Left and right arrow keys. A modified version of my earlier project showing an independently moving object as well as the tree. The butterfly moves back and forth in front of the tree. For enemies, projectiles etc this movement in 'real world coordinates' is what you need to do. So scrollx is just an adjustment to the real world position of things to indicate where in our window it would appear. So the tree doesn't move in the real world but our view of the world has changed.
