Extracting the centerlines of polygons

In several recipes in Chapter 4, Working with Vector Data – Advanced Recipes, we explored extracting Voronoi polygons from sets of points. In this recipe, we'll use the Voronoi function employed in the Using external scripts to embed new functionality to calculate Voronoi polygons section to serve as the first step in extracting the centerline of a polygon. One could also use the Using external scripts to embed new functionality to calculate Voronoi polygons—advanced recipe, which would run faster on large datasets. For this recipe, we will use the simpler but slower approach.

One additional dependency is that we will be using the chp02.polygon_to_line(geometry) function from the Normalizing internal overlays recipe in Chapter 2, Structures That Work.

What do we mean by the centerline of a polygon? Imagine a digitized stream flowing between its pair of banks, as shown in the following screenshot:

If we wanted to find the center of this in order to model...