# Procedural Geometry

> Use when generating or reviewing the GEOMETRY of a game — procedural terrain, meshes, structures, and scatter. Covers noise as the substrate (Perlin/Simplex/value noise, fractal Brownian motion, domain warping), terrain generation (heightfield vs voxel/SDF, marching cubes, dual contouring, hydraulic/thermal erosion, LOD/chunking), structural generation (L-systems, shape grammars, wave function collapse / model synthesis, constraint-based layout), mesh topology correctness (watertight/manifold meshes, inverted normals, winding order, degenerate triangles/UVs, runtime mesh performance and caching), and vegetation/scatter (Poisson-disc/blue-noise placement, slope/altitude/biome masking, GPU instancing). Also use to diagnose terrain that looks like "noise lumps", structures that read as grid-aligned or obviously generated, scatter that clumps or grids, or generated meshes with black/inverted faces and broken collision. Triggers on "procedural terrain", "terrain generation", "Perlin noise", "Simplex noise", "fBm",

- Skill: `rondorkerin/procedural-geometry` (Agent Skill, multi-file: 3 files)
- Install (CLI): `npx skillmds@latest add rondorkerin/procedural-geometry`
- Raw SKILL.md: https://api.skillmd.com/api/skills/rondorkerin/procedural-geometry/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: AI & ML
- Author: rondorkerin (https://skillmd.com/u/rondorkerin)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/rondorkerin/procedural-geometry

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# Procedural Geometry

How to generate the *geometry* of a game — terrain, meshes, structures, and scatter — so it is **correct** (watertight, manifold, properly wound, performant) and **varied** (perceptually distinct, not parametric oatmeal). Geometry generation is engineerable math: noise as a substrate, modulation and erosion for believability, constraints and hand-placed anchors for "designed-looking" structure, blue-noise for scatter, and topology validators as the non-negotiable correctness floor.

> **Tier:** technical craft (→ `gamestack-core`). Engine-agnostic geometry-generation math underneath generated terrain, structures, and meshes.

## When to use this

- Generating terrain (heightfield or voxel/SDF), and deciding which representation the design actually needs
- Choosing and wiring a mesh-extraction (marching cubes vs dual contouring), erosion, and LOD/chunking pipeline
- Generating structures with L-systems, shape grammars, or wave function collapse / model synthesis
- Authoring the **mesh topology validator** (winding/normals, manifold, degenerates, watertight)
- Scattering vegetation/props with blue-noise placement, environmental masking, and instancing
- Diagnosing terrain that looks like "noise", structures that read as generated, or meshes with black faces / broken collision

## Scope

This skill owns **the math and algorithms that produce geometry** — meshes, terrain, structures, scatter. It is deliberately narrow. Adjacent concerns live in sibling skills:

- *What content means and whether it's perceptually varied at scale* (quests, lore, items, the oatmeal problem for **content**) → `procedural-generation`. This skill is the geometric statement of the same hybrid (hand-anchor + constrained fill); that skill is the narrative one.
- *Making a generated level legible, navigable, gated, and paced* → `level-design`. This skill is the **geometry engine underneath** it — grammars/WFC produce the rooms; `level-design`'s principles make them readable. Geometry serves legibility; it does not replace it.
- *Gating generated output for perceptual sameness and intentionality* → `procgen-review`. Its **oatmeal test** runs on the geometry this skill emits; the topology validator here is the *correctness* floor below that *quality* gate.
- *Rendering, culling, and draw-call budgets for the meshes produced* → `3d-graphics-and-rendering`. *Texturing/shading them* → `shaders-and-vfx`.
- *Macro/world spatial layout, biomes as places, exploration pull* → `open-world-design` (this skill is the terrain math underneath that layer).

## How the pieces fit

- **`GUIDE.md`** — the cited *why*, in five sub-domains: noise as the substrate (Perlin/Simplex/value, fBm, multifractal/ridged, domain warping, Worley/3D-noise caves); terrain generation (heightfield vs voxel/SDF, marching cubes vs dual contouring, erosion, LOD/chunking); structural generation (L-systems, shape grammars, WFC/model synthesis, hybrid hand-anchor); mesh topology correctness (winding/normals, manifold/watertight, degenerates, runtime threading/caching); and vegetation/scatter (blue-noise, masking, instancing). Each rule carries an exemplar + source, a **test-for** criterion, the named failure mode, and the **procedural/headless implication**.
- **`CHECKLIST.md`** — Do/Don't + machine-checkable **Test-for** criteria, grouped by sub-domain. Written to be enforced as validators in a generation loop.

## The two ideas to anchor on

> **1. A single noise field is oatmeal; structure comes from modulation, not octaves.** Plain fractal Brownian motion is the canonical "fake terrain" look — equal roughness everywhere, no ridgelines, no drainage. Believability comes from *modulating* it (multifractal/ridged noise, domain warping) and from **erosion simulation** that carves the connected valleys and ridges a process would. More octaves add detail, not structure.

> **2. "Looks designed, not random" is a constraint problem.** Grammar- and constraint-based methods (L-systems, shape grammars, wave function collapse / model synthesis) get their authored quality from **hand-authored rules and tilesets**, not from the algorithm. The designer's craft moves into the constraint set — and the most reliable path at scale is **hybrid**: hand-place the silhouette landmarks and set pieces, let the generator fill between them under constraints.

> **Why this matters doubly for a generator:** a human sees a black inside-out face, a grid-aligned forest, or 10,000 identical buildings and feels "off." An autonomous generator has no eyes. Author the **mesh topology validator** (winding/manifold/degenerate/watertight) and the **representation, erosion, crack-free, and instancing contracts** as inviolable; let generation vary inputs *inside* them (frequency, octaves, density masks, seeds); and gate every batch through `procgen-review`'s oatmeal test for *geometric* sameness — because parametric variation of one formula produces perceptually identical results.

Start with `GUIDE.md`, then apply `CHECKLIST.md`.