Look development in Maya 2027 (Arnold)

Expert look dev is a material that holds under any light a lighter throws at it: correct data in (every map in the right color space through the right plug), physically meaningful base values (measured metal colors, glass tinted by depth, scattering at real scale), then deliberate imperfection, judged on a neutral turntable with reference balls, one change per render. Nothing is done before a render has been looked at. 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-maya-expert (execution channel, review loop, 2027 version traps).

Status (2026-09-24): scripts/mx_shade.py is not yet run in Maya: 122 offline checks passed; the Maya layer ran against a fake maya.cmds (logic only). Once Maya is installed: tests/code/maya-lookdev/run_all.sh, then fix the attribute tables from probe_lookdev.json.

Stance (the expert delta)

• Every map is a three-part contract: color space, grayscale plug, destination. Sarkamari's table [ZtEiVa3MPLg 00:27:44]; J Hill's two questions, "what color space is it and is it black and white" [mpk6IurOWbs 00:21:43]. Base, subsurface and emission color are color (sRGB for 8/16-bit, linear Rec.709 for EXR); everything else is Raw. outAlpha with Alpha Is Luminance off reads no data (Raycast [vPHhVrxxThU 00:19:14]). Maya 2027's rules tag .tx, .hdr and .exr as Raw, so a base color .tx or EXR arrives wrong: override it, Ignore Color Space File Rules on (Maya 2027 help).
• Under ACEScg an HDRI is linear Rec.709, not Raw. J Hill and Raycast set Raw, correct only in a linear-sRGB rendering space; the Arnold ACES page calls a linear HDRI read as ACEScg incorrect. Colors typed from linear-sRGB tables convert to the rendering space too [added: swatches are rendering-space numbers, Maya 2027 help].
• Metals are data, not taste. OpenPBR base color is F0 and specular color the F82 edge tint, both from the Arnold table; IOR does nothing on metals (Arnold OpenPBR doc; official intro [tEUiIBApw-U 00:02:51]). Generalists leave specular color white.
• Scale is physics. Subsurface radius and transmission depth are world lengths. Glass color lives in depth, not saturation: light tint, lower depth to deepen (Arvid [cpMBRIWwghg 00:18:13]). Skin scatters about 1 mm at real size (J Hill, scale 0.1 in cm [mpk6IurOWbs 00:18:31]).
• Perfect reads as CG: "just look at the reflections". Ranged noise on roughness, subtle micro bump, car-paint waviness on the coat normal, never the base (Arvid [cpMBRIWwghg 00:35:49, 00:43:25]); J Hill's "subtle variation" layers [mpk6IurOWbs 00:23:24]. Polished metal and glass also get smudge or fingerprint roughness, the doc's own example (OPBR § Specular Roughness; Arvid [cpMBRIWwghg 00:25:11]), as clean as the reference.
• Build layered shaders by killing every lobe, then adding one at a time, one or two changes per render (Arvid [cpMBRIWwghg 00:36:52]; J Hill [mpk6IurOWbs 00:51:43]).
• Hero close-ups displace; props use normal maps. The normal-mapped nose "looks flat to me and fake" (J Hill [mpk6IurOWbs 00:28:39]). Bake float EXR with zero at 0 so micro detail adds, match the zero value, pad the bounds.
• Judge on a lookdev rig, then under the reference's light. Camera carrying chrome ball, gray ball and a diffuse chart; asset spins, then the HDRI (Raycast [vPHhVrxxThU 00:05:11, 00:07:43]). A hero product is judged against its photo, lit like the photo first (Arvid [cpMBRIWwghg 00:02:44, 00:36:21]); a Painter transfer under Painter's HDRI and focal length (Sarkamari [ZtEiVa3MPLg 00:13:10]).

