Chapter 3: Lighting Renders for Paintover & Material IDs

Created by Sarah Choi (prompt writer using ChatGPT)

Lighting renders for paintover & material IDs

Purpose: why light plates and IDs matter

In hybrid 2D/3D prop workflows, lighting renders (“light plates”) and material IDs are your guardrails. A clean lighting plate establishes believable form and lets paint sit naturally; material IDs give you surgical control for fast, non‑destructive edits. Together, they let concept artists iterate at speed and production artists maintain discipline for downstream lookdev, UI, VFX, and marketing. The goal is the same on both sides: a readable prop that survives translation between blockouts, scans, kitbash parts, and final PBR shading.

Start with the blockout: light the mass, not the noise

Before scans and kitbash details enter the scene, light the blockout. Use a neutral, studio‑style rig that makes the silhouette and major planes legible at three reads (5 m / 2 m / 0.5 m). Keep bevels honest so the light rolls the way manufactured edges will. If the blockout reads with two values and a highlight, you can safely layer in scan texture and kitbash micro‑detail without losing control.

A dependable studio rig (for concept and production)

A simple three‑light layout solves 80% of use cases:

  • Key: large area source 30–45° off camera, slightly above horizon; soft shadows; intensity ~1.0 (relative).
  • Fill: broad, dimmer source on the opposite side; intensity ~0.3–0.5; no specular if possible.
  • Rim/Edge: narrow source from behind to carve the silhouette; intensity ~0.7; slightly harder. Add a top bounce (HDR dome at 0.2–0.3) to hold form in occluded areas. For metallic props, keep the key large so highlights aren’t pin‑hot. For dielectric plastics, increase roughness in the base material to avoid chalky banding. Lock exposure in linear or ACEScg; your plate should be physically plausible so paintovers don’t fight tone mapping.

Linear workflow and tone mapping

Work in a linear pipeline. Render to 16‑bit (or 32‑bit EXR if available) with a known view transform (sRGB or ACES). Turn off any “contrast” or “filmic” look inside the renderer for the beauty plate you’ll paint on; export a separate preview if art direction prefers a stylized look. Consistency here saves hours: with linear plates, dodge/burn and soft‑light layers behave predictably; PBR values mapped later won’t surprise you.

Core render outputs (per prop)

At minimum, export:

  • Beauty (neutral): key/fill/rim + dome, no color grading.
  • Shadow/AO: either a pure AO pass or a light‑linked contact shadow plate for grounding.
  • Normal (tangent or world): for relighting tricks and quick rim additions.
  • Curvature: to guide edge wear and highlight paint.
  • Cavity: micro occlusion for dust/oil placement.
  • Z‑Depth: for atmospheric separation and defocus in paintovers.
  • Material ID / Cryptomatte: stable masks by material slot or object group.
  • Wireframe (on shaded): for callouts and topology proof when needed. Export 16‑bit PNG/TIF for plates and EXR for technical passes. Keep resolution modest (2–4k) unless marketing requests more; speed matters for iteration.

Building Material IDs that won’t betray you

Treat IDs like a “coloring book” for the prop:

  • Author IDs at the material‑slot level (e.g., METAL_BODY, RUBBER_GRIP, GLASS_LENS) rather than per‑object names that may change.
  • Use high‑contrast flat colors with no antialiasing where possible for crisp selections; if using Cryptomatte, export with consistent seed so masks stay stable between versions.
  • Keep an ID legend in the PSD/EXR as a labeled swatch strip.
  • For kitbash parts, reassign donor materials to your house slots so all screws, for example, pick up the same ID and can be mass‑edited.
  • When importing scans, split materials by functional surface (painted metal vs raw steel vs rubber overmold) instead of by scan islands; this makes paintovers follow real‑world logic.

Scan‑aware lighting

Scans often contain residual baked shading and uneven albedo. Neutralize before lighting: equalize albedo in a texture editor (remove gradients and highlights) and keep normal strength reasonable (0.5–0.8) to avoid double‑shadowing. Under your studio rig, do a “honesty pass”: if a scan’s bumps catch light unrealistically, reproject only the micro detail onto a cleaner form or reduce the bump range in roughness instead of normal.

Kitbash consistency under light

Nothing calls out kitbash faster than mismatched specular language. Normalize:

  • Edge radii: align bevel widths so rim lights are coherent.
  • Roughness ranges: set families (e.g., anodized aluminum 0.25–0.35; painted steel 0.45–0.6; nitrile rubber 0.7–0.85).
  • Fastener logic: unify screw head families so highlights align and rhythm reads intentional. Tweak these before rendering plates; fixing in paint is slower and easy to forget in handoff.

