Unreal world building (levels, open worlds, landscape, PCG)

Version 0.1 (2026-09-24), part of Unreal Engine Expert Skills v0.1. Target: UE 5.8 on macOS Apple Silicon.

Expert level here means a world sized by rule, placed by tier of importance, streamed and budgeted from the first map, and changed by script region by region, never by hand-selecting thousands of actors. The environment artist decides what stays manual, what is baked and what is generated at runtime, then proves every choice with a count, a trace or a screenshot.

REQUIRED BACKGROUND: unreal-expert (channels, review loop, 5.8 traps). Toolkit: scripts/ue_world.py (sys.path[:0] = ["<skills>/unreal-world-building/scripts", "<skills>/unreal-expert/scripts"]; import ue_world as W) with ue_run, ue_review, ue_audit, ue_stat. Not yet run in Unreal: the offline layer passed tests/code/unreal-world-building/test_world_offline.py; [verify] names are resolved by W.probe(). A written plan names the engine call behind each W.* step (procedures P0 table).

Stance (the expert delta)

• Start from the integrated setup, then measure. File > New Level > Open World gives World Partition, OFPA, Data Layers, HLOD and outdoor lighting: "just use the template" (Sam Deiter, EEf07ggFWRw [00:20:40]). One runtime grid: more grids "can negatively impact performance" (World Partition doc). WP and OFPA from day one held 100,000 actors (Sam Dark, Subnautica 2, AalP65lrtpo [00:19:20]).
• Place by tier of importance. Hero assets and key areas by hand; shrubs to trees and rocks pre-baked; only grass-class detail generated at runtime on the GPU (CD Projekt RED and Epic, icIFFlOyob4 [00:04:23]-[00:07:05]). Collision and far visibility force persistent placement (Unreal Sensei, 5ju7wyvZGBI [00:54:16]). Bake what players do not perceive as varied; preview procedural output in the editor (Dark [00:07:56]). Decide what stays manual first (Matt Oztalay, TbNZ4GKaTow [00:32:26]).
• PCG is authored like materials. Template graphs with parameters, Graph Instances per use (Oztalay [00:12:18]); Is Partitioned is "pretty mandatory" [00:22:34]. Output inherits the PCG actor's Data Layer and HLOD Layer: assign them before generating (PCG doc). Grids: unbounded for parameters, 128 m for landscape reads, 32 m for scatter; every CPU to GPU transfer costs; layer weights do not exist on the GPU (icIFFlOyob4 [00:08:04] [00:23:16] [00:24:12]). Pop-in is generation latency, judged in Test builds [00:27:10] [00:27:44]. Bad data must be loud: Warn on Data Missing Attribute (Oztalay frame 00:10:47).
• A landscape is sized, not picked. Valid sizes (2017, 4033, never 2048), at most 1024 components, 2x2 sections, edit layers (Landscape doc). Z scale and actor Z come from the exported heights (Z = span / 512, actor at the midpoint), never a declared relief: 250 m on paper reached 314 m and clipped 123,356 pixels in our GREEN test [added]. The material contract, built by unreal-materials, is checked here: Opaque with Opacity Mask wired, one LB Alpha Blend base, a Layer Info per name (Landscape doc); triplanar only on the steep layer, anti-tiling on large layers, low specular (Sensei [00:11:12]-[00:15:36]).
• Nanite by default, construction still matters. "Enabled wherever possible" (Nanite doc), but porous kitbashes, hard-edge splits, seams and noisy vertex colors stop simplification: 250 MB vs 12 MB, same look (Epic, 6igUsOp8FdA [00:02:46]-[00:08:04]). Distance culling does not apply to Nanite (Nanite doc). Tessellation has no collision: per layer, center 0.5 on ground, 1 on vertical rock; dicing rate 2 [00:22:58] [00:29:53]. Foliage opaque and modeled (Quixel, aZr-mWAzoTg [00:01:46]).
• Streaming structure follows content. One HLOD layer per family: Instancing for foliage, Merged Mesh for buildings, Simplified when merged is still heavy (doc; Deiter [00:41:13]). Level Instance when content changes with gameplay, Packed Level Actor for static sets [00:30:59]. Runtime Data Layers only for gameplay state (doc). Teleport targets need a Streaming Source and, for runtime PCG, a generation source (doc; icIFFlOyob4 [00:36:44]).
• World-scale work is scripted, regional and validated. Scripts, never Outliner select-and-drag; whole-map jobs as builder commandlets region by region (Dark [00:20:23] [00:20:55]). Builders generate, then HLODs; generated actors stay out of changelists (Oztalay [00:26:23] [00:29:07]). Rules that documents only request become validators (Dark [00:23:04]).
• Kits are commitments. Footprint is the maximum extent, pivots frozen, standard door widths, core pieces first, hero last (Burgess and Purkeypile, QBAM27YbKZg [00:06:14] [00:07:51] [00:22:34]). Village paths join the nearest built path (Oztalay [00:20:01]); fences follow a spline grammar [00:18:51].