Establish first

Input	Changes	Default when silent
Target	Arnold stills or sequence vs a game engine (shade for export, judge in the engine)	Arnold
Shader model	OpenPBR for new assets (default since 2026, MaterialX and USD); aiStandardSurface when the scene, a Painter "Arnold (AiStandard)" export or the pipeline uses it	OpenPBR
Hero or background	per-material shaders, displacement, breakup vs one textured shader (Raycast [vPHhVrxxThU 00:11:20])	hero if it fills the frame
Texture source	Painter preset, normal convention, UDIMs, bit depth, displacement zero and scale, whether AO is meant to be used	OpenGL normals, AO unused
Rendering space	mx_shade.cm_info(); converts typed colors	ACEScg, Maya-default config
Scale	SSS, transmission, displacement, noise scales	real size in cm
Look reference	photos per material; HDRI: neutral studio, the sequence's graded HDRI for plates (Raycast [vPHhVrxxThU 00:01:58]), Painter's for a transfer	procedural studio HDR [added]
Camera	85 mm portrait lens (J Hill [mpk6IurOWbs 00:05:10]); longer for products [added]	85 mm, 24 + 24 frames

Workflow

All stages run headless through scenario-maya-expert's mx_run.py --plugins mtoa; import mx_shade as sh. Arnold RenderView is GUI only: iterate with batch renders you open.

1. Intake. mx_validate.validate(profile="model"), mx_audit.audit(mesh) for UVs (every face mapped, clean UDIM tiles), real-world size from the bounding box, closed outward meshes for SSS and transmission (Arnold docs). GATE: validate clean, UVs complete, size matches the reference.
2. Lookdev scene. ld = sh.lookdev_scene(["asset_GRP"], hdri=path, frames=24): turntable locators, skydome at HDRI width (cap 4096), HDRI tagged linear Rec.709, camera with the reference strip hidden from secondary rays. GATE: frame 1: chrome ball shows the HDRI the right way round, gray ball neutral, close to the chart's neutral 5 patch.
3. Material plan. One shader per material on heroes, named <asset>_<part>_MTL / _SG, assigned by a name table so remodels keep shaders (Arvid [cpMBRIWwghg 00:01:37]). GATE: every mesh off the default shading group.
4. Texture hookup. ts = sh.parse_texture_set(folder); sh.build_material(ts, "case", meshes=[...], texture_set="watch_case"). Options: normal_mode aiNormalMap (default), bump2d, hybrid; tangent_space="mikk" on 2027.1+ for Painter bakes, A/B it on a bevel [verify]; ao="multiply" only to match a Painter or real-time look. Pre-bake .tx (TX Manager or maketx) for heavy assets and batches; file nodes stay on the source images and spaces (Raycast [vPHhVrxxThU 00:13:04]). GATE: sh.verdict(sh.lint_scene())["pass"]; a render under Painter's HDRI and focal length matches a Painter screenshot.
5. Base values, one lobe at a time. saved = sh.isolate_lobes(shader, keep=("base",)), render, add specular, coat and the rest; sh.apply_preset(shader, "steel_polished") for table materials. GATE: per-material crop at final samples; highlight size and edge tint against the photo.
6. Surface detail. Normal map for props; displacement for hero organics: sh.add_displacement(sg, "disp.exr", meshes, scale=s, zero=0.0, iterations=2) (3 to 4 for finals). GATE: polygon budget printed, no clipped black patches, A/B crop normal vs displacement.
7. Breakup. sh.roughness_breakup(shader, lo, hi), sh.smudge_roughness(shader) (scan or cell noise, added on top), sh.noise_bump(shader, height, scale), sh.coat_waviness(paint); per-object masks (dial, indices, case parts) as mtoa_constant_ user data, no extra textures: sh.user_mask(shapes, "mask") (Arvid [cpMBRIWwghg 00:23:04]). GATE: on the turntable, highlights break and travel across panels, never wobble.
8. Special materials. sh.skin(...), glass presets plus sh.set_transmission_depth, sh.car_paint(...), sh.hair_shader(...); checks in references/critique.md. Transmissive media that touch or overlap (crystal on a gasket, liquid in glass) get a Dielectric Priority: higher wins, glass 3, inclusions 2, liquid 1 (OPBR § Dielectric Priority). GATE: lint dielectric_priority clean.
9. Turntable review and handoff. sh.render_frames([1, 7, 13, 19, 25, 31, 37, 43], out) writes PNGs and a sheet; judge with the critique; sh.handoff_report(path), new saved version. GATE: lint passes, sheet reviewed, not-verified list written.

