Another typical feature of 2D games that is deeply related to the DrawThread is parallax backgrounds.

The idea of a parallax background is to have an image that moves slower than the elements in the foreground, giving the impression of depth.

To make this effect better, we can use multiple background images at different speeds. This is commonly used in 2D scrollers. For example with trees in the near plane and mountains and clouds at the very back.

For YASS, we will use a star field that moves down slowly as a background.

We are going to use the same convention of units as we used for the sprites. The image we use for the background should be designed to be tiled, vertically in our case. This means that the end of the image fits with its beginning, so they are placed one after another and have continuity.

It should also be designed in a way that it is larger than the biggest screen. We can consider...