assert_debug fails 'not possible! UniqueSegmentsIter guarantee it'

[SauvageThomas]

Summary

Hi, I'm using IOverlay through the geo crate and I had a panick with a fail assert when offsetting a polygon.

Here is the fail assert:
https://github.com/iShape-Rust/iOverlay/blob/main/iOverlay/src/mesh/outline/builder.rs#L162

I'm not well versed in polygon manipulation ; is it a bug in the crate or is the polygon just not well formed enough for this operation ?
I'm working with generated polygon through algorithms on images, I try to pre-process them a bit, but some can have weird shapes.

Steps to reproduce

I managed to make a minimal reproductive exemple:

use i_overlay::mesh::outline::offset::OutlineOffset;
use i_overlay::mesh::style::{LineJoin, OutlineStyle};

fn main() {
    let shape = vec![vec![
        [53.0f64, 42.0],
        [27.0, 74.0],
        [26.0, 75.0],
        [35.0, 66.0],
        [53.0, 42.0],
    ]];

    let style = OutlineStyle::new(1.0f64).line_join(LineJoin::Round(1.0f64));

    shape.outline(&style);
}

Expected behavior

It should not panic

Actual behavior

It panics

Environment

• iOverlay version: 6.0.0
• Rust version: 1.95.0
• OS: W11

Additional context

[NailxSharipov]

Hi @SauvageThomas
Thank you for your report and your brilliant short case. It was very helpful.

I've done some research, and yes, this was my mistake after switching to the new buffering logic in version 4.5.0.

The code ignore/skip collinear points for optimization purpose, that's why assert double control it by cross product 0 equlity.

And this led to the same incorrect effect as missing caps at the ends.

Bug

Must be

I made a patch #75

And since it's a very short patch I am going to add it to

4.5.2
5.0.1
6.0.1

[NailxSharipov]

I'd like to acknowledge something else. This degenerate case I just fixed isn't possible in a valid polygon. And the documentation states this buffering api is only allowed for valid polygons.

Offsetting a polygon works reliably only with valid polygons. Ensure that:

No self-intersections.
Outer boundaries are counterclockwise, holes are clockwise—unless main_direction is set.

If polygon validity cannot be guaranteed, it is recommended to apply the simplify_shape operation before offsetting.

[SauvageThomas]

Awesome, thanks for the quick response! I'll take some time to apply more validation to the polygons.