Numbers

Item	Value	Relative to, source
Steel F0 / F82	0.669 0.639 0.598 / 0.789 0.823 0.870	linear sRGB, Arnold OpenPBR table (12 metals in METALS_OPENPBR)
Skin	subsurface 1, radius scale (1, 0.35, 0.2), IOR 1.4, radius 1 mm	J Hill [mpk6IurOWbs 00:17:25]; doc example 3.7 mm is deeper
Skin oil	coat roughness about 0.1, mask from inverted roughness	J Hill [mpk6IurOWbs 00:32:49]
Pore chatter	alligator cell noise, amplitude 0.003 to 0.004 in displacement	J Hill [mpk6IurOWbs 00:34:48]
Subdivision	catclark, 2 while working, 3 to 4 final; each level x4 polygons	J Hill; Arnold subdivision doc
Displacement zero	0 for float bakes, 0.5 for mid-gray maps	J Hill [mpk6IurOWbs 00:26:59]
Glass	IOR 1.5, roughness 0, light tint, depth near object size	Arvid [cpMBRIWwghg 00:17:40]; [added] depth start
Dispersion	off on flat, parallel-faced crystal (no visible fringe) [added optics]; small scale with the material's Abbe on faceted gems or close bevels	OPBR "gems only"; digest P9
Rubber	linear base 0.01 to 0.04, roughness 0.2 to 0.4 via smoothstep range	Arvid [cpMBRIWwghg 00:06:25, 00:10:47]
Leather	alligator cell noise bump, scale 400 to 500, very subtle, roughness about 0.2	Arvid [cpMBRIWwghg 00:04:33]
Brushed metal	roughness about 0.15, anisotropy up, specular samples 3, rotation map mip bias -8, smooth tangents + 1 iteration	Arvid [cpMBRIWwghg 00:15:43]; SS doc
Car paint	flakes 0.001 to 0.025 scale, flake roughness 0.2, coat 1, coat-normal noise at 3, 25, 80, 200 mixed about 0.01	Arvid [cpMBRIWwghg 00:39:01 to 00:47:46]
Thin film	Standard Surface 0 to 2000 nm; OpenPBR micrometers, colors fade above 1	Arvid [cpMBRIWwghg 00:31:47]; [tEUiIBApw-U 00:12:35]
Hair	melanin 0.2 blonde, 0.5 brown or red, 1 black; roughness 0.2; IOR 1.55; shift 0 to 10; diffuse 0; tints white	Arnold Standard Hair doc
Skydome resolution	HDRI width (default 1000)	Arnold lights doc

Quality gates

• Code: sh.verdict(sh.lint_scene())["pass"] (zero errors). The lint covers the texture contract (spaces, plugs, UDIM, normal routes, missing files, .tx), physical plausibility (weights, F82, glass tint and depth, dielectric priority, thin film, opacity, anisotropy, smudges), subdivision and displacement, and hygiene. Rules: references/critique.md.
• Visual: the turntable sheet and crops, judged with references/critique.md: breakup visible and moving, metal edge tint, glass tint deepening with thickness, ear backlight and a non-waxy nose; fireflies that survive more samples are data (Raycast [vPHhVrxxThU 00:33:43]).

