polygon_to_line function not working if a GeoInterface Polygon has holes.

[BG-AIMS]

I have run into an issue where polygon_to_line produces an error if a polygon has holes and has been created with GeoInterface methods. If the polygon originates from ArchGDAL creation then polygon_to_line does work even if the polygon has holes.

For context I found this issue as I have ArchGDAL polygons that are simplified with GO, and polygon_to_line works with these polygons (even if there are holes), however after using GO.buffer on these polygons an error is encountered if the polygon has holes.
Below is example code:

import GeoInterface as GI
import GeometryOps as GO
import ArchGDAL as AG
using GLMakie
using LibGEOS

# GeoInterface
x = [-5, -5, 5, 5];
y = [-5, 5, 5, -5];
multipoint = GI.MultiPoint(GI.Point.(zip(x, y)));

x = [-10, -10, 10, 10, -10]
y = [-10, 10, 10, -10, -10]
lines = GI.LineString(GI.Point.(zip(x,y)));

ring1 = GI.LinearRing(GI.getpoint(lines));
polygon1 = GI.Polygon([ring1]);
poly(polygon1)

hole = GI.LinearRing(GI.getpoint(multipoint))
polygon2 = GI.Polygon([ring1, hole])
GO.polygon_to_line(polygon2)

poly(polygon2)
GO.polygon_to_line(polygon1)


# ArchGDAL
x = [[-10, -10], [-10, 10], [10, 10], [10, -10], [-10, -10]]
polygon1 = AG.createpolygon()
ring = AG.createlinearring(x)
AG.addgeom!(polygon1, ring)
poly(polygon1)

h = [[-5, -5], [-5, 5], [5, 5], [5, -5], [-5, -5]]
polygon2 = AG.createpolygon()
hole = AG.createlinearring(h)
AG.addgeom!(polygon2, ring)
AG.addgeom!(polygon2, hole)
poly(polygon2)
GO.polygon_to_line(polygon2)
GO.polygon_to_line(GO.simplify(polygon2; number=8))
GO.polygon_to_line(GO.buffer(GO.simplify(polygon2; number=8), 0.5)) # my context use case

Below is the error message encountered on line 22, and is the same error as the final line (my use case for context):

[asinghvi17]

I see. The implementation from ArchGDAL is buggy in that it does not recognize linear rings on output (though it does require them on input, as you saw). This probably needs an implementation of coerce(LineStringTrait(), ::LinearRingTrait, geom) to ensure that all geometry is able to fit into the multilinestring.

Thanks for the bug report!

[asinghvi17]

Why do you need polygons as linestrings, though? If it's just for plotting you can simply lines(polygon) and it will work. Or do poly(polygon; color = :transparent, strokewidth = 2, strokecolor = :red) for example.

[BG-AIMS]

Why do you need polygons as linestrings, though? If it's just for plotting you can simply lines(polygon) and it will work. Or do poly(polygon; color = :transparent, strokewidth = 2, strokecolor = :red) for example.

I am looking to get each edge line between pairs of polygon vertices (so I can see which polygon line segment is closest to a point). I used something similar to GO.LineString(GO.Point.(linestring.geom)) which worked until this issue with hole polygons was encountered. Let me know if there's an easier way to go about this.

Note that I do also need the line segments from the holes in a polygon so I currently do Linestring(Point.()) separately for each linestring if the polygon has holes.

Along the lines of my desired output:

[rafaqz]

Isn't polygon_to_line internals? Its not exported. I don't think it should be documented either, that should be a comment.

Its also pretty wrong, this just can't work most of the time. First we should always return a MulltiLineString for type stability, and the we should check if the contents are LinearRing or LineString and I guess convert them?

    GI.ngeom(poly) > 1 && return GI.MultiLineString(collect(GI.getgeom(poly)))

Maybe... just maybe... LineString in GeoInterface should be allowed to wrap any LinearRingTrait object so this can be allocation free. If its just a GI.LinearRing it can simply switch to LineString, otherwise it can wrap. Then LinearRing can do the reverse.

[BG-AIMS]

I've realised for my use case I can use something like GO.LineString(GO.Point.(GI.coordinates(polygon))) instead of going through polygon_to_line().

[asinghvi17]

GI.getpoint will probably be more efficient than GI.coordinates, but otherwise yes!

[rafaqz]

Yeah, never used GI.coordinates if you can help it