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

[Regina Obe]

+1 

 

From: postgis-devel [mailto:postgis-devel-bounces at lists.osgeo.org] On Behalf Of Paul Ramsey
Sent: Tuesday, February 07, 2017 9:02 PM
To: PostGIS Development Discussion <postgis-devel at lists.osgeo.org>
Subject: Re: [postgis-devel] RFC - Orientation checking and forcing functions

 

+1 from me

P.


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