I'm trying to write a 2D vertical scrolling background for a shooter.  My implementation is a bit different.  I have one BIG texture (1600X4194).  Game screen resolution is 1024X768.
我尝试着为我的射击游戏开发一个2D的垂直滚动背景，不过我的实现方法有点另类，呵呵，我使用了一个大背景(1600X4194).  游戏屏幕的分辨率是1024X768.。

I start by drawing a rectangle the size of the game window at the bottom of this texture and slowly move that rectangle up until it get to the top then I wrap.  It works for the most part except I have one weird issue.  Take a look at this line:
游戏的实现方法是在游戏里画出哪个大背景图片的一部分，接着慢慢的移动，当移动到尽头的时候再调头重复画，周而复始。用这种方法几乎没有太大问题，不过有个奇怪的问题，请看下面一句：
th = mytextureinfo.Height / 2 - (screenheight/12);

I would think I could just do:
我想我是否应该使用下面的代码呢？
th = mytextureinfo.Height;

But that doesn't work.   At the start of the game I'm definitely drawing a rectangle outside the bounds of my texture.  Why is that?  I'm using origin (0,0).  I initialize y = mytextureinfo.Height - screenheight.  Any ideas?Here is the complete class:

但是问题依然存在。在游戏一开始，我画的矩形背景超出那个大背景图片(1600X4194)的边界. 到底怎么回事？ 请问各位有什么建议吗？ 下面是具体的实现代码，请各位推敲改进。

 public class ScrollingBackground
    {
        public int speed = 1;
        private Texture2D mytexture;
        private TextureInformation mytextureinfo;
        private int screenheight, screenwidth;
        private Rectangle frame = new Rectangle();
        private Rectangle frame2 = new Rectangle();
        private Vector2 Pos = new Vector2(0f, 0f);
        int x, y;
        int tw, th;

        // ScrollingBackground.Load
        public void Load(GraphicsDevice Device, string TextureName)
        {
            mytextureinfo = Texture2D.GetTextureInformation(TextureName);
            mytexture = Texture2D.FromFile(Device, TextureName);
            screenheight = Device.Viewport.Height;
            screenwidth = Device.Viewport.Width;
            tw = mytextureinfo.Width;
            th = mytextureinfo.Height / 2 - (screenheight/12);
            Reset();
        }

        public void Reset()
        {
            x = (tw / 2) - (screenwidth / 2);
            y = th - screenheight ;           
            frame.X = x;
            frame.Y = y;
            frame.Width = screenwidth;
            frame.Height = screenheight;

            frame2.X = x;
            frame2.Y = y;
            frame2.Width = screenwidth;
            frame2.Height = screenheight;

        }
        // ScrollingBackground.Update
        public void Update()
        {
            y -= speed;
            if (y < screenheight * -1)
            {
                Reset();
                return;
            }
            frame.Y = y;
            if (y < 0)
            {
                frame2.Y = th + y;
                frame2.Height = y * -1;
            }

           
        }
        // ScrollingBackground.Draw
        public void Draw(SpriteBatch Batch)
        {           
            // Draw the first texture
            Batch.Draw(this.mytexture, Pos, this.frame, Color.White);
            if (y < 0)
            {
                //draw second texture
                Batch.Draw(this.mytexture, Pos, frame2, Color.White);
            }
        }
    }