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name: scenario-unreal-lighting-rendering description: 'Use when lighting or rendering in Unreal Engine 5.8: physical light units and exposure (EV100, lux, lumens), Lumen GI and reflections, MegaLights, virtual shadow maps, sky, fog and clouds, post process and color grading, the path tracer, or Movie Render Graph stills. Triggers: "Lumen is noisy", "MegaLights noise", "the interior is too dark", "light leaks", neon signs, a night or dusk scene, "fireflies", "path tracer vs Lumen", "render a still", EXR settings.' license: MIT

Lighting and rendering in Unreal Engine 5.8

Expert lighting in Unreal is a calibrated camera, physical light values, and a renderer that sees the same scene you do; everything else is art direction layered on top and listed as such. Each step is proved with a representation view and a number before the lit image is trusted; noise is fixed in the system that owns it. 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).

Status (2026-09-24): scripts/ue_light.py is not yet run in Unreal; 200 pure, 65 editor-layer and 20 procedure checks pass offline against a fake unreal. First editor run: tests/code/unreal-lighting-rendering/job_00_probe_lighting.py.

Stance (the expert delta)

• Camera before lights. Default auto exposure "childproofs" the engine: a 10 lux and a 100,000 lux sun look alike. Chrome ball at the subject, Exposure Compensation 0, Min/Max EV100 clamped to the conditions, then lights; emissive last (Gobey nlbJwMoj1Dg [00:02:38] [00:11:40]; Faucher BGoaPyfZlYg [00:01:34]). Judge through Game Settings exposure, never the viewport EV100 override (Gobey [00:21:56]).
• Physical values remove problems; cheats go on top and in the report. Real lumens removed about 80 percent of Lumen flicker (Argyriou Q1whHlGJB_o [00:14:41]); albedo is lighting, at most 0.8, floor 0.02 [added] (path tracer doc; Gobey [00:07:30]; Faucher X5zVhc5ahl0 [00:22:24]). Every light has a visible fixture and a source shaped like it (Faucher 0GYyHDuaPcg [00:15:14] [00:15:47]). Moons, fills and boosts are fine when listed (cheat_inventory): the path tracer will not show them.
• Emissive is the look, lights do the lighting. Small bright emissive is noisy, screen-space and clamped harder since 5.6 (r.Lumen.ScreenProbeGather.MaxRayIntensity 10): a neon sign gets a rect light fitted to its face, in front of it (Faucher 1e6oOiKh91U [00:12:30]; Gobey Q&A [00:37:33]; MegaLights doc). Under Lumen, small emissive meshes get Emissive Light Source, optionally a Lumen-only boost of 5 to 10 through a Ray Tracing Quality Switch [verify node] (Campbell BKaAzhMHJZ0 [00:14:09]); the path tracer ignores that switch and double counts a tube plus its light (path tracer doc).
• The renderer sees a proxy; look at it. Lumen Scene and Surface Cache (black = screen traces only, pink = no cards, often a one-mesh room), MegaLights shadow caster mismatch, VSM cached pages, each against the lit view from the same camera (Faucher [00:07:21] [00:06:42]; Lumen, MegaLights, VSM docs).
• MegaLights noise is stolen samples, not light count. Hidden lights take up to 20 percent of samples (50 after failed reprojection), a directional up to 50. Tight radius, cones, barn doors, no light inside geometry, clusters merged, the sun on deferred plus VSM (Narkowicz and Costa dmmN8_c8Tb0 [00:10:37] [00:16:40]). Alpha masks are ignored by its ray tracing by default (MegaLights doc).
• Shadow method follows what moves. Local lights stay ray traced in gameplay: every character crossing a VSM local light invalidates its pages, while ray tracing pays per visible pixel with free penumbras (Campbell [00:26:25] [00:31:42]). VSM for the sun and for content ray tracing cannot represent, with a written reason.
• A dark interior is a tone-mapper problem first; light before you grade. Toe 0.55 to 0.3 and Local Exposure Shadow Contrast 0.6 before any fill (Gobey [00:14:58] [00:15:32]); then the ladder: direct light, one light's indirect boost, Diffuse Color Boost, global exposure last (Faucher 0GYyHDuaPcg [00:08:14] [00:13:42]). Glossy surfaces need a direct light: Lumen indirect gives little specular (Faucher [00:10:28]).
• For the still, the path tracer is ground truth and samples are its only quality knob. On the deferred path, samples buy motion blur and anti-aliasing only; Lumen or shadow noise is fixed at the light or the PPV (Faucher X5zVhc5ahl0 [00:07:43]; fVg5ihB8Wdc [00:09:15]; Comly, demystifying-mrq).

