Scenario Blender Hair

Hair and fur grooming (hair curves, Blender 5.2)

Expert grooming means a few evenly spaced guides that set direction and silhouette. Stacked, masked procedural layers add density, clumping, frizz, length variation and strays, judged in a lit render at full density and in pose. Write guides as data (no curve brush stroke can be scripted in 5.2.1) and make every look decision through the Essentials hair node-group assets. 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-blender-expert (execution channel, review loop, 5.2 API changes).

Stance (the expert delta)

• Guides get 2 points until every direction is right (Schmidbauer). More points make combing tangle and clip. Use 3 for short fur, 8 only for long hair. Bystedt resamples long human hair to 15 points before shaping.
• Space guides evenly, and always mask Interpolate (Schmidbauer). Children spawn over the whole surface, and a child far from any guide "does whatever". Gaps turn into bald spots that grow under deformation.
• Density is counted per square meter and must be huge. Radius drops with it (Thommes). Long fur uses 1,000,000/m² at about 0.2 mm radius. Work at a Viewport Amount of 4%.
• Randomize per curve, not per point (Thommes). Give 80% of strands no frizz. Strays are 10% of curves with strong noise.
• Trim with Replace Length off plus a Random Offset (Schmidbauer, Thommes). It is the cheapest realism gain. Mask it, or short-fur regions lose their guides and go bald.
• Grow a human hairstyle on a separate hair cap split along the part (Bystedt): body topology stops mattering, interpolation stops crossing the part. The part still needs extra density and a lift.
• Shape works the same way on every asset: 1 affects the tip only, 0 the whole strand, negative the roots. Use a negative Frizz Shape so curl and clump tips survive (Thommes, Bystedt).
• Judge renders at full density, and in pose (Thommes, Schmidbauer). A harsh mesh deformation "looks ten times worse" in the hair.

Establish first

Finish topology and non-overlapping UVs before grooming (hair binds to UV space) and apply scale: density counts the mesh's local area (verified).

Workflow

1. Prepare the surface. H.uv_report(surface): stacked UVs make curves ignore the rig, UDIMs are fine, an unmerged mirror seam needs UV Merge by Distance (Schmidbauer). Humans: a hair cap split into islands along the part. Animals: separate the mouth interior. Masks: vertex groups (fast) or textures (detail on low-poly meshes). GATE: overlap_faces == 0, scale applied, one mask per region.
2. Plan flow and objects. Turn the draw-over into regions (vertex groups) with one flow direction each. Schmidbauer: one curves object per landmark (face, cheeks, ears, neck, body, tail, feet) for performance. Thommes: one object with masks. Bystedt: split front and back where combing one disturbs the other. GATE: written list of regions, flows, objects.
3. Guides. hair = H.new_groom(surface), P, N = H.sample_roots(surface, spacing, mask=...), H.add_guides(hair, P, N, length, points=2, flow=..., flow_blend=...) (snaps, writes surface_uv_coordinate). Later: H.subdivide_guides or H.resample_guides(hair, 15). Brush substitutes: a direction field for Comb, droop for gravity, delete and re-add guides for Density and Add; Bystedt's "grow long, comb, shrink back" is long guides plus Trim. GATE: in H.groom_report, roots_off_surface and spacing_gaps_2x are 0 and length_outliers is 0. A guides-only render shows every direction right.
4. Density. Add Set Hair Curve Profile first, then Interpolate Hair Curves with a Density Mask. Bystedt paints 50% gray and 100% at the part, then remaps. The 5.2 Interpolate also takes a Mask Texture image. GATE: H.count(hair) (full density) is within budget, root_area_fraction is well under 0.5, and a render shows no bald spots.
5. Procedural layers, each masked, A/B-tested by toggling show_viewport and rendering.
   • Long fur (Thommes, preset H.FUR_STACK_LONG, applied with H.build_stack): Noise, Curl, strays, Frizz with negative shape, Clump, Clump 2, final Frizz (Cumulative off), Trim, Rotate.
   • Animal (Schmidbauer): Trim, Clump (cheeks, Tip Spread, Clump Offset), low Frizz, each with its own vertex group.
   • Human (Bystedt): Noise on guides (Cumulative off, Offset per Curve on), Roll at the tip only, then per population Interpolate, Profile, Clump on the groomed guides; wide and thin layers need different seeds.
   • Per-curve randomness: H.random_curve_mask(hair, "frizz_mask", 0.8) then input "attr:frizz_mask"; root-to-tip ramps: H.along_curve_attr. GATE: each layer earns its place in an A/B render.
6. Part and hairline (human). Leave Part by Mesh Islands on (default): in the test, 21% of children near the part crossed it with islands off and 0% with islands on. For the lift, H.distance_weights(cap, "part_lift", "part", radius, "bump") feeds Displace Hair Curves (Surface Normal on, small distance, Shape near 0, Factor attr:part_lift). GATE: in a top view there is no scalp gap at the part and the crown is not flat.
7. Shading and render. Use H.hair_material(name, "CYCLES", melanin=(root, tip)) or "EEVEE" with explicit colors, then H.setup_render(scene, engine, close_up). GATE: H.review([surface, hair], out, engine) passes references/critique.md.
8. Deformation. Key 2 or 3 extreme poses and render each frame from the same camera. GATE: no bald spots or clipping in any pose.
9. Games: cards. Run H.add_cards(hair, width, material=H.card_material(...)), then convert a copy with object.convert(target='MESH') and export glTF. Put a textured hair cap under the cards. GATE: triangles within budget, UVs in the half tile for cards, roots on the scalp.