Hello im doing the Ashley tutorial of the platformer,when i add a 2d background found on the web instead of increase the size of it,the background is always duplicated..i post here a pic of the problem.
That's exactly how a Tiled Background is meant to look, dependent on original image (https://www.scirra.com/manual/118/tiled-background).
The Tiled Background object will repeat the original image to whatever dimensions you tell it to, so if your original image is only 200 by 200, then you will obviously see a repeating pattern for such a large Layout size.
The trick is to choose a background that doesn't have such an obvious repeatable pattern. I only ever use either a single colour or a gradient when using Tiled Background for a background.
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There are plenty of ways to change this:
If you want one big background (stretched) then you open an image editor such as paint.net and stretch the image and save it manually. Alternately find a background with a higher resolution.
If you want a background that you can adjust the resolution of and have it stretch whenever, you have to use a sprite (or other similar plugin, such as a 9 patch). However note that this is less computationally efficient, and drawing to the screen is what takes up the most processor time.
If you want a background that fills the screen and never moves, then set the background's parallax to 0 and make sure the source image and tiled background object are both the same resolution as the screen.