Chapter 1: Global vs Local Illumination for Concepting

Created by Sarah Choi (prompt writer using ChatGPT)

Global vs. Local Illumination for Environment Concept Artists (Concepting + Production)

Light is the most persuasive storyteller in an environment. Players understand mood, depth, material, and route primarily through illumination—long before they consciously register color or detail. For concept artists, the practical distinction between global illumination (GI) and local illumination is crucial. GI governs the broad wash: skylight, bounce, and ambient energy that ties the world together. Local illumination governs the focused, authored accents—lamps, torches, screens, signs—that carve readable beats and guide the player. This guide treats both the concepting side (discovering look and mood) and the production side (making scenes game‑honest and shippable) in equal measure.

1) The Working Definitions You Can Draw With

Global illumination (GI). The indirect, environment‑wide light that results from major sources (sun/sky, overcast dome, cavern mouth) bouncing off large surfaces and filling shadows. In games, GI may be baked (lightmaps), probe‑based, or real‑time approximations. In painting terms, think overall key, bounce color, and ambient occlusion (AO).

Local illumination. Direct, authored lights (streetlamps, lanterns, neon, consoles, emissive props, muzzle flashes) and their immediate spill. In painting terms: key pools, practicals, accents, and specular peaks.

Ambient occlusion (AO). Contact darkening where light cannot reach; a local modifier that anchors objects to surfaces.

Color bleeding. The tinting of nearby surfaces by a colored surface under GI (e.g., red rug warming a white wall near the floor).

These are not engine‑specific definitions—they are the visual levers you manipulate in thumbnails, keyframes, paintovers, and callouts.

2) Why the Split Matters to Concept Artists

Treat GI as the glue and local lights as the grammar. The glue makes materials feel like they inhabit the same space (shared key, shared hue family for bounce), while the grammar imposes read order (where the eye goes first, then second). If your concept reads only because of a few bright local lights while the GI is incoherent, the scene will fall apart in production lighting changes. Likewise, a perfect GI with no local hierarchy will be pretty but indecisive.

3) Concepting‑Side: Establishing the GI First, Then Writing with Locals

Start with a value‑only pass that defines the GI envelope: horizon brightness, sky vs. ground bounce, and interior ambient level. Ask: high‑key, mid‑key, or low‑key? Overcast (soft, low contrast) or hard sun (crisp, directional)? In caves and ships, define the base darkness and the color of the few bounces (rusted steel = warm brown bounce; icy rock = cool cyan bounce). Once the GI is stable, place local lights to express beats—arrival, interaction, hazard, exit—so they read even if post‑processing is muted.

When color enters, assign bounce hues by material: ochre stone lifts warm mids; forest floors cast greenish up‑fill; oceanic scenes push blue‑green bounce on lower planes. Reserve saturated local colors (neon, magic, UI panels) for gameplay‑critical accents so they do not dilute the GI logic.

Practical steps for concepting

  • GI thumbnails: 12 small studies exploring sky conditions and enclosure (open plaza vs. canopy vs. arcade). Keep to 3–5 tones.
  • Bounce maps: Paint a simplified pass that shows where warm vs. cool bounces live; annotate materials responsible.
  • Local light grammar: Choose 2–3 practical types (e.g., 3 m bollards, 1 m sconce band, 0.5 m floor reflectors) and show consistent spacing/cadence at human heights.
  • Color script: Establish how GI keys change across beats (dawn → noon → storm), then layer local accents that preserve route clarity in each state.

4) Production‑Side: Making GI and Locals Survive Engine Reality

On live levels, performance and pipeline choices constrain light. Many projects use hybrid lighting: baked/global solutions for the bulk of the scene plus a small budget of dynamic local lights for interaction. Your paintovers should assume this reality.

What to control in paintovers

  • Value separations that don’t rely on hard shadows: If cascaded shadows are expensive or limited, use albedo/roughness and GI contrast to carve forms.
  • Roughness as a value tool: Broader, brighter highlights (lower roughness) can lift the path band without extra lights; matte backgrounds prevent competing sparkle.
  • Emissive ladders: Replace sun streaks at night with consistent emissive rhythms (lantern pools every 6–8 m, reflective paint at stair noses).
  • Probe/volume hints: Suggest where reflection probes or GI volumes should be denser (tight courtyards, glossy markets) to stabilize material reads.

Communicate constraints

  • Light count budgets: Propose clusters of locals rather than a field of singles; group lights so they can be instanced and culled cleanly.
  • Bake‑friendly intent: If lighting will be baked, avoid relying on moving shadows for storytelling. Stage mood with GI color and emissives.
  • Fog/vfx interactions: High particle density can lift GI values and crush contrast; recommend localized effects that bracket paths rather than blanket fills.

5) Reading and Painting GI: The Four Big Cues

  1. Sky contribution: Open skies push cool top‑down fill; overcast collapses directionality, raising shadow floors. Interiors with skylights create spotlight‑like GI pools.
  2. Ground bounce: Sun hitting bright ground warms undersides; dark ground (wet stone, soil) reduces up‑fill—use this to thicken mood.
  3. Enclosure: Arcades, canyons, and narrow alleys trap light and color; big halls bleed light unevenly (brighter near openings). Paint the gradient.
  4. Material albedo: Light surfaces amplify GI; dark absorb it. In concept, exaggerate these relationships slightly to telegraph behavior that production can match with PBR values.