Establish first

Input	Changes	Default when silent
Platform, frame rate	Lumen scalability, MegaLights, HWRT	console 60 fps: Lumen High; budgets from scenario-unreal-performance; Mac numbers are proxies
What this Mac runs (probe)	every plan	M5 Max, macOS 26.5.1: Lumen HWRT and MegaLights Experimental, Nanite and VSM Beta, path tracer supported (deltas file)
Deliverable	exposure mode, still plan	gameplay plus one still; direct delivery (tone-mapped PNG plus EXR)
Conditions the player walks between	EV100 bracket	CONDITIONS: interior 4, blue hour 6.5 [added], low sun 7, moonlit 1
Style	physical vs art-directed moon and sun	physical base; film moon and a 15 degree sun floor only for readability (Faucher; Oakley rX0wZZxpB-U [00:19:16])
Key cameras; glass and emissive fixtures in shot	fog scattering, cheats, glass and double-count audits	bookmarks from the brief; front layer only in a PPV around hero glass

Workflow

Editor Python via Epic's MCP server or ue_remote; import ue_light as L; full code in references/procedures.md. Screenshots are latent: one capture per call.

1. Probe and settings (P0, P1). probe(), render_settings_report, ini_patch after a backup; extended luminance range only in a new project. GATE: probe JSON read.
2. Calibrate the camera (P2). spawn_calibrator at the subject, then one unbound PPV_Global: apply_exposure(ppv, exposure_plan("gameplay", ("interior", "blue_hour"))), apply_look (Local Exposure mandatory with Lumen GI; Film only here). GATE: ppv_lint no fail; HDR view shows each target inside the clamps.
3. Exterior (P3). build_exterior_rig(exterior_rig_plan("blue_hour", key_camera_forward=...)): Sky Atmosphere, sun at 120,000 lux below the horizon, real-time Sky Light, volumetric fog with black inscattering, Scattering Distribution from the key camera's angle to the light (0.9 into it, 0 from the side: Faucher 1LfiYtKDsac [00:16:06]). Steady lamps scatter above 1 for halos, flickering ones 0. GATE: scene_lint clean; sun out of MegaLights.
4. Practicals, neon, openings (P4, P5). FIXTURES lumens and kelvin, radius from tight_attenuation_radius capped by the room, source radius, length or size matching the fixture, one rect per shelf, neon_sign_light in front of each sign, opening_rect_light for the window; emissive via emissive_for_stops, then emissive_strategy (Emissive Light Source, optional boost). GATE: light_lint no warn in the gameplay profile (fixture, shape, VSM, clusters), or a written reason.
5. Does the renderer see it (P7 to P9). Lumen views vs lit; room_shell_lint on pink; MegaLights dump and mismatch view; masked casters; VSM cached pages. GATE: no black or pink where light must bounce; no hidden light in the dump; VSM red share near 0; noise judged in motion.
6. Look (P10). Ambient shape first, then the focal hue, then "the promise of more": light leaving the frame, a lit side street (Oakley [00:07:34] [00:09:19] [00:08:44]); the shadow-lift ladder in order; grade last; white-balance direction by A/B screenshot. GATE: frame_report B3 to B8.
7. Review loop (P11, P12). Per bookmark: log_mark, capture, frame_report(frame, brief, log_text=log_since(...)), one change, logged in iterations.jsonl. The 5.8 exposure-range warning fails the frame; glossy regions must hold a highlight; walk the player path for adaptation. Timings use P12's measure preset (async compute, dynamic resolution off) and go to scenario-unreal-performance. GATE: no fail.
8. The still (P13). Scene audit first: Base Color buffer through albedo_check, emissive_fixture_pairs (A/B r.PathTracing.EnableEmissive 0, Indirect Emissive off), glass_lint. Then still_plan("path_traced", "final", glass=True, sky_atmosphere_visible=True, emissive_fixtures=...), mrg_lint clean, set_cine_camera_exposure (Manual metering, Apply Physical Camera Exposure), queue_mrg_still. Still-only cheats via still_only_light_plan on their own lighting channel. Offline: gray card within 1/3 stop on the linear EXR, compare_ground_truth, 100 percent crops; a render that differs from the viewport walks RENDER_DIFF_TRIAGE. GATE: C1 to C11 of critique.md.
9. Handoff (P14). lighting_manifest(path, ctx): lights, PPVs, lint, cheats, emissive pairs, masked casters, cvars.

