[QGIS-Developer] Feedback on introducing a new (abstract) class: QgsSimpleCurve

[Even Rouault]

Le 28/05/2026 à 00:05, Greg Troxel via QGIS-Developer a écrit :
> Germán Carrillo via QGIS-Developer<qgis-developer at lists.osgeo.org>
> writes:
>
>> In the context of the crowdfunded project to improve Circular Arcs support
>> in GEOS/QGIS, between Opengis.ch and Dan Baston from iSciences, we'd like
>> to ask you for technical feedback at an early stage of the QGIS
>> implementation.

+1 from me for QgsSimpleCurve. Obviously strongly biased as the author 
of GDAL's OGRSimpleCurve.

If QgsSimpleCurve appears in the SIP Python bindings, it might be 
appropriate to have a word in the doc about it being an implementation 
detail not covered by standards.

An alternative approach could have been to have QgsSimpleCurve as a 
member of QgsLineString & QgsCircularString, using composition rather 
than derivation, but that would likely be a rather verbose approach, 
probably not much better than copying&pasting code between both classes.

> I know there are various specifications and standards about kinds of
> features.   I would like to see written up the high-level information
> about what's going on, including:
>
>    What are the relevant standards?
>
>    For each, are there aspects that are not implemented in
>    qgis/geos/gdal/postgis/?
SQL/MM could possibly have exposed such a class, but I guess that's 
mostly seen as an implementation detail for implementations to avoid 
code duplication, rather than something impacting operability.
>    Do programs in the osgeo stack implement object types that  are not in
>    the standards?
Isn't that's > 90% of the job of making useful software, unless your 
software is just about making a reference implementation of a standard? 
Standards cover only the boring aspects :-)
>    If the proposal is about an object type that's not in the standards,
>    is there text available that is basically a proposed diff to each
>    standard to add it?   Is there intent to actually propose that, or is
>    this going down the path of diverging from the standard?
Personal opinion here: participation to standard working groups is time 
consuming and implies many year involvement due to slow motion / 
bureaucracy / opacity in group dynamics. That's not an activity 
structured for small/medium businesses (more for big commercial players 
that want to push their thing so they can later tick their 
implementation is compliant in call for tenders, or government agencies 
that have dedicated staff for interoperability topics), unless you are 
really fond of that or have significant funding.
> I see you address gdal/geos but I don't see postgis mentioned.
PostGIS is C, so inheritance is not a natural concept there. PostGIS's 
liblwgeom has identical layout for struct LWLINE 
(https://github.com/postgis/postgis/blob/master/liblwgeom/liblwgeom.h.in#L480) 
and LWCIRCSTRING 
(https://github.com/postgis/postgis/blob/master/liblwgeom/liblwgeom.h.in#L504) 

> An inheritance diagram would help to be sure I'm following.  And
> explaining what the rules are for class membership.  I'm a little
> worried about multiple inheritance and can't tell that you are intending
> to really avoid that.
See GDAL RFC 49 : 
https://gdal.org/en/stable/development/rfc/rfc49_curve_geometries.html#new-cass-hierarchy 

>> Moreover, other projects like GDAL/OGR [1] and GEOS itself [2], already
>> have SimpleCurve classes to ease implementation, even if a SimpleCurve
>> class is not part of the SQL/MM standard.
> is Simple Features also relevant?
SQL/MM is the ISO extension of Simple Features adding curve geometries. 
Old draft at 
https://web.archive.org/web/20071212154300/http://jtc1sc32.org/doc/N1101-1150/32N1107-WD13249-3--spatial.pdf 
, otherwise pay 100 + CHF to ISO. Although I see at the bottom of 
https://www.ogc.org/standards/sfa/ there's apparently an ongoing effort 
to reconcile both.

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