为什么当我在 xna 4.0 中启动游戏时我的球很模糊

我正在使用此代码，但我仍然不明白为什么我开始游戏时球模糊，有人可以帮助我吗??, 当用户点击时，我只是试图给我球，但我不知道为什么它很模糊，我仍然在检查代码，请帮忙

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 Critters
{
    /// <summary>
    /// This is the main type for your game
    /// </summary>
    public class Game1 : Microsoft.Xna.Framework.Game
    {
        GraphicsDeviceManager graphics;
        SpriteBatch spriteBatch;
        Texture2D ball;

        Vector2 playerPosition;
        Vector2 direction;
        Vector2 destination;        
        float speed;
        float playerPosX;
        float playerPosY;

        MouseState oldState;
        public Game1()
        {
            graphics = new GraphicsDeviceManager(this);
            Content.RootDirectory = "Content";
            this.IsMouseVisible = 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
            graphics.PreferredBackBufferWidth = 700;
            graphics.PreferredBackBufferHeight = 500;
            graphics.IsFullScreen = false;
            graphics.ApplyChanges();
            Window.Title = "Darrells Game";

            playerPosY = graphics.GraphicsDevice.Viewport.Height / 10 * 8;
            playerPosX = graphics.GraphicsDevice.Viewport.Width / 2;
            playerPosition = new Vector2(playerPosX, playerPosY);
            destination = playerPosition;

            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()
        {
            // Create a new SpriteBatch, which can be used to draw textures.
            spriteBatch = new SpriteBatch(GraphicsDevice);

            ball = Content.Load<Texture2D>("orb");
            // TODO: use this.Content to load your game content her
        }
        /// <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
        }

        protected override void Update(GameTime gameTime)
        {
            // Allows the game to exit
            if (GamePad.GetState(PlayerIndex.One).Buttons.Back == ButtonState.Pressed)
                this.Exit();

            if (Vector2.DistanceSquared(destination, playerPosition) >= speed * speed)
            {
                MovePlayer();
            }
            else
            {
                playerPosition = destination;
            }

            MouseState leftState = Mouse.GetState();
            MouseState rightState = Mouse.GetState();             
            if ((leftState.LeftButton == ButtonState.Pressed && rightState.RightButton == ButtonState.Pressed))
            {
                int mouseX = leftState.X;
                int mouseY = leftState.Y;
                destination = new Vector2(mouseX, mouseY);
                speed = 8.0f;
            }
            else if (leftState.LeftButton == ButtonState.Pressed)
            {
                int mouseX = leftState.X;
                int mouseY = leftState.Y;
                destination = new Vector2(mouseX, mouseY);
                speed = 2.0f;
            }

            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);

            spriteBatch.Begin();
            DrawPlayer();
            spriteBatch.End();
            // TODO: Add your drawing code here
            base.Draw(gameTime);
        }
        private void DrawPlayer()
        {
            Vector2 textureCentre = new Vector2(ball.Width / 2, ball.Height / 2);
            spriteBatch.Draw(ball, playerPosition, null, Color.White, 0f, textureCentre, 0.8f, SpriteEffects.None, 1f);
        }
        private void MovePlayer()
        {
            {
                direction = destination - playerPosition;
                direction.Normalize();
                playerPosition += direction * speed;
            }
        }

    }
}