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Active Ragdolls (Physical Characters)

Active ragdolls move beyond “canned” animations by using a physics-driven skeleton. Instead of just playing a clip, the ragdoll uses PD Controllers to force its limbs to follow an animated target, resulting in characters that stumble, react to impacts, and feel truly physical.

1. The Dual Skeleton System

To make an active ragdoll, you need two versions of the character:

1. The Ghost (Kinematic): A standard invisible skeleton that plays traditional animations. It has no physics.
2. The Body (Physical): A collection of Rigidbodies and Joints (the Ragdoll) that exists in the world.

2. Driving Limbs with PD Controllers

The “Active” part comes from driving the Body’s joints to match the Ghost’s rotations.

• Target: The rotation of the bone in the animated “Ghost.”
• Current: The rotation of the physical Rigidbody bone.
• Control: We use Joint Drives (which are essentially PD Controllers) to apply torque.

3. The “Keep Upright” Force

The hardest part of an active ragdoll is preventing it from collapsing into a pile of meat.

• We apply a specific “Upright Torque” to the hips.
• This torque calculates the error between the character’s transform.up and the world Vector3.up and forces the hips to stay vertical.

4. Why use Active Ragdolls?

• Procedural Reaction: If a character walks into a wall, their arm naturally bends and pushes back.
• Dynamic Impacts: When hit by a projectile, the character flinches physically rather than playing a “Hit” animation.
• Emergent Fun: Used famously in Gang Beasts, Human Fall Flat, and TABS for their “drunken,” physical feel.

5. Implementation Strategy

// Simplistic Hip-Leveling Logic
void FixedUpdate() {
    Quaternion uprightError = Quaternion.FromToRotation(transform.up, Vector3.up);
    rb.AddTorque(uprightError.x * kP, uprightError.y * kP, uprightError.z * kP);
    
    // Joint Drives handle the rest of the limbs
    foreach(var joint in limbs) {
        joint.targetRotation = ghostLimbs[i].localRotation;
    }
}

6. Summary

• Physics first: All movement is done via forces/torques.
• Hybrid approach: Mixes the control of traditional animation with the reactivity of physics.
• The Core: Requires stable PD/SPD controllers to prevent the limbs from oscillating.