// Movement configs

    // Trail configs
    trail_size_scale: f32,

            // Movement configs

            // Trail configs
            trail_size_scale: 0.5,

        .add_system(update_trails)

struct Trail;
#[derive(Component)]

    let scale = rapier_config.scale;

                    extents: Vec2::new(w, h) * rapier_config.scale * 2.,

                    extents: Vec2::new(w, h) * scale * 2.,

    let shape_ball = shapes::Ellipse {
        radii: Vec2::new(0.35 * rapier_config.scale, 0.25 * rapier_config.scale),

    let shape_ball = shapes::Circle {
        radius: 0.3 * scale,

        .insert(RigidBodyPositionSync::Discrete);

        .insert(RigidBodyPositionSync::Discrete)
        .with_children(|commands| {
            let mut color = Color::ORANGE;
            color.set_a(0.5);
            commands
                .spawn()
                .insert(Trail)
                .insert_bundle(GeometryBuilder::build_as(
                    &shapes::Polygon::default(),
                    DrawMode::Fill(FillMode::color(color)),
                    Transform::default(),
                ));
        });

#[derive(Default)]
struct Position(Vec2);
fn update_trails(
    mut trail_paths: Query<(&mut Path, &Parent), With<Trail>>,
    mut previous_pos: Local<Position>,
    transforms: Query<&Transform, Changed<RigidBodyVelocityComponent>>,
    rapier_config: Res<RapierConfiguration>,
    constants: Res<Constants>,
) {
    let scale = rapier_config.scale;
    let size = 0.3 * scale;
    for (mut path, parent) in trail_paths.iter_mut() {
        let pos = if let Ok(transform) = transforms.get(parent.0) {
            transform.translation.truncate()
        } else {
            continue;
        };
        let delta = previous_pos.0 - pos;
        previous_pos.0 = pos;
        if delta == Vec2::ZERO {
            continue;
        }
        let trail = delta.normalize() * (size * constants.trail_size_scale + delta.length());
        let polygon = shapes::Polygon {
            points: vec![
                trail,
                Vec2::new(trail.y, -trail.x).normalize() * size, // 90 degrees clockwize
                Vec2::new(-trail.y, trail.x).normalize() * size, // 90 degrees counterclockwize
            ],
            closed: true,
        };
        *path = ShapePath::build_as(&polygon);
    }
}