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

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. If a sibling skill named here is missing from your available skills, ask the user to install it (npx skills add scenario-labs/skills --skill <name>); unattended, proceed from tool schemas and flag the gap.

REQUIRED BACKGROUND: scenario-unreal-expert (channels, review loop, 5.8 traps). Toolkit: scripts/ue_world.py (sys.path[:0] = ["<skills>/scenario-unreal-world-building/scripts", "<skills>/scenario-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 scenario-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].

Establish first

Input	Changes	Default when silent
Platform, frame budget	Nanite, foliage path, layers; PCG time, grass density	PC and consoles, Nanite on, 60 fps (scenario-unreal-performance)
Size, quad size, height source	layout, Z scale, actor Z	W.plan_landscape, 1 m quads; Z fitted to the heights
Fastest traversal, teleports	loading range, streaming and generation sources	ask gameplay; recorded as unknown
Terrain features, Experimental tolerance	Landscape vs Mesh Terrain; Nanite Foliage, PVE, FastGeo	Landscape; no Experimental feature for shipping
Manual vs procedural, source control	graph scope, who commits generated PCG	hand: village layout, river, roads, hero areas; builders own generation

Workflow

Each stage is an ue_run job or a live call; code in procedures (P-numbers).

1. Brief as a world spec (P1), like W.U1_EXAMPLE_SPEC; W.probe() once per install. GATE: W.check_world_spec(spec)["ok"], info findings answered.
2. Map (P2). W.new_open_world_map(path) from the template; old levels through W.convert_args(..., report_only=True) first. GATE: W.wp_facts_check(W.world_facts()) clean; PIE screenshot with wp.Runtime.ToggleDrawRuntimeHash2D.
3. Terrain (P3, P4). DCC heightmap or the blockout W.valley_heightmap; W.export_heightmap fits Z and actor Z to the real range. Creation is one Landscape mode step (Location Z = actor_z_cm, Scale Z = z_scale) unless an MCP tool does it [verify]. Edit layers Base, Sculpt Details, Paint, Splines, Patches. River (Water Body River or a Splines-layer spline) before heavy painting. Then the landscape builder. GATE: W.heightmap_clip_gate empty (0 clipped pixels) before import; W.landscape_facts() matches plan, scale and location; W.landscape_material_check clean; Layer Debug, cliff close-up, grass in sun, mannequin at 2, 10 and 100 m.
4. Organization (P5, P6). Editor Data Layers per family; HLOD layers from W.hlod_plan; Streaming Source components at teleport targets. GATE: W.data_layer_check, W.hlod_actor_check clean.
5. Village and kits (P7). W.kit_piece_check, W.door_width_check; houses as Level Instances or Packed Level Actors; clutter as tagged assemblies (W.tag_actors); paths by W.village_paths into landscape splines; fences by grammar strings. Version the map before breaking an instance. GATE: no kit errors; no z-fighting or obvious repetition from the paths.
6. Nanite (P8). W.nanite_construction_check(W.obj_construction_stats(obj)) on DCC exports; W.batch_nanite(paths, dry_run=True), read, run; W.tessellation_check with per-layer centers. GATE: no translucent Nanite mesh; Overdraw over the canopy; feet on displaced ground.
7. Baked PCG: forest and rocks (P9). Template graph (layer filter warning on missing data, slope density, Extents Modifier into Difference for paths, Distance thinning at banks, understory differenced against trees, Self Pruning, species table, collision on trunks only), checked by W.pcg_graph_lint; W.spawn_pcg_volume(..., hlod_layer=, data_layer=). Debug one volume (D) first. GATE: W.pcg_component_check clean; counts against W.expected_points; top-down and gameplay-height shots, nothing on paths or curated areas.
8. Runtime grass (P10). W.grass_rings() (dense to 128 m on 32 m cells, sparse to 256 m on 128 m cells, far edge recolored); mask from grass types without meshes through Generate Grass Maps; a placeholder in every asset array; editor viewport as generation source; landscape cache; pcg.FrameTime set. GATE: W.runtime_grass_check and W.find_gpu_transfers clean; W.generation_lead_check with latency measured on a Test build.
9. Builds (P11). for name, args in W.build_pipeline(map, graphs): ue_run.run_commandlet(uproject, name, args): landscape, PCG, HLOD. GATE: exit 0 each; HLOD actors per layer counted; W.changelist_check, then submit from the editor.
10. Review and handoff (P12 to P14). W.tour(...) with W.REVIEW_CONSOLE passes, ue_review.image_checks on every shot, then look; W.world_rules_check(W.world_census(), vocab); W.budget_sheet, W.lighting_handoff. GATE: critique rubric passes; nothing reported done without a capture.

