Extracting centerline of a Complex-Polygon in PostGIS/Python

Question

Any idea how to extract the centerline of the following polygon (Fig1) using PostGIS functions or Python modules? I was trying Voronoi Function but thats not helpful for this kind of shape. I want something like Fig2 blackline.

Answer 1

This may be a wrong tool for you, but you can do it simply with OpenJUMP and Skeletonizer plugin. Plugin can be downloaded from http://sourceforge.net/projects/jump-pilot/files/OpenJUMP_plugins/More%20Plugins/JUMP%20Skeletonizer%20Plugin/

Zip file contains a good manual. Plugin is meant for streams and water bodies but your polygon mimics a stream to me.

This is your polygon as I digitized it. Notice vertices in the center lines of inputs/outlets.

Entry points digitized on a separate layer. Points must snap to vertices on the polygon boundary.

Use the Skeletonizer tools and fill in the fields

Get the result

Answer 2

If your polygon is raster, digitize it.

Once your polygon is really a polygon :), a first solution lies, like @WhiteboxDev said, it's the straight skeleton, but it has the problem that, besides the centerline you want, it also returns a lot of segments radiating from that line to the border of your polygon. But if you have PostGIS 2.2+, you can use ST_ApproximateMedialAxis() instead, which is what you want.

Answer 3

The library pygeoops has an implementation to calculate a centerline of a polygon: pygeoops.centerline. It also removes short branches, simplifies the result,... to get a "clean" result.

Sample script:

import pygeoops
import shapely

polygon = shapely.from_wkt("POLYGON ((0 0, 0 8, -2 10, 4 10, 2 8, 2 2, 10 2, 10 0, 0 0))")
centerline = pygeoops.centerline(polygon)

Result:

Disclaimer: I'm the developer of pygeoops