# Expression Muscle-Onset Reference (Cal, 6/6)
**For the FC morph timing. Jun's principle: expressions almost never happen in one frame — a specific muscle INITIATES, others follow, and the gradualness (right onset order + speed) is what reads as human. This doc = what manifests FIRST per expression, so the warp/morph sequences each one from the right place.**

Grounded in FACS (Facial Action Coding System — AUs map to muscles) + facial-animation research. See sources at bottom.

## The rule
For each expression define: (1) the INITIATOR (fires first), (2) the follow sequence, (3) onset SPEED. Warp/morph the initiator first, stagger the rest, never fire all AUs on the same frame.

## Per-expression onset

**Smile / happy** (AU6 + AU12) — onset MEDIUM, gentle.
1. **Zygomaticus major INITIATES** — mouth corners pull up + OUT (AU12). On a closed mouth the lip *center* stays still; the corners lead.
2. Cheeks raise (AU6, orbicularis oculi outer) — the "apples" lift.
3. **Orbicularis oculi (eye-squint) follows LAST = the Duchenne tell.** A real smile crinkles the eyes a beat after the mouth; a fake one doesn't.
   - Bonus realism: posed smiles tend to lead slightly on the LEFT side (asymmetry).
   - (This is the order the smile POC already uses.)

**Surprise** (AU1+2+5+26) — onset FAST.
1. **Frontalis INITIATES** — brows shoot UP + arch (AU1+2), eyes widen (AU5, levator palpebrae) — near-simultaneous, fast.
2. Jaw drops, mouth opens (AU26) a hair AFTER the eyes. Eyes/brows lead, mouth follows.

**Anger** (AU4+5+7+23) — onset FAST, sharp.
1. **Corrugator INITIATES** — brows pull DOWN + together (furrow, AU4); lids tighten (AU7) — the glare forms first.
2. Lips press/flatten then thin (AU23). Brows lead, mouth hardens after.

**Sadness** (AU1+15, +AU4 inner) — onset SLOW, "softening."
1. **Inner frontalis INITIATES** — INNER brow corners lift (AU1) into the oblique sad shape — this is the signature, and it leads.
2. Lip corners pull down (AU15, depressor anguli oris).
3. Upper lids droop + gaze drifts down/away (the gaze system, not warp). Slow, sagging.

**Fear** (AU1+2+4+5+20) — onset FAST.
1. Brows up + together (AU1+2+4 combined — raised AND drawn in) + eyes widen (AU5) initiate.
2. Lips stretch horizontally (AU20) after.

**Disgust** (AU9+10) — onset MEDIUM.
1. **Levator labii INITIATES** — NOSE wrinkles (AU9) first.
2. Upper lip raises (AU10), mouth may open. Starts at the nose, spreads down.

**Contempt** (AU12+14, unilateral) — onset MEDIUM.
1. **ONE-sided lip corner** tightens/lifts (AU14) — asymmetry IS the expression. Half the mouth, not both.

## Onset-speed cheat
- FAST / sharp: surprise, anger, fear (immediacy).
- MEDIUM: happy, disgust, contempt.
- SLOW / softening: sadness, thoughtfulness, regret, tenderness.

## Morph implication (how this drives the rig)
- The INITIATOR muscle = the first warp/key to move; give it a head start (~3-6 frames) before the followers.
- Eyes/brows are the most reliable emotion carriers (esp. for subtle/spontaneous reads) — never animate the mouth alone.
- The Duchenne rule generalizes: the eye component lagging the mouth slightly is what sells *genuine* vs *posed* for the warm expressions.
- Match onset SPEED to the emotion (don't ease a surprise as slowly as a sadness).

Sources: FACS (Ekman/Friesen); [PMC: Spatiotemporal Dynamics of Posed Smiles](https://pmc.ncbi.nlm.nih.gov/articles/PMC10525663/); [animseeds: 7 universal expressions for animation](https://www.animseeds.com/post/the-seven-basic-universal-expressions-for-animation); [Animation Mentor: animating facial expressions](https://www.animationmentor.com/blog/tutorial-how-to-animate-facial-expressions-and-emotion/). Links: [[malin_avatar_brow_spec]], [[fc_hybrid_rig_spec]], [[project_malin_avatar]].