Function stroke_line 

pub fn stroke_line(
    coords: &[GeoCoord],
    half_width: f64,
) -> (Vec<[f64; 2]>, Vec<u32>)

Expand a polyline into a thick triangle-strip ribbon.

Each input vertex is extruded along its local normal (perpendicular to the tangent) by +/- half_width, producing two output vertices. Adjacent quads are then connected with two triangles each.

Tangent computation

Vertex position	Tangent source
First	Forward difference to next vertex
Interior	Central difference (average of prev->curr and curr->next)
Last	Backward difference from previous vertex

At sharp bends the averaged tangent may cause the ribbon to narrow or self-intersect. A miter-limit strategy is planned for a future release.

Returns

(positions, indices) where:

• positions – [lon, lat] pairs in degree space, two per input vertex (left and right of the centreline).
• indices – triangle indices into positions (6 per segment).

Edge cases

Input	Result
Fewer than 2 coordinates	(empty, empty)
half_width <= 0.0	Degenerate zero-area ribbon (positions collapse onto the centreline)
Coincident consecutive vertices	Zero-length tangent yields zero normal – the two extruded vertices coincide, producing degenerate triangles that are invisible on screen

Example

use rustial_engine::stroke_line;
use rustial_engine::GeoCoord;

let line = vec![
    GeoCoord::from_lat_lon(0.0, 0.0),
    GeoCoord::from_lat_lon(0.0, 1.0),
];
let (positions, indices) = stroke_line(&line, 0.01);
assert_eq!(positions.len(), 4); // 2 vertices * 2 sides
assert_eq!(indices.len(), 6);   // 1 segment * 6 indices