How can I seamlessly loop a background texture in XNA?

Question

Using XNA, I made a looping background with several textures (whose widths are over 1024px). There are fewer than five of them. All the pictures are different, held in a list and their positions are looped. When it reaches the end of the loop, it flickers.

In my Update method I do this:

dt = (float)gameTime.ElapsedTime.TotalSeconds;
foreach(BackGroundObject bgo in myList)
{
    bgo.Position += speed * dt;
}

So that it works based on gameTime. Then, in my drawing function is this code:

foreach(BackGroundObject bgo in myList)
{
    spriteBatch.Draw(bgo.Picture, bo.Position, Color.White);
}

Here is the BackGroundObject class. The game holds a list of BackGroundObjects and Update changes their positions. If any object goes off screen, its position is set again; a BackGroundObject is loaded once and the game always uses same BackGroundObjects. MayDraw is a property of the picture's position. I.e. If in the screen or not.

public class BackGroundObject
{
    public Texture2D Picture;
    public Vector2 Position, DrawPosition;
    public bool MayDraw;

    public BackGroundObject(Vector2 position, Texture2D picture)
    {
        this.Picture = picture;
        this.Position = position;
        this.MayDraw = true;
    }

    public virtual void Draw(SpriteBatch spriteBatch)
    {
        if (MayDraw == true)
        {
            spriteBatch.Draw(Picture, Position, Color.White);
        }
    }
}

Does the flicker problem occur because of the large image sizes? Could I do this with a shader instead?

My background code works on with a graphic card with 60 FPS, but on a onboard graphic card with 30 FPS.

I have 4 background layers, each layer has 4 pictures, in total about 15pictures are drawen with widths of 1920.

Answer 1

It's unclear to me what BackGroundObject is, and how its implemented -- it's possible the problem lies in there, or elsewhere, as the code you've provided doesn't seem terribly suspect. The biggest thing that stands out to me is that you never seem to actually loop (reset) the position of the background images at all. The "Position" value will increase without bound, unless there's a modulus operation that occurs in the setter for that property or something.

You also seem to render all the background images every frame, which is potentially wasteful as probably at most two (given their sizes) will be onscreen at once.

You may want to check out this resource on 2D scrolling background in XNA.