LineString buffer not working as expected

[tsuz]

Previous: #215

What

Initial, generated and output data

print	image
print("initial data", json)

expected (red line)	observed

         if let json = String(data: collection, encoding: .utf8) {
           print("initial data", json)
        } 

        for feature in featureCollection.features {
            if case let .lineString(lineString) = feature.geometry {
                linestrings.append(lineString)
                if let data = try? JSONEncoder().encode(lineString) {
                    let json = String(data: data, encoding: .utf8)
                    print("item json", json)
                }
                guard let buf = try? lineString.buffer(by: 1).geometry else {
                    return
                }
                if let data2 = try? JSONEncoder().encode(buf) {
                    let json = String(data: data2, encoding: .utf8)
                    print("item json 2", json)
                }
            }
        }

Can anyone see what I'm doing wrong here?

[macdrevx]

In GEOSwift, everything is just math, so since your input geometry is in lat/long coordinates, buffer(1) means that you're trying to buffer by 1 degree of latitude and 1 degree of longitude, which, based on the description is not what you want. Try using a much smaller value.

[macdrevx]

That said, I'll admit that I'm confused by the output shape. I'll take a closer look at that.

[tsuz]

@macdrevx

Thanks for the fast reply!

I assumed the buffer(width: Double) was in meters. After updating this to a smaller value, it worked as expected!

guard let buf = try? lineString.buffer(by: width).geometry else {
  return
}

(*note the original data of the two items below are slightly different but it's in a similar lat/lng)

width=0.0001	width=0.0005

Thanks again!

[macdrevx]

I did a little more investigation into this issue and discovered that GEOSwift is passing 0 for the quadsegs parameter of geos's buffer function which is leading to some of those flat edges around the outside of the buffered corners. In another location internally, GEOS uses a value of 8 as a default, which results in a result that is closer to what you might expect on first glance. I'll push an update with this change.

[macdrevx]

Here's the before and after:

quadsegs=0	quadsegs=8

[macdrevx]

Updated in https://github.com/GEOSwift/GEOSwift/releases/tag/8.0.2