Planning

Overview

The city is a continuous 3D world — no tile-based simulation grid underneath. Grids are a UI aid, drawn only in contexts where people reason spatially in units: building plots, city blocks, and the design-time surveying/road-sketching mode. They disappear in normal play.

Design-Time Surveying

• A dedicated top-down/ortho mode for terraforming and road layout, echoing SimCity 4’s survey tools.
• Shows a light construction grid plus terrain contour lines; road centerlines snap to grid + contour intersections with adjustable tolerance.
• Toggled off (or faded to near-invisible) once back in the free 3D camera / play mode.

Plots & Blocks

• A block is a polygon bounded by road-bundle frontage edges, not a fixed tile region.
• A parcel/lot subdivides a block along its frontage, sized in world units (cm, as plain float64 — see Coordinate precision), snapped to a light alignment grid rather than forced onto fixed tiles.
• This keeps blocks irregular and organic, like real subdivided city blocks, while still giving players clean alignment guides while editing.
• Default alignment grid is a 10m cell with a 5m snap tolerance.

Road-bundle frontage edges are defined in Arc-Spline Transport.

Parcel Subdivision

Subdivision co-generates parcels with the road graph so every buildable lot has street access and neighboring lots do not claim the same ground. The pipeline follows Chen, Song & Ortner's co-generation idea adapted to our stack: never emit overlapping candidates and repair them afterward — if two parcels would share an interior point, the generator that produced both is wrong.

Closed blocks

When frontage edges form a closed loop, the network yields a block. The block interior subdivides along frontage (extractBlocks → subdivideBlock). Each child inherits or receives a frontage edge; landlocked children are not emitted.

Open networks & junctions

Roads that do not enclose a loop subdivide road-facing: each side of a centerline gets a strip of parcels along its offset frontage (subdivideRoadFrontage). Where two or more open roads share a centerline endpoint, independently subdivided strips would leave a gap or overlap at the corner. Before ordinary frontage subdivision runs, junction closure builds that corner by construction:

1. For each interior corner between angularly adjacent offset arms, emit a wedge parcel whose outline is the two frontage offsets plus the corner that joins them (subdivisionMode: "junction").
2. Trim each road's ordinary frontage by the arc-length those wedges already cover (trimStart / trimEnd), so the first road-facing parcel meets the wedge at the miter rather than overlapping it.

Wedge reach is capped at the ray-intersection (miter) distance between the two frontage lines (and further by half a typical frontage width at the call site). A fixed reach past the miter produces a self-intersecting bowtie.

Mid-span crossings (a stem meeting a through road's interior, or two through centerlines crossing away from shared endpoints) are the same algorithm with a different junction detector. Until that detector exists, pairs that are not closed by construction use a conservative drop-both safety net based on real polygon intersection — not a centroid-in-polygon heuristic, which misses long thin road-facing overlaps whose centroids lie outside each other.

Invariants

• Every parcel has street frontage (see Data Model).
• No generate → detect-overlap → delete → synthesize-patch loop at junctions.
• Contiguous single coverage around endpoint junctions (cross, Y, multi-way, tight corners at or above the minimum approach angle) is the acceptance bar; geometry optimization may refine vertices only after topology is already valid.

Data Model

Every parcel must have street frontage to be buildable and valuable. Landlocked parcels (no street access) are legally and economically unusable. Therefore, frontageEdgeId must never be null or undefined.

Parcel Constraints & Construction Grid

The dedicated Parcels surveyor tool allows players to inspect and customize individual platted lots without affecting the surrounding road network or triggering zoning actions.

• Local Construction Grid: When maxLotCoverage < 1.0, building footprints are anchored flush against frontage and snapped to a 50cm grid cell in parcel-local model space (canonical rectangle) before 3D extrusion and mapping back to world coordinates.
• Bowtie Elimination: Each parcel records its immutable basePolygon from initial subdivision. Setback edits recalculate one-shot against this base geometry using bisection scaling if requested insets would cause self-intersecting polygon folds.
• Inside-Curve Radial Clamping: Along curved roads, rear lot lines converge radially toward the center of curvature; depth is clamped safely short of the focal singularity ($R - 50\text{cm}$) to prevent ray-crossing.

Saved Plans

A plan is a named, saved set of sketched transport networks — a snapshot the player can restore into the current sketch rather than a submitted plat. Distinct from a plat (the block/parcel/zoning document a Surveyor session submits): a plan only holds the transport-line sketches used to refit those curves later.

Terrain Integration

Survey mode integrates with the terrain system to provide visual and cost-based feedback:

• Visualization: Terrain contours (10m interval), construction grid, elevation labels, and toggled slope ramp render beneath road sketches.
• Grade Evaluation: When a road sketch is committed, terrain grade is evaluated per-segment (max 6% freeway, 8% arterial, 12% local). Violations flagged; total cost adjusted for slope.
• Terrain Snapshot: Plan stores a compressed baseline heightmap state at creation time, enabling consistent re-rendering if terrain regenerates and supporting undo/redo within survey sessions.
• Camera Easing: Entering survey mode smoothly eases camera from orbital perspective to fixed ortho top-down (500ms ease). Exiting eases back to prior orbital pose (target + yaw/pitch).

Undo/Redo in Survey Mode

Full undo/redo stack using Memento pattern (immutable snapshots + caretaker history):

• Each sketch commit creates a memento capturing plan state + terrain snapshot.
• Memento creation debounced every 500ms using fastest available wasm compression (jsr).
• History limited to 30 entries to prevent unbounded memory growth.
• Keyboard shortcuts: Ctrl+Z (undo), Ctrl+Y (redo).
• Undo reverts sketches and restores terrain snapshots; redo re-applies with grade re-evaluation.

Future Work

• Parking Lot Geometry & Rendering: Visualizing and reserving leftover space behind building footprints as parking lots (tracked in Parking Tool).
• Parking Minimum Validation: Validating that unbuilt lot area accommodates required parking stalls.
• Multi-Parcel Bulk Editing: Copy/pasting setback and coverage constraints across multiple selected parcels.
• 3D Viewport Grid Visualization: Overlaying construction grid lines on selected parcels in the 3D viewport.
• Height / Story Caps: Adding per-parcel maxFloors constraints into building massing heuristics.
• Zoning Rule Integration: Defaulting setbacks, coverage, and parking minimums from municipal zoning codes.