Numbers

Item	Value	Relative to, source
Sun, moon	120,000 lux at zenith (0.545 degrees); moon 0.26 lux	Epic sky doc
Look targets (EV100)	sunlit 14, cloudy 10, low sun 7, interior 4, moonlit 1, moonless -2	Gobey chart [00:12:02]; targets, not meter readings
Fixtures	candle 12 lm 1,900 K; decorative 300; interior 1,000; exterior 10,000; street 2,500 to 10,000 lm	Gobey chart; Argyriou [00:13:36]
Units	1 cd = 625 unitless = 12.6 lm (point) = 3.14 lm (rect) = 1.76 lm (spot, 44 degrees)	exposure doc
Exposure	EV100 = log2(N^2 / t x 100 / ISO); B = L / 2^(EV100 + comp); f/2.8, 1/50, ISO 800 is EV100 5.6	exposure doc
Auto exposure	Low 70 to 80, High 80 to 95 percent; Speed Up 3 above Speed Down 1	exposure doc; Gobey frame [00:21:52]
Local Exposure, Film	contrast 0.6 to 1 (default 0.8); Toe 0.55 default, 0.3 for dark interiors	exposure doc; Gobey
Albedo	0.02 [added] to 0.8 path traced, 0.9 Lumen	path tracer doc; Faucher 1e6oOiKh91U [00:13:35]
MegaLights	20 percent of samples to hidden lights (50 after failed reprojection); sun up to 50	SIGGRAPH [00:10:37] [00:16:40]
Path-traced still	1 temporal, spatial 256 draft to 1024 or more final; glass 10 bounces; Max Path Intensity default	path tracer doc; Faucher X5zVhc5ahl0 [00:16:47]
Deferred still	spatial only, odd (9 draft, 15 final, 31 thin bright), AA None above 8, Motion Blur Amount 0, warm-up 250	Faucher fVg5ihB8Wdc [00:06:01] [00:10:58]; 1e6oOiKh91U [00:09:56]

Quality gates

• Measurable: ppv_lint, light_lint, scene_lint, glass_lint, room_shell_lint, mrg_lint, cvar_lint without fail; frame_report on every bookmark with the log since capture (blank frames and the exposure-range warning fail); albedo_check on the Base Color buffer; gray card from linear EXRs; compare_ground_truth within a stop outside listed cheats; red_fraction on the VSM cache view; temporal_flicker on warm-ups. Thresholds marked [added] are calibrated per project on one approved frame.
• Visual: each bookmark full size and as a thumbnail beside its representation views; a camera move for noise; inner corners for leaks; glossy highlights; the still at 100 percent; in the order of references/critique.md.

