Blender Facial Shapekeys

---
name: blender-facial-shapekeys
description: Facial rigs and shape keys in Blender 5.2 - ARKit 52 blendshapes, visemes for lip-sync, sculpting expressions, splitting symmetric shapes into Left/Right, mirroring, corrective and combination shapes, drivers and control rigs, bone+blendshape hybrids, export to engines (Unity, Unreal, Roblox dynamic heads, VRM/web). Use for faces, expressions, lip-sync, or any shape-key work.
license: MIT
metadata:
  author: blender-agent-skills
  version: "0.1.0"
---

# Facial Rig & Shape Keys

The execution model is in `blender-core`. The body rig is in `blender-character-rigging`.

## Choose the approach
| Approach | Use | Notes |
|---|---|---|
| **Blendshapes (ARKit 52)** | games, VTubers, mocap (iPhone/Live Link Face), web/VRM | the industry interchange set; each shape is sculpted |
| **Bones** (jaw, eyes, lids, brows, lips) | stylized, low memory, mobile | jaw + eyes as bones are almost always right |
| **Hybrid** | most production game heads | jaw/eye/tongue bones + blendshapes for everything soft |
| **Rigify face** (`human` metarig) | film-style keyframe animation inside Blender | 100+ face bones, heavy for engines |

## Procedure (blendshapes)
1. The head mesh needs clean loops around the eyes and mouth (`blender-retopology-uv`), with eyes, teeth and tongue as separate objects parented or weighted to bones.
2. `scripts/shapekeys.py -- --mesh Head --create arkit` (or `--create visemes` for the 15 Oculus visemes) creates empty, correctly named keys.
3. Sculpt each key (live: sculpt mode on the active key, `use_shape_key_edit_mode`). Symmetric expressions can be sculpted once, then `--split eyeBlink` gives `eyeBlinkLeft/Right` with a soft blend across X=0 (`--falloff` m).
4. `--mirror browDownLeft` builds `browDownRight` (topology-independent mirror).
5. `--report` lists unsculpted keys, the max offset per key, and missing ARKit names. A shape is done only when every key moves and nothing is left over.
6. Optional: `--drivers FaceCtrl` adds a controller with 0–1 custom properties driving each key, for keyframing in Blender.
7. Check the extremes and **combinations** (jawOpen + mouthSmile, eyeBlink + eyeSquint) with `look`. Fix them with corrective combination keys driven by the product of two keys.

## Shape key rules
- Shapes are **deltas from Basis**. Never change the Basis topology after creating keys (adding or removing verts destroys every key).
- One action per key. No head or neck movement inside face keys (bones do that). Keep eyelids sliding over the eye surface, not through it (check with `blender-asset-qa` zfighting on lid/eye).
- Range 0–1. `slider_max` can be raised for exaggeration, but engines clamp to 0–1.
- Relative keys (`use_relative=True`, the default) are the only kind that exports.
- Name exactly per spec (case-sensitive): `eyeBlinkLeft`, not `EyeBlink_L`. Engines map by name.
- Keys on meshes with Subdivision: sculpt at level 0, and keep the subdivision modifier **after** the shape keys (they're evaluated first anyway).
- Applying modifiers on a mesh with shape keys fails. Use `modifier_apply_as_shapekey` for deform modifiers, or apply them before creating keys.

## Correctives and combination shapes
- Corrective for a bone pose: sculpt in the posed state, then drive with a `TRANSFORMS` variable (`ROT_X`, `LOCAL_SPACE`, mapped to 0–1).
- Combination: key `jawOpen_mouthSmile` with driver `SCRIPTED` expression `a*b`, from the two key values (`SINGLE_PROP` `key_blocks["jawOpen"].value`). Simple expressions don't need Python auto-run.

## Engine notes
- **Unity**: FBX blendshapes. In the Model importer, set Blend Shape Normals = Calculate (or Import if smoothing was exported). ARKit names map to the AR Foundation face-tracking samples.
- **Unreal**: morph targets ("Import Morph Targets"). Live Link Face / MetaHuman Animator use ARKit names, and the Pose Asset maps the curves. No leaf bones.
- **Roblox dynamic heads**: FACS-based poses with Roblox-specific names, authored as animation frames on a head rig (Creator Hub "Dynamic heads" spec). Build from the official Blender template. ARKit sculpts can be reused by renaming/mapping. Head ≤ 4,000 tris.
- **VRM / web (three-vrm, ReadyPlayerMe-style)**: ARKit 52 + visemes as glTF morph targets (`export_morph=True`). Keep the morph-normal export on only if shading needs it (it doubles the size).
- Drop unused keys before export. Every key costs memory per vertex (glTF sparse accessors help).

## Gotchas
- `ob.shape_key_add(from_mix=False)` copies the Basis. `from_mix=True` bakes the current mix (useful for combos, dangerous by accident).
- `shape_key_mirror(use_topology=False)` needs a mesh symmetric on X. Run it on the **active** key (the script handles this).
- Driver variables on shape keys use the Key ID (`ob.data.shape_keys`), not the mesh: `fc = kb.driver_add("value")`.