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 <mtncl...@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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