Clipping two polygons without adding a new point at the intersection

[jblestang]

Is is possible to clip two polygons without adding an extra point where the two polygons do intersect?

[jblestang]

I am creating a weather routing simulation for a boat. With the wind sped and wind angle and bearing of the boat I can create an isochron (points where the boat can be after a given amount of time).
at step 1 : I have a list of 360 points
at step 2 : I have a list of 360 isochrons with each item being itself an isochron of 360 points.

the isochron at step 2 is the union of all isochrons of step 2. This problem being an exponential one, by clipping two isochrons I want to keep the outer enveloppe of the isochrons. Unfortunately when clipping points are being added at the intersection of the two polygons.

I was wondering if this behaviour could be made an option or if it was possible to know which points are real points and which one are added by the clipping .

[LarsSkiba]

https://stackoverflow.com/help/minimal-reproducible-example

Create a minimal example that demonstrates an undesired behavior and you will probably be helped.

[jblestang]

With the following Rust program

    let path_a = vec![(0.2, 0.2), (6.0, 0.2), (6.0, 6.0), (0.2, 6.0)];
    let path_b = vec![(5.0, 5.0), (8.0, 5.0), (8.0, 8.0), (5.0, 8.0)];

    // Functional API
    let r: Paths = union::<Centi>(path_a.clone(), path_b.clone(), FillRule::default())?;
    let r1: Vec<Vec<(f64, f64)>> = r.into();
    println!("a = {:?}", path_a);
    println!("b = {:?}", path_b);
    println!("r = {:#?}", r1);

I got the following output

a = [(0.2, 0.2), (6.0, 0.2), (6.0, 6.0), (0.2, 6.0)]
b = [(5.0, 5.0), (8.0, 5.0), (8.0, 8.0), (5.0, 8.0)]
r = [
    [
        (
            6.0,
            5.0,
        ),
        (
            8.0,
            5.0,
        ),
        (
            8.0,
            8.0,
        ),
        (
            5.0,
            8.0,
        ),
        (
            5.0,
            6.0,
        ),
        (
            0.2,
            6.0,
        ),
        (
            0.2,
            0.2,
        ),
        (
            6.0,
            0.2,
        ),
    ],
]

The first point of r is a point added by the clipping algorithm, I would like to be able to remove this extra added points.

[philstopford]

There are no superfluous points in this output. There are two insertion points due to the union, which you can see below:

One is at (5.0, 6.0) and the other is that (6.0, 5.0)

I don't see how else you expect this result to be represented.

[jblestang]

Yes of course, my point is that I would like to be able to identify the intersection points in the final output so that I can remove them.

[philstopford]

You can set USINGZ when you build the library, and then use the set Z callback for that. You can use it to set the intersection points' Z value to -1 and then walk the result to remove those intersection points. With USINGZ, you can also explicitly set the Z coordinate to any value you like and then see the origin of the points in the output. The intersection points will have their Z value set by the callback.

References:

https://www.angusj.com/clipper2/Docs/USINGZ.htm
https://www.angusj.com/clipper2/Docs/Units/Clipper.Engine/Classes/Clipper64/Properties/ZCallback.htm