[geos-devel] GEOS C API for Polygonal Coverage operations?

[Martin Davis]

I've started to build out operations on Polygonal Coverages in JTS ([1],
[2]).  These will be ported to GEOS (started here with CoverageValidator
etc in C++ [3]). Hopefully these will be exposed in PostGIS as well (and of
course other downstream projects).

Here's some examples of coverage operations and their geometric signatures:

- Validation: List of Polygons -> List of linear geometries for invalid
locations
- Cleaning: List of Polygons -> Coverage
- Union: Coverage -> Polygonal geometry
- Simplification: Coverage -> Coverage

A key question is how to expose these operations in the GEOS C API.  I see
two options:
1) Model a Polygonal Coverage as an array of simple Polygons (and possibly
MultiPolygons)
2) Provide a Polygonal Coverage datatype (which might contain internal
topology)

So far I've been favouring #1, both for the C API and for the underlying
C++ and JTS APIs.  Reasons are:

- Simplicity of use and implementation
- Some operations (such as validation and cleaning) have to operate on
non-coverage lists of polygons anyway
- A potential advantage of having a Coverage data model is that coverage
operations could be chained without needing to convert back to the simple
polygon representation - but it's not clear to me that this will be very
common (and this is not an option for PostGIS, and maybe other downstream
projects).
- It seems more in the spirit of the Simple Features philosophy (if that's
a thing)

Any thoughts or comments on this?

If Model #1 is used, what would the C API look like?  Accept a Geometry
array with a size parameter, and return a Geometry array?  It will likely
be good to have some support functions to free Geometry arrays too.

[1]
http://lin-ear-th-inking.blogspot.com/2022/07/polygonal-coverages-and-operations-in.html
[2]
http://lin-ear-th-inking.blogspot.com/2022/08/validating-polygonal-coverages-in-jts.html
[3]
https://github.com/libgeos/geos/commit/62c928c9f37957c62fab8db69e6c8efd26ce4085
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