6) Local Lights as Readability Devices

Local lights should say verbs: enter, climb, loot, hide, beware. Define mounting heights and color/temperature roles (e.g., warm = safe/service; cool = hazardous/tech; amber blink = interactable). Keep accent pools within the player’s cone of vision and at eye/hand height where possible. In production paintovers, place lights to eliminate ambiguity at decision points (forks, vertical junctions) and to outline traversal edges (rail tops, ladder feet, mantle lips). Avoid peppering highlights on non‑interactive clutter.

7) Color Bleeding and Material Storytelling

Use color bleed to explain world logic: a copper roof warms adjacent plaster; stained glass paints ritual color on stone; bioluminescent algae cools ceilings. In concept, make bleed directional (downcast from windows, upcast from floors) and proportional to source saturation. In production, ensure bleed does not create false positives—do not place strong red/green bleed where it could be mistaken for danger/safe coding unless intentional.

8) AO, Contact Shadows, and Believability

AO is the visual contract that objects touch. In concept, paint AO sparingly but decisively at feet‑to‑floor, prop‑to‑wall, and detail creases. In production, if baked AO gets lost under strong GI or exposure changes, request decals or material darkening at contact lines rather than extra point lights. Clean, believable AO improves readability by separating layers without high contrast everywhere.

9) Weather, Time of Day, and Multi‑State Locations

Design for states, not shots. Provide paired lighting keys: noon vs. dusk, clear vs. storm, intact vs. damaged. Show how GI shifts (cool overcast fill vs. warm sunset bounce) and how local systems adapt (street lantern cadence, window emissive bands). In production paintovers, preserve the same readability cadence across states by keeping route‑band values consistent via different means.

10) Genre Emphasis

  • Shooters: Clear cover tops via strong top‑edge contrast; GI should not flood cover planes to the same value as backgrounds. Local lights bracket lanes and call flank entries.
  • Platformers: Step faces carry lighting bands that count jumps; avoid GI that flattens vertical edges. Local accents mark grapple points and safe landings.
  • Stealth/Horror: Low‑key GI with strong shadow pockets. Local lights create islands of safety and reveal patrol routes; keep flicker and strobe within comfort guidelines.
  • Open‑World RPGs: Macro GI color scripts per biome; locals differentiate cultures (paper lanterns vs. sodium lamps) and ownership.

11) Common Pitfalls (and Fixes)

  • Mid‑gray mush: No value hierarchy; everything sits in the same band. Fix: Re‑key the GI, add matte backgrounds, and concentrate locals on verbs.
  • Specular lies: Gloss over‑brightens non‑interactive clutter. Fix: Raise roughness or lower spec on background materials; move sparkle to the path band.
  • Color crutch: Composition works only because of saturated lights. Fix: Prove readability in grayscale; re‑stage massing and GI.
  • Over‑lighting: Too many locals erode mood and performance. Fix: Consolidate lights into readable pools; remove accents that don’t support decisions.
  • Inconsistent bounce: Walls change tint illogically shot‑to‑shot. Fix: Lock material albedos and sky model; annotate bounce sources.

12) Exercises (Concepting + Production)

  1. GI only: Paint a corridor scene readable in three tones with no direct lights. Then add one local system (wall sconces) and compare.
  2. Bounce map: Take a plaza mid‑day and at golden hour; produce two bounce maps showing different warmth and shadow floors.
  3. Night parity: Re‑light a day keyframe for night using emissives and reflectors only; keep path clarity identical at thumbnail scale.
  4. Material swap: Repaint a scene swapping bright ground/dark walls to dark ground/bright walls; observe GI changes and readability impact.
  5. Probe hint sheet: Create a quick callout page showing where GI/reflection probes should cluster for a glossy market interior.

13) Hand‑Off Checklist (Concepting + Production)

  • Scene value key declared (high/mid/low) and proven in grayscale
  • GI envelope established (sky/ground bounce, enclosure gradients) with brief notes
  • Local light grammar defined (types, heights, spacing, temperature roles)
  • Readability parity shown for alternate states (day/night, weather)
  • Roughness/reflectance targets annotated for key materials
  • AO/contact shadow expectations visible at touch points
  • Wayfinding via emissive/reflective cadence documented
  • Performance‑friendly notes: consolidate lights, avoid reliance on moving shadows if bake‑heavy pipeline

Conclusion

Think of global illumination as the breath of the world and local illumination as its speech. On the concepting side, you set the breath—key, bounce, and color family—then script the speech with selective, purpose‑driven accents. On the production side, you preserve that language under technical limits—budgeted lights, stable GI, disciplined roughness, and state‑to‑state parity. When GI and locals are designed together, players feel oriented, the mood lands, and the world reads as physically true—no matter the time of day, weather, or platform.