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

[Paul Ramsey]

+1 from me

P.

> On Feb 7, 2017, at 4:55 PM, Daniel Baston <dbaston at gmail.com> wrote:
> 
> 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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