# Native Road Centre Lines Design

## Architecture

The native compiler adds one independent polygon layer:

```text
canonical bidirectional segment + trimmed OSM centreline
  -> repeated 2m x 0.25m dash polygons
  -> native-road/layers/center_lines.geojson
  -> Workbench / native Blender adapter / Cesium
```

It does not read osm2streets `center_lines.geojson`. The legacy layer is a
visual baseline only.

## Placement

- A candidate must have exactly forward and backward native roads for the same
  `segmentId`, neither be `highway=service`, and have a valid trimmed OSM
  centreline.
- Dashes use the legacy observed dimensions: 2m length, 0.25m width, with a
  deterministic 2m gap. The first dash starts at a fixed segment-local offset
  so rebuilds do not drift.
- The line is trimmed with the same `junctionPlans` cutback used by native lane
  centreline generation. Any dash intersecting a crosswalk or stop line is
  omitted, preserving the control-marking priority already established.
- One-way and service segments have no generated feature. A degenerate source
  line produces a diagnostic rather than malformed geometry.

## Contracts

`layers/center_lines.geojson` is a Polygon FeatureCollection. Every feature
has `native_id`, `segment_id`, `directional_road_ids`, `osm_way_ids`,
`dash_index`, `dash_length_m`, `dash_gap_m`, `placement_rule`, and
`provenance="native-road-center-line/v1"`.

`compiled.json.layers.centerLines`, comparison count
`nativeCenterLineFeatures`, and `build-area.js` required native records use the
same filename. `catalog.NATIVE_ROAD_LAYERS` maps source `center_lines` to the
existing `center_lines` material layer.

The Workbench loads this source into the marking overlay, exposes a Chinese
toggle, and identifies it as `道路中心虚线` on selection.

## Compatibility

The change is additive within `native-road/`; `--road-provider osm2streets`
remains unaffected and is rollback. A native Blender build treats a missing
layer as an input error rather than silently omitting it.