Hair and fur grooming (XGen Interactive Groom, Maya 2027)

Expert grooming sets the flow on a few guides first, then a clump hierarchy with loose roots and varied profiles, then breakup only where the reference shows it (tips, strays), judged in close, side-lit renders with the real hair shader. Interactive Groom brushes have no stroke API, so the agent grooms with data (guides from a flow sheet wired into the stack, masks written as images, modifiers set by value) and judges with numbers from exported strands plus renders. Code: scripts/mx_groom.py (import mx_groom as G). 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): every Maya call here is not yet run in Maya (2027 not installed). The pure layer of mx_groom passes offline (tests/code/maya-groom/test_mx_groom_offline.py). Names not quoted from the 2027 help resolve at run time from G.NAME_CANDIDATES.

Stance (the expert delta)

• Flow first, on the guides, copied from arrows drawn over the reference (Fernandez). Flow is pressure: neighbors share the compression curve, and opposing streams meet through a straighter band. Short fur turns sharply, long hair slowly. Decide whether the flow or a clump owns each big shape.
• Roots loose, every clump different, two levels (Fernandez). No attraction over about the first 14 % of the length, or the base pinches into bald lines. 3 to 5 small clumps per big one; a third level only for long hair or manes. Vary size and profile (G.CLUMP_PROFILES: spike, onion).
• Mask and tightness are different knobs (Fernandez 05, 06). The mask decides triangle versus square: square clumps are tight small clumps inside a looser big clump, both masks lowered. Tightness (IG Clump strength [added]) 0 keeps the clump's shape without converging; mask 0 removes it. Straight guides make them look alike, so test on noisy guides (G.clump_follow).
• The level that gets the noise decides what breaks (Fernandez 07). Clump-level noise (the secondary Clump's Noise, tip half) breaks the big clumps and keeps the small ones and the roots; a Noise modifier breaks the small clumps too, so it goes on few hairs (Giovannini: 0 on the body, 3 on a 5 % share). Vary the frequency; fray only on few hairs. Stray clumps come from holes in the primary's mask, not a new system.
• The hairline reads real through thinner edge strands plus transition and peach-fuzz descriptions, not density alone (Hadi Karimi). One density map is shared by every description; each hair type gets its own description.
• A short fill layer hides the scalp (Giovannini): body guides cut to 2/3, so long hair need not be "crazy dense".
• Motion drives a few wires, never the hair (Flood). Export the wires from the Linear Wire's inGuide. Update the Reference State at the rest pose on frame 1. Curve to Spline gets Align to Normals off.
• Standard Hair realism comes from melanin, roughness and IOR (Arnold hair doc). Keep diffuse 0, tints white and indirect scales at 1, and use ribbon mode (Min Pixel Width only works there). Blond hair gets Extra Depth. Cost has four knobs (MPW, transparency depth, specular samples, AA); opacity below 1 costs significantly.

Establish first

Write the answers to brief.json before any node.

Input	Changes	Default when silent
Target	Arnold strands, Unreal strands, or cards (MtoA renders no XGen Card primitives)	Arnold strands
Hair types	one description each: body, fill, fringe, strays, brows, upper and lower lashes, transition, fuzz. Close-up face: rule peach fuzz in or out (Hadi)	body + brows
Style	realistic: random clump points, Randomness > 0, melanin. Stylized: clumps on the guides (FlippedNormals), flat Base Color with Melanin 0 [added]. Both: a point on the defined-to-soft range, same silhouette (Fernandez 07)	realistic
Motion, and who owns it	static follow; keyed or simulated wires; rig or CFX department (Flood) or the groom file (2027 help)	follows the scalp
Still or animated	still likeness: grow on the head (Hadi). Animated or real time: scalp cap with clean UVs	scalp cap
Camera distance	Scattering Mode: Accurate for close-ups (10 to 30 % slower), Adaptive for mixed shots	Adaptive
Budget	spline count and GPU memory (xgmSplineQuery), Unreal sim guides	state a number
Reference	flow arrows and clump counts come from it	ask; else write the assumed look down
New or inherited	Interactive Groom for new work; legacy XGen only for an inherited asset or non-spline primitives	Interactive Groom

Workflow

Headless mayapy (mx_run --plugins mtoa,xgen,abc, in that order) covers guides, masks, Alembic, Unreal tags, lint, analysis and Arnold reviews; the GUI bridge covers the menu queries and whatever the probe shows does not evaluate headless (IG computes on the GPU). references/procedures.md P16 is the whole job as one script with every gate asserted (G.gate raises).

