Rendering the scene

At one point, we have to reflect about how the game is rendered on the screen. How do we draw all the entities, the scenery, and interface elements (such as a health bar)? A simple option is to have different sequential containers through which we iterate. For each element, we call a possible Entity::draw() function to draw the corresponding entity on the screen. We only have to make sure that objects that appear behind others (such as the scenery background) are drawn first.

Relative coordinates

The sequential rendering approach works well for many cases, but makes it difficult to handle an entity relative to another one. Imagine we have a formation of airplanes, where one is the leader and the rest follows it. It would be nice if we could set the position of the following airplanes dependent on the leader, in the sense of "plane A is located 300 units behind (below on the screen) and 100 units right of the leader" instead of "plane A is located at position (x, y) in...