[postgis-devel] ST_LineToCurve output type

[Sandro Santilli]

On Tue, Aug 13, 2013 at 12:36:02PM -0700, Paul Ramsey wrote:
> On Tuesday, August 13, 2013 at 12:32 PM, Sandro Santilli wrote:
> > I noticed that ST_LineToCurve returns linear-type geometries when it doesn't
> > detect any curve in input. Given curved types can legitimately represent lines
> > (compoundcurve only containing linestring elements) wouldn't it be more
> > appropriate to always return curves ? It would make a nice invariant, IMHO.
> > 
> > Any reason NOT to do that ?
> Because it's for un-stroking arcs, quite specifically, as your recent restriction on the kinds of inputs it would accept emphasizes. If the input is not an unstroked arc, we shouldn't muck with it. 
> 
> Take a compoundcurve. It's got arcy things in it, it's got linear things in it. Run curvetoline on it. Now it's just liney things. Now run linetocurve on it. It should now be a mixture again of liney things and curvey things.
> 
> Take a linestring. It's just liney things. Run curvetoline on it. It's still just liney things. Run linetocurve on it. It's still just liney things.
> 
> I think the current behavior makes sense.

Uhm, I actually kind of agree about it.
But then again, take a compoundcurve that only has liney things,
run CurveToLine, run LineToCurve and you end up with a linestring
when you started from a compoundcurve.

I guess this is similar to the "multi" status of some inputs 
lost during overlay operations. That's what we added ST_Multi for,
should we have an ST_Curve to do a similar thing ? Or an ST_ForceCurve
kind of casting function ?

--strk;