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

[Germán Carrillo]

Hi All,


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.


We've come to the conclusion that in order to avoid code duplication in the
QgsGeos class, when importing/exporting LineString or CircularString
geometries from/to GEOS (used, e.g., in QgsGeos::fromGeos()
and QgsGeos::asGeos() methods), we would need to introduce a new abstract
class called QgsSimpleCurve.

This new class would inherit from QgsCurve and would be the base class for
QgsLineString and QgsCircularString. Namely, importing/exporting
QgsCircularString geometries would largely benefit from the existing
import/export code for QgsLineString, so we'd be reusing as much code as
possible.

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.


An alternative would be to keep using QgsCurve as the base class, and add
some methods to it that would be ONLY used by
QgsLineString/QgsCircularString, ensuring that QgsCompoundCurve returns
exceptions when those methods are called.
Other alternatives are also welcome for the discussion, just let us know.


Since this decision is one of the building blocks for improving Circular
Arcs support in QGIS (e.g., for integrating a brand new Geometry Splitter
from GEOS, expected for the 3.15 release), we'd like to kindly ask for your
feedback before proceeding with a PR, so that we can assess early whether
the community estimates this move as risky or as a natural step for the
QGIS project.

Also, let us know if a QEP would be needed here, or if we could go on with
this discussion and then with a well isolated PR.
Thank you in advance.


Regards,

Germán
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[1] https://gdal.org/en/stable/doxygen/classOGRSimpleCurve.html
[2]
https://github.com/libgeos/geos/blob/main/include/geos/geom/SimpleCurve.h
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