Native Lane Markings Design

Architecture

The native compiler remains the source of truth. It derives two new polygon layers alongside its existing surface, sidewalk, lane-centerline, and connector outputs:

canonical directed roads + lane centerlines + junction cutbacks
    |
    +-- lane separators: paint polygons between adjacent same-direction lanes
    |
    +-- direction arrows: repeated through-arrow template on directed lanes,
    |   outside the reserved junction marking zone
    |
    +-- turn arrows: tested existing template, anchored to an incoming lane
          only when that lane has an explicit supported turn:lanes value

No OSM2streets rendered geometry is consumed by this path. The existing arrow template library is reused only as a geometry/style asset, so the native lane ID and OSM tags remain the evidence for placement.

Contracts

layers/lane_separators.geojson: polygon FeatureCollection. Each feature records native_id, directed road_id, adjacent lane indices, source OSM ways, and native-road-lane-separator/v1 provenance.
layers/turn_arrows.geojson: polygon FeatureCollection. Each feature records its native_id, road_id, lane_id, OSM way IDs, direction, lane index, maneuver, template asset, placement distance, and placement provenance.
Unsupported turn values, a missing usable incoming-lane segment, or an insufficient pre-junction placement distance create a diagnostic and no arrow geometry.
layers/direction_arrows.geojson: polygon FeatureCollection. Each feature records the native lane and directed road, OSM way IDs, a stable sequence index, its distance along the lane, and native-road-direction-arrow/v1 provenance. It uses the tested through template but is not a turn claim.
The workbench serves both layers, draws them separately from its current centerline/connector debug layer, and selects them by native_id.
catalog.NATIVE_ROAD_LAYERS maps the two native sources to the existing lane_separators and lane_arrows_webscale Blender materials. The native adapter does not extend the osm2streets scene-layer registry.

Placement

An incoming lane is oriented in driving direction. A turn arrow is sampled from that lane's endpoint backwards by the configured safe distance, staying outside the junction cutback. Its template basis uses the sampled lane tangent; therefore it is on and aligned with the lane rather than the OSM centerline or a screen-space direction. Multiple template rings remain separate polygons.

Lane separators are narrow polygons centered between adjacent lane centerlines on a single directional carriageway. They stop at the same junction cutbacks as the lane centerlines. A one-lane direction produces none.

Direction arrows are sampled at a fixed road-scale interval along the same directed lane centerline. Their candidates exclude both endpoint buffers and the turn-arrow reserve at the incoming end. This preserves a readable repeated direction cue without overlapping a turn instruction at a junction.

Compatibility And Rollback

All new files are additive under native-road/layers/. The existing osm2streets/QGIS layer contract and package/ are unchanged. Selecting --road-provider osm2streets remains rollback. Missing native marking files are a native Blender build error rather than a silent omission.

Risks

OSM turn tags can be incomplete or incompatible with the inferred lane count. These are diagnostics, not guessed arrows.
Very short approaches can have no safe position before the cutback. They are skipped with a source-traceable diagnostic.
Blender and Cesium need a real native build to verify the mesh/material contract, not only GeoJSON unit tests.

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