Posted by dxli on Oct 17, 2024; 6:10pm
URL: https://forum.librecad.org/cannot-snap-intersection-of-two-inscribed-ellipses-tp5725436p5725469.html

I will quickly add fixes to 2.2.1:

1, if no intersection is found, check for tangent conditions;
2, since the tangent condition is not actually decidable with rounding errors, without a topology layer, we are going to allow false interactions, but avoid missing real tangent points.

Due to rounding errors, a tangent line of a circle may see random noises in position relative to the circle. We will accept tangent points whether 0, 1, 2 intersections are found. It means even if the line is actually 1e-6 from closest point on the circle, a tangent point will be identified. Depending on the tolerance level, we are going to report intersections from two close but not contacting curves.

The basic idea, it's acceptable to report a false interaction, but not acceptable to miss a real one.

If there's no objection, I will add this to 2.2.1 and quickly release 2.2.1 as the latest stable.

Tonyeltigresa wrote

Thanks for the detailed reply!

I wont pretend to understand all of it, but I'm sure that at some point it will  be of use to someone who does.

I've no idea how typical my own requirements are, but I'd imagine the geometry I'm trying to draw is somewhat common. It would be great if snap to intersection worked reliably with inscribed ellipses because it's really the only bug I've encountered that has a material impact on the programs usability.

I'll try the workaround and report back.