# Ep.11 — Simu Bone · Spring Magic Integration

https://youtu.be/0yHhCZqgYZ4?si=YObinszdcfC3bhtr

## Overview

Describes how to properly attach **secondary simulations** like **Spring Magic** (hair, smartphone chains, chains) to a BipedPlus rig.  
The key is to **separate "skin bones" from "controllers"** and organize them with **Space Groups**.

## Spring Magic and BipedPlus — Why Be Careful

| Standard BipedPlus approach | Spring Magic-friendly approach |
|---------------------|------------------------------|
| **Offset + Controller** (empty node in between) | **Bones only** in a **simple chain** |
| Maintains **position·rotation at 0** after connection | Head → Bone → Bone **without intermediate nodes** |

- Spring Magic may **not work correctly** if there are **offsets (intermediate groups)**.
- Attaching bones directly under Head without offsets **works**, but conflicts with BipedPlus **Layer·Skin separation rules**.

> If you use a **different simulation tool**, **test yourself** whether it allows offsets. If it does, you can keep the standard BipedPlus approach.

---

## Recommended Structure (video — hair example)

### Step 1: Skin Bones (what gets sent to game)

- Create a **bone chain** like Hair1, Hair2… in the **Skin hierarchy** (extending from the head).
- **Weights** will be assigned here later.
- **Do not place in the Controller Layer.**

### Step 2: Controllers (what animators and sims operate)

1. **Clone** the same structure → **Unparent**.
2. **Name controllers** per team convention (e.g., Ctrl_Hair1, Ctrl_Hair2…).
3. Use **box display** etc. in viewport to **distinguish from bones**.
4. **Edit Utils → Quick Connect:**
   - **Controller list** ← Ctrl_Hair items
   - **Bone list** ← Skin Hair items
   - **Find** → **Local Matrix Auto Parent** → connect
5. Verify that moving the **head** causes the **entire Hair chain** to follow.

### Step 3: Space Group — Solving "Where to place it"

**What is a Space Group?**  
An **empty group** that has the **exact same position·rotation·scale** as the **reference bone** (video: **head skin bone**).  
(The video explains the concept using the **Pelvis**, then uses the **head** as the reference for the hair example.)

**Why it's needed**

- Placing sim controllers in the **Skin Layer** → they mix with game bones on **FBX export**.
- Using **Controller Layer** only → they may not move **with the head**.
- Placing only Ctrl under **Space Group** → they **follow the head** without **mixing into the export**.

**Creation steps (video)**

1. Select **Skin Head** (head skin bone).
2. **Create Space Group** (menu in Figure/Edit area — check location per version).
3. **Disable the Space Group's shape (curve)** — make it invisible (may already be off at 100% scale).
4. Verify the Space Group **moves exactly like the head**.
5. Make **Ctrl_Hair\*** children of the **Space Group** → place in **System Layer**, etc.

### Step 4: Layer Organization

| Contents | Layer |
|---------|--------|
| Ctrl_Hair* | **Controller** |
| Skin Hair* | **Skin** |
| Space Group | **System** |

---

## Running Spring Magic

- The video covers up to **structure** — Spring Magic **execution** can be done separately.
- With a **simple bone chain + Ctrl under Space Group**, the goal is to align with a **standard Spring Magic** workflow.

## Rigging Info Integration

1. **Scan** → custom **Ctrl_Hair\*** → **Rigging Info Add**.
2. **Update**.
3. **Ani Utils Reset All** — verify that Hair also returns to the **default pose**.

## Scale Warning

- Quick Connect + **Scale keys** → possible **skin distortion** (manage together with Ep.7 Stretch/Squash).