osmWorkflow/.trellis/tasks/08-17-native-road-center-lines/design.md

Native Road Centre Lines Design

Architecture

The native compiler adds one independent polygon layer:

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.