ST_IsPolygonCCW for invalid inputs

[Paul Ramsey]

I chickened out and fiddled my patch to the old behaviour. I'm not sure
there is a correct answer (maybe empty should be counted as correctly
oriented regardless of the desired orientation? maybe the use case of the
test is primarily viewed as to find incorrectly oriented things, suitable
for fixing (but in that case, why is the test-and-fix logic also embedded
in the c routines for fixing)
Anyways, I think it's a poorly specified piece unfortunately and probably
not a lot of use to anyone, as long as the fixing functions work (which
they do now).
P.

On Wed, Oct 15, 2025 at 6:55 AM Greg Troxel <gdt at lexort.com> wrote:

> Paul Ramsey <pramsey at cleverelephant.ca> writes:
>
> > Grrr, I'm trying to fix https://trac.osgeo.org/postgis/ticket/5754
> which on
> > the surface is quite simple, but it actually exposes a bit of a rats nest
> > of bad logic.
> >
> > I have a good fix that mostly works, but now I have a "big" regression:
> the
> > old function returned "true" when handed non-polygonal inputs.
>
> The spec for ST_IsPolygonCW
>
>   https://postgis.net/docs/ST_IsPolygonCW.html
>
> says
>
>   Returns true if all polygonal components of the input geometry use a
>   clockwise orientation for their exterior ring, and a counter-clockwise
>   direction for all interior rings.
>
>   Returns true if the geometry has no polygonal components.
>
> which is perhaps odd, but seems well defined.
>
> It seems intended that a geometry passes ST_IsPolygonCW unless there is
> a polygon which failes to be CW.
>
> > My feeling is that a geometry is "clockwise" if it is (a) a polygon or
> > polygonal and (b) the outer ring is clockwise and (c) the inner rings are
> > anti-clockwise. Otherwise it's not oriented. (My changes for this work
> > mostly centered around the fact that a geometry can have one of three
> > orientations: clockwise, anticlockwise, and none. That made the logic, I
> > think, more straightforward to understand.)
>
> That certainly makes sense.
>
> > So, this change is for 3.7 and I'd like to change the behaviour for
> > non-polygonal inputs to the ST_IsPolygonCCW and ST_IsPolygonCW functions.
>
> So you want to change the docs to say
>
>   Returns true if all components of the input geometry use are polygons,
>   and each has a clockwise orientation for their exterior ring, and a
>   counter-clockwise direction for all interior rings.
>
>   Returns false if any component of the input geometry is not a polygon,
>   or if there is a polygon that does not meet the rules in the first
>   paragraph.
>
> I wonder why the specification was written the way it was.
>
>
>
>
>
>
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