XNA C＃2D滚动背景3或更多

Question

I've trying to create a scrolling background with 3 background, but everything When the 3rd start to come out. It creates a giant blue screen(Default background of the game) infront of the 3rd and after the 3RD it doesn't show any background. I have no idea how fix this, and I already know it something simple. I've got the 3RD one to work, but .

Code that I'm trying to use.

 public void Update(GameTime gameTime)
    {
        bgPos0.Y += speed;
        bgPos1.Y += speed;
        bgPos2.Y += speed;

        if (bgPos0.Y >= 950)
        {
            bgPos1.Y = -950; 

            if (bgPos1.Y >= 950) // Doesn't go fully down.
            {
                bgPos2.Y = -950;

                if (bgPos2.Y >= 950)
                {
                    bgPos0.Y = 0; //(Try to change it to -950 still doesn't work. I guest it due to that bgPos0 is set to 0, 0)
                }
            }
        }
    }

And the Vector2 code for the pos are

        bgPos0 = new Vector2(0, 0);
        bgPos1 = new Vector2(0, -950);
        bgPos2 = new Vector2(0, -1900); // Could be the large -1900 number that destroying the code. To make it not work.

So how do I fix this? I wish I could fix it right now, but I can't for some reason.

Answer 1

I don't think you want to have the if-statements nested. The code you posted constantly moves the second background to -950 for as long as the first is past 950. Since the first is constantly moved back to -950 it shouldn't ever manage to move past 950, and so it never gets into the last one. I think what you probably want to do is something more like:

public void Update(GameTime gameTime)
{
    bgPos0.Y += speed;
    if(bgPos0.Y > 950) {
        bgPos0.Y = -950;
    }

    bgPos1.Y += speed;
    if(bgPos1.Y > 950) {
        bgPos1.Y = -950;
    }

    bgPos2.Y += speed;
    if(bgPos1.Y > 950) {
        bgPos1.Y = -950;
    }

}

[EDIT]: As an aside, the number in question isn't nearly large enough to cause problems. XNA's Vector2 class stores the x and y components as floats, and the maximum value for a float in C# is somewhere around 3.4e38 or so, and accurate to 7 digits according to MSDN.