================= label_centerlines

This tool runs with Python 3.6 and reads Polygon/MultiPolygon datasets such as i.e. the geographic regions_ from Natural Earth_ and extracts smoothed centerlines for better label placement. This method is used to create the label layer of EOX Maps_.

.. _geographic regions: http://www.naturalearthdata.com/http//www.naturalearthdata.com/download/10m/physical/ne_10m_geography_regions_polys.zip .. _Natural Earth: http://www.naturalearthdata.com/ .. _EOX Maps: http://maps.eox.at

To do so, it a Voronoi diagram_ is created to get the polygon skeleton where the skeleton centerline is selected and smoothed.

.. _Voronoi diagram: https://en.wikipedia.org/wiki/Voronoi_diagram

.. figure:: img/centerline.gif

Steps:

1. Extract outline.
2. Segmentize outline to get more evenly distributed outline points.
3. Extract points.
4. If there are too many points, simplify the segmentized outline and Extract points again.
5. Create Voronoi diagram.
6. Select all Voronoi edges which are inside the source polygon.
7. Determine the best line.
8. Smooth line.

Installation

clone repository and run

.. code-block:: shell

pip install -r requirements.txt
python setup.py install

CLI

.. code-block:: shell

$ label_centerlines --help

Usage: label_centerlines [OPTIONS] INPUT_PATH OUTPUT_PATH

  Read features, convert to centerlines and write to output.

  Multipart features (MultiPolygons) from input will be converted to
  singlepart features, i.e. all output features written will be LineString
  geometries, not MultiLineString geometries.

Options:
  --version                       Show the version and exit.
  --segmentize_maxlen FLOAT       Maximum segment length for polygon borders.
                                  (default: 0.5)
  --max_points INTEGER            Number of points per geometry allowed before
                                  simplifying. (default: 3000)
  --simplification FLOAT          Simplification threshold. (default: 0.05)
  --smooth INTEGER                Smoothness of the output centerlines.
                                  (default: 5)
  --max_paths INTEGER             Number of longest paths used to create the centerlines.
                                  (default: 5)
  --output_driver [GeoJSON|GPKG]  Output format. (default: 'GeoJSON')
  --verbose                       show information on processed features
  --debug                         show debug log messages
  --help                          Show this message and exit.

API

.. code-block::

>>> from label_centerlines import get_centerline
>>> help(get_centerline)

get_centerline(geom, segmentize_maxlen=0.5, max_points=3000, simplification=0.05, smooth_sigma=5, max_paths=5)
Return centerline from geometry.

Parameters:
-----------
geom : shapely Polygon or MultiPolygon
segmentize_maxlen : Maximum segment length for polygon borders.
    (default: 0.5)
max_points : Number of points per geometry allowed before simplifying.
    (default: 3000)
simplification : Simplification threshold.
    (default: 0.05)
smooth_sigma : Smoothness of the output centerlines.
    (default: 5)
max_paths : Number of longest paths used to create the centerlines.
    (default: 5)

Returns:
--------
geometry : LineString or MultiLineString

Raises:
-------
CenterlineError : if centerline cannot be extracted from Polygon
TypeError : if input geometry is not Polygon or MultiPolygon

License

MIT License

Copyright (c) 2015, 2016, 2017, 2018 EOX IT Services_

.. _EOX IT Services: https://eox.at/

(see LICENSE file for more details)