idea: ensure more road smoothness — reduce unwanted waviness from terrain unevenness

[nigel-hall-codes]

What we're exploring

Roads in the baked SF map can become wavy / undulating where the real-world road is actually smooth. The waviness is an artifact of the underlying terrain heightfield being uneven (noisy contour interpolation, sparse elevation samples, steep-grade conform logic), which gets transferred onto the road geometry.

We want roads to:

• Follow terrain on genuine slopes and grade changes (real hills should still feel like hills), but
• Stay locally smooth — suppress high-frequency vertical noise the road shouldn't have.

Why it matters

Wavy roads hurt drivability (vehicle physics bounce), visual realism, and the parked-car / building conform that piggybacks on road height.

Directions to investigate (not decisions)

• Smoothing/filtering the sampled heightfield along the road centerline (moving average, Savitzky–Golay, spline fit) before generating road mesh.
• Distinguishing genuine grade change from noise (slope thresholds, curvature limits, max vertical acceleration along arc length).
• Improving the heightfield source quality (contour interpolation, cache resolution .rNNN.heightcache).
• How road smoothing interacts with the steep-grade conform work in fix: road mesh breaks on steep slope (Lombard) — faceting + buildings float over gaps #219 and with building/parked-car conform.

Relevant code (starting points)

• Assets/Scripts/Pipeline/CarRoadSpawner.cs
• Assets/Scripts/Pipeline/ChunkStreamer.cs
• Assets/Scripts/Pipeline/HeightField.cs
• Bake pipeline: python/sfmap_bake