestEffect.View = Matrix.Identity;
            alphaTestEffect.Projection = halfPixelOffset * projection;
            // set up stencil state to always replace stencil buffer with 1
            stencilAlways = new DepthStencilState();
            stencilAlways.StencilEnable = true;
            stencilAlways.StencilFunction = CompareFunction.Always;
            stencilAlways.StencilPass = StencilOperation.Replace;
            stencilAlways.ReferenceStencil = 1;
            stencilAlways.DepthBufferEnable = false;
            // set up stencil state to pass if the stencil value is 1
            stencilKeep = new DepthStencilState();
            stencilKeep.StencilEnable = true;
            stencilKeep.StencilFunction = CompareFunction.Equal;
            stencilKeep.StencilPass = StencilOperation.Keep;
            stencilKeep.ReferenceStencil = 1;
            stencilKeep.DepthBufferEnable = false;
            rt = new RenderTarget2D(GraphicsDevice, shape.Width, shape.Height,
                                   false, SurfaceFormat.Color, DepthFormat.Depth24Stencil8,
                                   0, RenderTargetUsage.DiscardContents);
        }
        protected override void UnloadContent()
        {
        }
        protected override void Update(GameTime gameTime)
        {
            // Allows the game to exit
            if (Keyboard.GetState().IsKeyDown(Keys.Escape))
                this.Exit();
            // TODO: Add your update logic here
            base.Update(gameTime);
        }
        float angle = 0f;
        private Texture2D back;
        Vector2 pos = new Vector2(400, 300);
        float cloudscale = 0.25f;
        protected override void Draw(GameTime gameTime)
        {
            // set up rendering to the active render target
            GraphicsDevice.SetRenderTarget(rt);
            // clear the render target to opaque black,
            // and initialize the stencil buffer with all zeroes
            GraphicsDevice.Clear(ClearOptions.Target | ClearOptions.Stencil,
                                 new Color(0, 0, 0, 1), 0, 0);
            spriteBatch.Begin(SpriteSortMode.Immediate, BlendState.Opaque,
                       null, stencilAlways, null, alphaTestEffect);
            spriteBatch.Draw(shape, Vector2.Zero, null, Color.White, 0f,
                             Vector2.Zero, 1f, SpriteEffects.None, 0f);
            spriteBatch.End();
            spriteBatch.Begin(SpriteSortMode.Immediate, BlendState.AlphaBlend,
                  null, stencilKeep, null, null);
            //for (int i = 0; i < Math.Ceiling(800 / (cloud.Width * cloudscale)); i++)
            //    for (int j = 0; j < Math.Ceiling(600 / (cloud.Height * cloudscale)); j++)
            //        spriteBatch.Draw(cloud, Vector2.Zero + new Vector2(i * cloud.Width * cloudscale, j * cloud.Height * cloudscale), null, Color.White, 0f,
            //         Vector2.Zero, cloudscale, SpriteEffects.None, 0f);
            spriteBatch.Draw(grass, new Vector2(rt.Width / 2, rt.Height / 2) + new Vector2(0f, -100f), null, Color.White, 0f,
         new Vector2(grass.Width / 2, grass.Height / 2), .85f, SpriteEffects.None, 0f);
            spriteBatch.Draw(cloud, new Vector2(rt.Width/2, rt.Height/2), null, Color.White, 0f,
                     new Vector2(cloud.Width / 2, cloud.Height / 2), 1f, SpriteEffects.None, 0f);
            spriteBatch.Draw(star, new Vector2(rt.Width / 2, rt.Height / 2) + new Vector2(0f, 100f), null, Color.White, 0f,
         new Vector2(star.Width / 2, star.Height / 2), .85f, SpriteEffects.None, 0f);
            spriteBatch.End();
            GraphicsDevice.SetRenderTarget(null);
            spriteBatch.Begin();
            spriteBatch.Draw(back, Vector2.Zero, null, Color.White, 0f, Vector2.Zero, 1f, SpriteEffects.None, 0f);
            spriteBatch.Draw(rt, pos + new Vector2(50f * (float)Math.Cos(MathHelper.ToRadians(angle)), 50f * (float)Math.Sin(MathHelper.ToRadians(angle))), null, Color.White, 0f, new Vector2(rt.Width / 2, rt.Height / 2), 1f, SpriteEffects.None, 0f);
            spriteBatch.End();
            // TODO: Add your drawing code here
            angle +=0.5f;
            base.Draw(gameTime);
        }
    }
}

你就不能把圆圈做大一点，让它下面没有东西可以显示，然后把它放在上面吗?