0. Probe and recipes (once per install). G.probe() headless and in the GUI. Menu actions with no documented command are read, not clicked: through the bridge, G.menu_recipe(name) queries the item's command string (cmds.menuItem(item, q=True, command=True)) and stores it; Attribute Editor buttons (Make Wires Dynamic, Reference State Update, Rebuild) go through G.button_recipe(name, node). Export Cache opens a dialog: write its @path@ template from G.recipe_source. GATE: probe JSON saved; recipe_status() shows the recipes the plan needs as recorded.
1. Scalp. G.scalp_check(scalp, for_unreal=...). GATE: no error: line, border edges present (the hairline), UVs unique.
2. Flow sheet and counts. From the reference write the flow sheet (base, whorl, part, direction, length regions) and count big clumps and small per big. GATE: G.check_flow_sheet(flow) == [].
3. Guides. g = G.make_guides(scalp, flow, spacing=..., cvs=8, collide=head), few first (FlippedNormals); symmetric=True only on symmetric topology (Hadi). GATE: G.gate(G.measure_curves(...)["verdict"]), penetration 0, discontinuities only at parts and whorls.
4. Descriptions and wiring. G.create_description(scalp, "hair_body") per hair type (recipe, renamed). G.wire_guides(desc, g["curves"]): a Guide modifier low in the stack interpolates the guides, fed by a Curve to Spline on its inGuide; it holds the region maps parts need. A Curve to Spline on the description itself turns each curve into one hair and disables everything below it: only for exact strands or caches. GATE: G.wiring_verdict: flow-sheet error of the hair drops after wiring, and the InGuide_base cache holds every guide.
5. Stack, density and masks. Bottom to top: base (CV count per Numbers), Scale, Guide, primary and secondary Clump (G.build_clumps), strand Noise on a share, length breakup, Collision. G.write_hairline_mask for the hairline (band=True: transition), one map shared across descriptions (G.connect_texture), G.patches holes in the primary's mask for stray clumps. GATE: G.gate(G.lint()["lines"]); the coverage pass (showthrough %) and spline count within budget.
6. Clumps and breakup, one layer at a time (Giovannini). Once per install, P5b's two tests settle the Clump Scale convention, which knob acts as tightness and what each noise level breaks. Export mid-stack caches, then G.hierarchy_from_states (3 to 5), G.breakup_report and G.measure_abc. Render Clump before and after Noise once each (2027 help). GATE: breakup_verdict and strand_verdict clean; a side-lit close-up passes references/critique.md, triangles and squares in the reference's proportion.
7. Shader and render. G.hair_shader(desc, preset=...) (replaces the default hairPhysicalShader), G.curve_render(desc, "ribbon", min_pixel_width=...), G.cost_sweep over the four knobs with flicker. Save before rendering XGen. GATE: G.review(...) sheet judged with the critique, no shader warn:.
8. Motion. Static follow, keyed wires (Flood) or dynamics (Linear Wire, Make Wires Dynamic). Caches are versioned. GATE: renders at the rest, mid and last frames with no penetration.
9. Deliver (Handoffs).

Numbers

Value	Relative to	Source
no clump pull over the first ~14 %	strand length, clump profile	Fernandez 02 [00:17:56]
spike: 0.01 at mid length; onion: mid bulge ~0.9	legacy Clump Scale ramp (radius reading)	Fernandez 02 [00:05:38], frames 00:06:00, 00:10:51
3 to 5 small clumps per big; 2 levels, 3 for long hair	clump hierarchy	Fernandez 03 [00:01:31], [00:09:11]
second clump density 7.5, clump noise 2, Cut random up to 10	his real-time scalp asset	Giovannini [00:09:43] to [00:11:25]
strays 5 % (up to 20 %), noise 3 on strays, 0 on body	share of hairs	Giovannini [00:09:28], [00:11:59]
noise mask 0.1 to 0.5 (1 kills clumping)	legacy Noise modifier	FlippedNormals [00:32:05]
20 CVs with noise; 24 to 36 for curls	modifier or base CV count	FlippedNormals [00:28:52]; 2027 help
random length 0.8 to 1.2; clump cut 0.2	balanced around 1; small-clump level	Fernandez 08 [00:09:33], [00:15:34]
melanin 0.2 blonde, 0.5 red or brown, 1.0 black	Standard Hair	Arnold hair doc
IOR 1.55 (outside 1.4 to 1.6 only wet); Shift 0 to 10 deg, 2.3 to 3.7 measured, 0 synthetic	Standard Hair	Arnold hair doc
Unreal: Sub Steps 40, Iterations 10, Collision Radius 3.5, Bend Scale tip 0.1	UE 5.3 era [estimate]	Giovannini [00:51:04] to [00:54:27]
Nucleus Space Scale 0.01	cm scenes	maya-version-deltas 2.8

Quality gates

In code. G.lint() (plug-in order, stack, clump wiring and profiles, noise placement, budget, shader), G.measure_abc(cache)["verdict"] (root pinch, profile variety, small per big, strays, tip noise, frequency and fray, length balance, flow, penetration), G.wiring_verdict, G.breakup_verdict, G.unreal_verdict(G.inspect_abc(abc)). G.gate turns any of them into an assertion. Thresholds marked [added] are this skill's defaults; each line cites its rule.

