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

[Martin Davis]

I probably should have started this discussion as a GitHub issue (better
formatting, linking, tracking etc).  I will try and move this content there
soon.

On Tue, Sep 6, 2022 at 2:37 PM Martin Davis <mtnclimb at gmail.com> wrote:

> 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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