Common mistakes

Mistake	Looks like	Fix
Lighting under default auto exposure	any value "looks fine"; flicker	exposure_plan, comp 0, then physical lights
Neon lit by emissive only	splotchy, crawling GI	rect light per sign; Emissive Light Source
Tube plus its light in a path-traced still	fixtures glow twice as bright as Lumen	A/B r.PathTracing.EnableEmissive 0; Indirect Emissive off
Sky light or exposure raised for a small window	splotches, flat gray interior	opening_rect_light sized to the opening
Many small lights, big radii, a sun indoors under MegaLights	noise and ghosting in motion	tight bounds, merge clusters, sun on VSM
Local lights on VSM over moving characters	constant invalidation, cost spikes	ray-traced shadows; VSM only with a reason
Grilles or plants under MegaLights	solid shadows once off-screen	model the holes, or EvaluateMaterialMode 1
One mesh for the whole room	pink surface cache, screen-space GI	split walls, floors, ceilings; Max Lumen Mesh Cards
Fill lights to rescue a dark interior; grading first	flat, view-dependent GI	toe and local exposure, then the ladder; grade last
Glossy bar top lit only by Lumen	dry, dead surface	a direct light it can reflect
More MRG samples for Lumen noise; even or mixed deferred samples	noise unchanged, blur on a still	NOISE_TRIAGE, still_plan, mrg_lint
HDRIBackdrop in a path-traced still	double lighting	remove it; Sky Light cubemap plus r.PathTracing.VisibleLights 2, which does nothing under Reference Atmosphere
Render differs from the viewport, cvars pasted	"the render is broken"	RENDER_DIFF_TRIAGE: Global Game Overrides first
Still-only rim left in the level	the cheat lights gameplay	own lighting channel, indirect 0, in the shot sequence

Handoffs

• Receives from scenario-unreal-world-building: the level, modular walls, floors and ceilings 10 cm or thicker, correct mobility, room volumes tagged room, fixture meshes tagged fixture:<name>, emissive meshes tagged emissive. From scenario-unreal-materials: masters exposing EmissiveIntensity (and LumenEmissiveScale behind a Ray Tracing Quality Switch), albedo 0.02 to 0.8, IOR glass on Surface ForwardShading. From scenario-unreal-vfx: particle lights (shadows off, Allow MegaLights).
• Delivers to scenario-unreal-cinematics: the lit level, lighting_manifest.json, the exposure plan (EV100, ISO and shutter for their aperture), still_plan presets, MRG variables, still-only cheat plans. To scenario-unreal-performance: light counts, shadow methods, measured passes. To scenario-unreal-materials and scenario-unreal-world-building: albedo, glass, masked-caster and room-shell fixes.

UE 5.8 notes

• MegaLights Production Ready (lighting channels, cloud shadows, mismatch views); directional lights excluded unless r.MegaLights.DirectionalLights 1; r.MegaLights.Debug, r.MegaLights.DownsampleMode.
• Lumen Lite at Medium (Beta); SSGI deprecated; RTGI gone since 5.4; detail tracing deprecated; r.Lumen.HeightFog 1 default.
• r.EyeAdaptation.CachedLightingPreExposure (default 4) unifies pre-exposure, with an on-screen warning when exposure leaves the supported range.
• ACES 2.0 is the SDR path; OCIO 2.5.1; grading scopes ShowFlag.VisualizeColorGrading.
• Path tracer on Mac (macOS 26.4+); back-face culling always on; HDRIBackdrop incompatible; Reference Atmosphere ignores Sky Lights.
• Movie Render Graph Production Ready: Basic config by default, Light Modifier, layer warm-ups, DWAA/DWAB EXR.
• Python: the Rotator's positional order is roll, pitch, yaw, so ue_light passes keywords [verify].

References

• references/procedures.md: P0 to P14, full code, each block with its test id and status. Load before writing lighting code.
• references/critique.md: setup, frame, render and budget rubrics mapped to code findings. Load at every gate.
• references/expert-notes.md: depth per expert with timestamps, disagreements and deciding conditions.
• references/gui-paths.md: menus and editors for a computer-use agent or a human.
• references/sources.md: every source, credential, URL, best timestamps, revision history.
• scripts/ue_light.py: the toolkit; its docstring lists every call and where it runs.