Visual. G.review(hair, out, head=head, scalp=scalp, closeups=[...]) renders the real shader: beauty (dome plus a key along the groom, Schneider), backlit (strays, halo), silhouette, coverage. Viewport captures only after G.viewport_aa(True) (FlippedNormals, Fernandez). Judge with references/critique.md: flow, then clumps, then strands (Fernandez).

Common mistakes

Mistake	Looks like	Fix
Guides not wired, or through Curve to Spline on the description	hair ignores the flow sheet, or one hair per curve	G.wire_guides (Guide modifier), then wiring_verdict
Clumped roots	bald lines between clumps at the scalp	root-loose Clump Scale ramp (Fernandez)
One clump shape everywhere, all triangles	"CG" regularity	Radius Variance, profiles, square clumps (Fernandez 02, 05)
Mask and tightness judged on straight guides	two knobs seem to do the same	noise below the Clump, G.clump_follow (Fernandez 06)
Strand noise to break big clumps; noise on the body	small clumps dissolve into frizz	clump-level noise, strand noise on a share (Fernandez 07, Giovannini)
One noise frequency, fray everywhere	fake, uniform frizz	vary frequency; fray on few hairs (Fernandez 05)
Strays as a new system only	strays unrelated to the clumps	holes in the primary's mask (Fernandez 03)
Extending strands	criss-crossing inside clumps	scale or cut at the small-clump level (Fernandez 08)
Jagged clumps, bad placement	stepped clump edges	raise Map Subdivision Level on every level; new Seed (2027 help)
Secondary density at or below primary	secondary clumps leak across cells	duplicate the primary, raise density, Use Control Map (2027 help)
Clumps crossing a part	clumps straddle the part	Guide region map as clump control map, or separate descriptions
Cache without Multiple Transforms and Write Final Width	curves wrong in other apps	turn both on (2027 help, Flood); check Epic's opposite advice
Default hairPhysicalShader kept	no indirect diffuse on the hair (Ai Kd Ind defaults to 0)	aiStandardHair, or raise Ai Kd Ind (2027 help)
Judging an aliased viewport	density and clumps misread	G.viewport_aa(True) first

Handoffs

• Receives from scenario-maya-retopology-uv: a scalp-only cap (open border at the hairline), unique UVs in 0 to 1, UV set map1 for Unreal, frozen, no history; it passes G.scalp_check.
• Receives from scenario-maya-animation: rig referenced, shots starting at the rest pose on frame 1, an Alembic of the animated body (-uvWrite, -stripNamespaces), and the wires cache when the rig owns the wires.
• Delivers to scenario-maya-lighting-rendering: a new groom version, aiStandardHair assigned and linted, ribbon settings, versioned per-shot caches, the review sheet and what was not verified. Loop in scenario-maya-lookdev when the hair must match the skin.
• Delivers to scenario-maya-pipeline-scripting (Unreal): <asset>_ue5.abc with groom_group_id, groom_guide (real guides, no fill) and groom_root_uv (UV set map1), a clean unreal_verdict on the reimport, and the Unreal start values (Numbers).

Maya 2027 notes

• Legacy descriptions do not work with Interactive Groom tools; convert one at a time, then Rebuild each (Giovannini).
• Default IG shader is hairPhysicalShader (Ai Kd Ind 0). Standard Hair gained scattering_mode in MtoA 5.6.0. Load MtoA before XGen.
• IG computes on the GPU; headless evaluation on Apple Silicon is unverified (probe). Parallel Maya known limitation: batch rendering XGen scenes "may produce incorrect results".
• Alembic is Ogawa only. Epic's scripts are Python 2 and API 1.0: use the mx_groom ports. No Arnold GPU on macOS; Arnold RenderView replaces Maya IPR. Ctrl means Control, not Command.

References

• references/procedures.md: code for every stage, P5b clump tests, P16 the whole job. Load before writing groom code.
• references/critique.md: the rubric for judging a groom from renders and numbers. Load at every gate.
• references/expert-notes.md: principles by expert, disagreements with deciding conditions. Load when choosing between options.
• references/gui-paths.md: menus, editors, brushes, hotkeys (computer-use agent).
• references/sources.md: every source, credentials, URLs, best timestamps.