World opacity is very handy to keep grass from growing onto places you want to keep clear, such as paths and rocks. Another way of using it is to make straight-sheared hedges. It does come with some artifactional mmmm-thingy in one case. I haven't found out why this didn't work well. Left is hedges as they are, right is clipped by world opacity mask. Opacity comes from a bitmap-grey map in a certain resolution. Perhaps simple shapes just work better.