Function to check or correct the "orient" of polygons

[jorisvandenbossche]

Shapely has a orient function to make a copy of a Polygon with "proper" orientation of the rings: https://shapely.readthedocs.io/en/latest/manual.html#shapely.geometry.polygon.orient (definition in the code: https://github.com/Toblerity/Shapely/blob/b21a964a56a86cf77c67b477db7e7c2e81d2ccb5/shapely/geometry/polygon.py#L391-L404)

In addition, Shapely also has a is_ccw (is counter clock wise) attribute on LinearRings: https://shapely.readthedocs.io/en/latest/manual.html#object.is_ccw

For this, there is a GEOS C API function: https://github.com/libgeos/geos/blob/eb4da1440836fedf503e1eb2c245595a0f33db5e/capi/geos_c.h.in#L351-L357 (although Shapely implements it manually by calculating the signed area)

[jorisvandenbossche]

Postgis also has a set of methods related to this: ST_ForcePolygonCCW, ST_ForcePolygonCW , ST_IsPolygonCCW , ST_IsPolygonCW (see eg https://postgis.net/docs/ST_ForcePolygonCCW.html)

[jorisvandenbossche]

Possible related to this is also GEOSNormalize_r (https://postgis.net/docs/ST_Normalize.html)

[caspervdw]

#126 --> implement is_ccw

[jorisvandenbossche]

It's potentially a bit more than just is_ccw, eg both Shapely and Postgis also have functions to convert to consistent orientation, in addition to checking the orientation.

[caspervdw]

Ok sorry I was a bit fast closing this then. Let’s investigate!

[jorisvandenbossche]

I opened an equivalent function in Shapely: shapely/shapely#1366, so let's close here.