We are going to use the following images to create the background for our game:
Images for the background
Note that all of these can be seamlessly tiled horizontally—this isn't a strict requirement, but a desirable property nonetheless, as the background looks nicer this way.
As mentioned in the description, the background is always in motion, disconnected from the rest of the game. This effect can be achieved in at least two ways:
Using the straightforward approach, we can just move a huge textured polygon (or any number of polygons) in the background. Creating seamless looping animation in this case may require a bit of work.
A more efficient way to achieve the same visual effect is to create a number of static polygons (one per layer) that span the whole viewport, and then animate just texture coordinates. With a tileable texture, this approach produces seamless and visually pleasing results, and amounts to less work overall—there is no need to reposition objects...