Numbers

Value	Setting	Relative to / source
Landscape 2 km	2017 x 2017, 63 quads, 2x2, 256 of max 1024 components	1 m quads (Landscape doc)
Z scale	span cm / 512 x 1.02; actor Z at the span midpoint	16-bit heights (doc); headroom [added]
Template grid	cell 25,600 cm, range 76,800 cm	profile first (Deiter [00:15:32])
PCG partition grid	25,600 cm	PCG World Actor (Oztalay frame 00:23:16)
Hierarchical grids	unbounded, 128 m, 32 m	runtime grass (icIFFlOyob4 [00:08:39])
Grass rings	dense to 128 m, sparse to 256 m	icIFFlOyob4 [00:15:30]
Instances per 32 m cell	10k to 20k fine	base PS5, 5.6 demo [00:30:26]
pcg.FrameTime	5 ms default; 2 ms in the demo	5.6 notes; base PS5 [00:26:24]
Height upload	about 0.5 ms game thread per 32 m cell	base PS5 [00:24:12]
Streamed instances	16 million hard limit	Nanite doc
Nanite construction	250 MB vs 12 MB, same look	6igUsOp8FdA [00:02:46]
Nanite Assemblies	5 to 10% of asset size; 200 to 1,000 wind bones	aZr-mWAzoTg [00:07:13] [00:09:51]
Dicing rate	2; slower below 2 and above about 5 or 6	6igUsOp8FdA [00:29:53]
Displacement center	0.5 ground, 1 vertical rock, per layer	collision surface [00:22:58]
Terrain specular	0.02 (Sensei); 0.5 as a cavity-driven ceiling (Cloward)	5ju7wyvZGBI [00:11:12]; 0L5Azq6ugyo [00:01:19]
Tick aggregation	above 10 to 15 placed instances	Dark [00:23:04]
Mobile landscape	3 layers, 16 samplers	ES3.1 (doc)

Quality gates

• Measurable: the workflow checks (all W.*_check, heightmap_clip_gate, pcg_graph_lint, find_gpu_transfers); ue_audit on imports; builders exit 0; stat unit, stat gpu per viewpoint; PCG time in Insights on a Test build; -trace=WorldStreaming; wp.Runtime.HLOD 0/1 A/B.
• Visual: runtime hash overlay without failed cells; Layer Debug without black spots; tiling against a mannequin; cliffs without stretching; no plastic sheen on grass in sun; feet on displaced ground; canopy overdraw; Hierarchical LOD Coloration and HLOD silhouettes; ring handover and pop-in on frame-by-frame video; nothing on paths, banks thinned; rocks grounded. Rubric: references/critique.md.

Common mistakes

Mistake	What it looks like	Fix
Z scale from a declared relief	plateaus and pits after import	fitted export, W.heightmap_clip_gate
2048 heightmap	import refuses or resamples	2017 or 4033
All layers height-blended; layer without Layer Info	black patches; layer never paints	base layer Alpha Blend; Layer Info, same name
Second runtime grid	streaming cost, no gain	one grid, HLOD for distance
Runtime Data Layers for tidiness; Level Blueprint references	hitches; actors always loaded	Editor Data Layers; Blueprint classes
PCG volume without HLOD or Data Layer	baked forest missing at distance	assign both before generating
Runtime graph invisible in editor	artists see nothing	Treat Editor Viewport as Generation Source
Layer weights in a GPU graph	empty grass mask	grass types without meshes, Generate Grass Maps
Empty artist asset array	grass streams collapse	one placeholder per array
Cull or draw distance on Nanite	no thinning, rings do not hand over	HLOD, generation radius per ring
GPU-only trees	trees missing from Lumen	GPU-only for small things
One ever-larger grass radius	instance explosion, visible edge	two rings, recolored edge
Porous kitbashed rocks	huge assets, holes at distance	connected base, tiling texture
Construction-script scatter	170,000 bodies, 7-minute map open (Oztalay [00:30:49])	PCG output, clutter collision off
Generated PCG actors checked in	contention, stale data	builders own generation
Select-all edits on a big map	editor stalls	W.region_batch or a builder

Handoffs

• Receives from scenario-unreal-pipeline-automation: meshes passed by ue_audit, standard kit pieces, DCC heightmaps with their height range; construction failures go back (and to scenario-maya-expert, scenario-blender-expert).
• Receives from scenario-unreal-materials: the landscape instance and layer list (blend types, steep layer, anti-tiling), RVTs with bounds, grass type names, the rock RVT blend, opaque foliage. Specular is theirs; the world artist reports sheen from the sun capture.
• Delivers to scenario-unreal-lighting-rendering: the map, World Bookmarks or tour_viewpoints, W.lighting_handoff (template lights, GPU-only roles outside the Lumen scene, HLOD far-field and distance-field flags, Data Layer states).
• Delivers to scenario-unreal-performance: W.budget_sheet and the tour path.
• Also: scenario-unreal-gameplay (streaming and generation sources, Asset User Data, tick aggregation); scenario-unreal-pipeline-automation (world rules for its validators); scenario-unreal-cinematics (pcg.FrameTime pauses, icIFFlOyob4 [00:28:51]); scenario-unreal-vfx (water shading).

UE 5.8 notes

• New WP maps use the runtime hash with partitions (5.4): read world_facts() before writing grid properties [verify].
• PCG on by default; PCG Python Data Processor, PCG Builder Volume; landscape cache opt-in since 5.6 (runtime CPU reads fail without it).
• Edit layers mandatory (5.7); landscape.ForceLayersFullUpdate replaces ForceLayersUpdate; WorldPartitionLandscapeBuilder (5.7); water brushes force a global merge.
• HLOD: Build HLOD for Selection or Region, bForceRayTracingFarField, Custom HLOD actors (5.7); Approximate Mesh absent from the saved doc [verify].
• Experimental: Mesh Terrain, Nanite Foliage, PVE, FastGeo, World Streaming Insights. Mac: Nanite and VSM Beta on M2 or newer; cook platform Mac.

References

• references/expert-notes.md: judgment by expert, with timestamps.
• references/procedures.md: P0 to P14, full code, engine calls behind the toolkit, tests, status.
• references/critique.md: the rubric for each gate.
• references/gui-paths.md: editor menus for every procedure.
• references/sources.md: sources, credentials, URLs, timestamps, revision history.