[postgis-devel] RFC - Orientation checking and forcing functions

[Daniel Baston]

Hi All,

I put together a pull request a couple of weeks ago that introduces four
functions for checking and forcing geometry orientation.  The intent is to
provide a less-ambiguous alternative to ST_ForceRHR, and an alternative to
ST_Orientation that does not depend on SFCGAL.  Strk asked that I bring
this up on the devel list for a review of the semantics.

The four proposed functions are.

ST_ForceCW
ST_ForceCCW
ST_IsCW
ST_IsCCW

The semantics are straightforward, in my opinion:

1.  ST_ForceCW is a synonym for ST_ForceRHR, which gives all exterior rings
a clockwise orientation and all interior rings a counterclockwise
orientation.  Non-polygonal geometries, including closed LineStrings, are
simply passed through.
2.  ST_ForceCCW is simply a shorthand for ST_Reverse(ST_ForceCW)
3.  ST_IsCW returns true if a geometry would not be modified by
ST_ForceCW.  It is essentially a synonym for ST_OrderingEquals(geom,
ST_ForceCW(geom)).
4.  ST_IsCCW returns true if a geometry would not be modified by
ST_ForceCCW.

A few notes:

ST_IsCW and ST_IsCCW both return false if a polygon has invalid ring
orientation (i.e., exterior and interior rings are all clockwise)
ST_IsCW and ST_IsCCW both return true for geometries that have no non-empty
polygonal components.

Relevant links:
PR: https://github.com/postgis/postgis/pull/122
Trac: https://trac.osgeo.org/postgis/ticket/3689,
https://trac.osgeo.org/postgis/ticket/3651

Dan
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