Common mistakes

Mistake	Looks like	Fix
Roughness or metal map in sRGB	glossy-plastic or dull metal	Raw + Ignore Color Space File Rules
Grayscale via outAlpha, Alpha Is Luminance off	map has no effect	outColorR, or Alpha Is Luminance on
Base color .tx or EXR left on the Raw rule	washed-out or oversaturated color	sRGB (8/16-bit source) or linear Rec.709
HDRI on Raw under ACEScg	over-saturated lighting and reflections	linear Rec.709 input space
Normal into a plain bump or straight into the shader	lumpy or flat normals	aiNormalMap, or bump2d Tangent Space Normals
DirectX normal read as OpenGL	relief inverted on one axis (dents read as bumps)	invertY on aiNormalMap (flip G on bump2d)
Smooth Mesh Preview plus Arnold iterations	render time explodes	displaySmoothMesh 0
Displacement zero value wrong	object inflated or shrunk	0.5 mid-gray maps, 0 float bakes
No bounds padding	black holes in the displaced render	padding from the maximum displacement
Saturated transmission color	black, ink-like glass	light tint, lower depth
Overlapping glass or liquid at the same priority (0)	wrong refraction in the overlap	Dielectric Priority, higher wins
Anisotropy at roughness 0 or faceted	brushing invisible or stepped	roughness > 0, smooth tangents, 1 iteration
Car-paint waviness on the base normal	coat reflections stay perfectly straight	coat normal only (Arvid [cpMBRIWwghg 00:43:25])
Layer shader created, displacement left on the old SG	displacement disappears	reconnect to the new SG (J Hill [mpk6IurOWbs 00:49:07])

Handoffs

• Receives from scenario-maya-retopology-uv: a version passing validate(profile="model"), UVs complete (0-1 or clean UDIMs), texel density stated, bakes named by channel with normal convention and tangent basis, displacement EXR with its zero value and scale. From Substance Painter: textures, preset name, normal format, UDIM flag, the HDRI Painter displayed. From scenario-maya-groom: descriptions to shade with aiStandardHair.
• Delivers to scenario-maya-lighting-rendering: <asset>_lookdev_v###.ma (never over the source) with named materials and shading groups; handoff_report() JSON (materials, textures, color spaces, .tx status, displacement, a passing verdict); the turntable sheet; sampling notes (SSS, transmission depth, anisotropy, flakes to protect from the denoiser); the not-verified list. Lighting owns render settings, AOVs, light rigs and denoising.

Maya 2027 notes

• OpenPBR is the default shader. Porting from Standard Surface (sh.port_standard_to_openpbr): thin film nm to micrometers, emission weight 1 to 1000 nits, SSS radius and scale roles swapped, sheen to fuzz, dispersion needs scale above 0, metal colors re-picked from the F82 table. "Convert All Standard Surface to OpenPBR Surface" skips shaders inside networks (MTOA-2560): lint after.
• Color names: default input sRGB Encoded Rec.709 (sRGB) since 2026.2, linear sRGB spelled two ways, "Utility - Raw" is ACES 1.x naming: always sh.space_name(role). Regenerate .tx after changing the rendering space (Brejon).
• normal_map.tangent_space_type mikk (MtoA 5.6.1.1, Maya 2027.1); interior_set replaces sss_setname; OpenPBR thin film energy change in 5.6.0 can shift hue on old assets; Standard Hair scattering_mode (5.6.0).
• Maya IPR does not work with MtoA 5+ (Arnold RenderView); command-line renders watermark without a license; "Use Autobump in SSS" is deprecated; nested dielectrics on by default change old glass-in-liquid scenes.

References

• references/expert-notes.md: depth per expert with timestamps, disagreements and deciders. Load when a material or decision is not covered above.
• references/procedures.md: test-backed procedures, hookup to handoff. Load before writing shading code.
• references/critique.md: the rubric for a turntable, a crop or a lint report. Load at every gate.
• references/gui-paths.md: Hypershade, Attribute Editor, Arnold and Painter paths for a computer-use agent or a human.
• references/sources.md: sources, credentials, URLs, timestamps.
• scripts/mx_shade.py: the toolkit (docstring lists every call).