concave_hull returns nonsense results for simple polygons

[wlinna]

I'm testing concave_hull with a simple polygon, and concave_hull returns very weird results. Here are the screenshots

Original polygon

Concave hull

Here's the test code

       let exterior = LineString(vec![
            Coord { x: 5.1734996, y: 1.4128553 },
            Coord { x: 5.1734996, y: 1.5378553 },
            Coord { x: -4.9266067, y: 1.5378562 },
            Coord { x: -4.9266057, y: -8.45025 },
            Coord { x: -4.8016057, y: -8.45025 },
            Coord { x: -4.8016057, y: 1.4128553 },
            Coord { x: 5.1734996, y: 1.4128553 },
        ]);
    
        let polygon = Polygon::new(exterior.clone(), vec![]);
        let hull = polygon.concave_hull(0.5); // Whether the concavity is 0.5 or 4.0 doesn't seem to matter

I'm using geo 0.27.0 with Rustc 1.72

[wlinna]

I tried this again with both geo 0.27 and geo 0.28 , and it looks like nothing changed with concave hull itself. Both versions result in following coordinates

-4.9266057 -8.45025
-4.8016057 -8.45025
5.1734996 1.4128553
5.1734996 1.5378553
-4.9266067 1.5378562
-4.9266057 -8.45025

However, I'm not sure anymore how I produced the original 3D model of the concave hull. When I generate it now, it looks like this

I checked the printed coordinates visually one by one, and they correspond with the generated triangle mesh.

So at least the algorithm produces a convex hull. But I can't make it produce the original shape.

[kirillolenev-dm]

Any updates about that? I have a similar experience when concave_hull provides something not close to expected result, concavity param doesn't change much.

[michaelkirk]

@kirillolenev-dm - can you provide a minimal reproduction?

e.g. the input you gave and the output you're expecting?

concave hull is a bit tricky because there's no rigorous definition for what the "correct" output should be. I'll say that @wlinna's first screenshot looks very concerning, not a hull at all, but seemingly that output can no longer be reproduced.

The second one at least looks like a hull, but admittedly doesn't look very concave.

I don't use these methods myself, but I'd be willing to look into any issues with it if you can gather more expected input/outputs.

[michaelkirk]

(sorry that this sat here so long!)