a-b-street/abstreet · error

at has weird geom along

Error message

{} at {} has weird geom along {:?}: {}

What it means

get_draw_car slices each past step's lane polyline to rebuild the car's rendered geometry and appends the pieces. If PolyLine::append fails (degenerate or disconnected geometry), rendering cannot continue and it panics with the vehicle id, time, step list, and the underlying polyline error.

Solutions

1. Inspect the printed vehicle id and last_steps; check the referenced lanes for degenerate geometry on the current map
2. Regenerate or fix the map geometry (avoid zero-length lanes) before replaying
3. Report as a rendering robustness bug; as a workaround, restart the sim on the unedited map

Example fix

// graceful alternative in caller code
match car.get_draw_car(map, now) {
    Ok(draw) => g.draw(...),
    Err(_) => {} // skip rendering this car instead of crashing the frame
}

// ensure the map has no degenerate lanes before replay
for l in map.all_lanes() {
    assert!(l.length() > Distance::ZERO && l.center_pts.num_points() >= 2);
}

match std::panic::catch_unwind(|| car.get_draw_car(map, now)) {
    Ok(draw) => g.draw(...),
    Err(_) => {} // skip this car's draw
}

When it happens

Trigger: get_draw_car() during rendering when a sliced lane piece cannot be appended — usually zero-length or single-point polyline pieces produced after extreme map edits or degenerate lane geometry.

Common situations: Viewing a replay/simulation after edits that created degenerate lanes; cars whose last_steps span lanes with broken geometry; long replays crossing modified intersections.

Understand the failure class

Background: "This is a bug, please report it": internal invariant violations, unreachable panics, and SNH errors explained — this error's family across 47 libraries.

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AI-assisted analysis of a-b-street/abstreet@0964f29315 (2026-09-13). Data as JSON: /api/errors/12367bd0d45fee8f. Report an issue: GitHub.

Appendix: source

Thrown at sim/src/mechanics/car.rs:120

            let mut i = 0;
            while leftover > Distance::ZERO {
                if i == self.last_steps.len() {
                    // The vehicle is gradually appearing from somewhere. That's fine, just return
                    // a truncated body.
                    break;
                }
                partly_on.push(self.last_steps[i]);
                let len = self.last_steps[i].get_polyline(map).length();
                let start = (len - leftover).max(Distance::ZERO);
                let piece = self.last_steps[i]
                    .get_polyline(map)
                    .slice(start, len)
                    .map(|(pl, _)| pl.into_points())
                    .ok()
                    .unwrap_or_else(Vec::new);
                result = match PolyLine::append(piece, result) {
                    Ok(pl) => pl,
                    Err(err) => panic!(
                        "{} at {} has weird geom along {:?}: {}",
                        self.vehicle.id, now, self.last_steps, err
                    ),
                };
                leftover -= len;
                i += 1;
            }

            if result.len() < 2 {
                // Vehicles spawning at a border start with their front at literally 0 distance.
                // Usually by the time we first try to render, they've advanced at least a little.
                // But sometimes there's a race when we try to immediately draw them.
                if let Ok((pl, _)) = self
                    .router
                    .head()
                    .get_polyline(map)
                    .slice(Distance::ZERO, 2.0 * EPSILON_DIST)
                {

View on GitHub (pinned to 0964f29315)