Photobash ethics for light plates

When you photobash materials on top of a light plate, respect origin and rights. Only use sources that allow derivative use, keep a source sheet (thumb, author, license, link) in your PSD, and avoid bashing museum or cultural artifacts without permission and context. If a photo dictates a unique cultural motif or proprietary UI iconography, annotate it—producers and legal need to know. If you bash scanned material from your studio library, cite the asset ID/version for reproducibility.

Paintover workflow (step‑by‑step)

  1. Prep: Import Beauty (bottom), AO/Shadow (multiply at 20–40%), Z‑Depth (as a mask for fog), Normal/Curvature/Cavity (as utility layers).
  2. Masking: Load Material IDs/Cryptomatte to create grouped masks per surface family.
  3. Global balance: Levels/Curves on Beauty to lock midtone contrast; keep blacks above 0–5% to preserve print/engine headroom.
  4. Material reads: Using IDs, adjust roughness cues (spec width/strength) with dodge/burn and subtle noise overlays. Add anisotropy hints on brushed metals with directional blur.
  5. Edge order: Paint highlight accents along primary edges first; then add wear using Curvature as a guide (low flow, big brush). Save grime and decals for last.
  6. Photobash: Within each masked group, overlay photographic albedo/roughness detail; repaint edges and shadow anchors so the photo truly belongs.
  7. Status & UI: Reserve screen‑space planes early; add emissive reads on separate groups with a 1–3 px bloom to preview VFX.
  8. Reality check: Downscale to gameplay size and grayscale—does the silhouette and affordance read? If not, reduce mid‑frequency chatter.
  9. Export: Deliver layered PSD plus flattened PNG; include the EXRs for IDs and technical passes.

Renderer and engine parity

Match your renderer to the target engine’s PBR model (metal/rough, GGX). If the project sits in UE/Unity, consider previewing the plate in‑engine before paint so your specular response matches reality. Disable tone‑mapper look LUTs for the paint plate; save those for presentation renders. For production handoff, include material parameter notes you implied in paint (e.g., roughness 0.3 on anodized rails) so lookdev can replicate.

Practical settings (portable defaults)

  • Sampling: enough to kill splotches, not enough to erase texture (e.g., 64–128 spp path tracing for plates).
  • Shadows: soft penumbra from area lights; enable contact refinement.
  • Camera: 50–85 mm FOV for orthos/turns; avoid wide‑angle distortion on plates meant for paint.
  • Background: mid‑gray (40–60%); it helps judge specular better than white or pure black.
  • Scale check: always include a small real‑world reference (10 cm cube) on early plates; remove for finals.

Common pitfalls checklist

  • Baked lighting left in scan textures causing double shadows.
  • Cryptomatte masks shift between versions due to changed object IDs—stabilize with material‑slot masks.
  • Inconsistent house radii creating incoherent rim highlights.
  • Tone‑mapped beauty used as paint plate—paint fights with the look LUT.
  • AO overused (muddy look); keep it subtle and value‑correct.
  • IDs authored per object name; kitbash swap breaks masks.
  • Decals/emissives painted without reserving geometry—VFX and UI have nowhere to live.

Handoff notes for downstream teams

Include: layered PSD (grouped by material), neutral Beauty PNG, EXR pack (Normal, Curvature, Cavity, AO, Z‑Depth, IDs/Cryptomatte), a one‑page ID legend, and a text note of implied PBR ranges (roughness/metalness), emissive intents, and any fake cheats you used (e.g., hand‑painted spec kick). Rigging wants pivots and zeroed transforms if they’ll animate; VFX wants emissive masks and suggested flicker logic; UI wants reserved planes and pixel densities; Audio appreciates material callouts (rubber, ceramic, painted steel) for Foley.

A 60‑minute exercise

  • 0–10 min: Light a blockout with a key/fill/rim rig; lock exposure.
  • 10–20 min: Assign tidy material slots (METAL_BODY, RUBBER_GRIP, GLASS, DECAL) and export IDs.
  • 20–35 min: Drop a scan grip and a kitbash latch; normalize roughness and radii.
  • 35–50 min: Render Beauty + passes; assemble a PSD with masks and do a focused paintover (edge accents, two materials refined, no decals yet).
  • 50–60 min: Downscale, grayscale check, annotate one page with ID legend and PBR notes. Save as a shareable package.

Closing

Light plates make your paint honest; material IDs make your edits responsible. If you light the mass first, normalize kitbash and scans, keep a linear pipeline, and package clean IDs, your 2D/3D hybrid props will iterate quickly without surprising anyone downstream. Discipline at this stage is the difference between a fast concept and a fast concept that ships.