Function miter_extension_between 

pub fn miter_extension_between(
    width: f32,
    incoming: [f32; 2],
    outgoing: [f32; 2],
) -> f32

miter_extension for a chain that carries its own tangents rather than a third point — a fitted arc/line chain, where the direction a neighbour arrives or leaves at is a property of the piece and not of any vertex.

incoming and outgoing are unit directions of travel through the joint. A G1 joint has them equal, so this returns exactly width there and a tangent-continuous run is byte-identical to one extended by the flat half-width; only the chain’s genuine corners move.