Vrm Springbone Physics

VRM SpringBone Physics Debugging

This skill covers common issues with VRM hair/clothing physics using @pixiv/three-vrm and how to fix them.

Common Symptoms

1. Hair flies upward or explodes outward on load
2. Hair sticks out horizontally like there's an invisible wall
3. Hair is stiff and doesn't move naturally
4. Physics works but starts from wrong position

Root Cause 1: Incorrect Delta Time (Most Common - 90%)

Problem

The vrm.update(delta) function expects delta in seconds, not milliseconds. If delta is too large, physics "explodes".

Diagnosis

// Add this to your animation loop
console.log('delta:', delta);
// Should be ~0.016 for 60fps, NOT 16 or larger!

Solution

// Correct implementation using THREE.Clock
const clock = new THREE.Clock();

function animate() {
    requestAnimationFrame(animate);
    
    let delta = clock.getDelta();
    // Clamp to prevent explosion on tab switch or lag
    delta = Math.min(delta, 0.05);  // Max 50ms
    
    if (vrm) {
        vrm.update(delta);
    }
    
    renderer.render(scene, camera);
}

Root Cause 2: SpringBone Colliders (Very Common)

Problem

VRM models have invisible spherical colliders (usually on head/body) that prevent hair from penetrating. Virtually ALL VRM models have oversized colliders, causing hair to appear stuck horizontally in mid-air.

Root Cause Analysis

Confirmed Facts

1. UniVRM Export Bug Exists (#673):

   • When colliders are on scaled objects, the radius doesn't normalize with the mesh
   • Gizmo shows correct size in editor, but exported collider is larger
   • Issue documented with reproducible steps

2. three-vrm Uses Radius Directly (source):

   const distance = length - objectRadius - this.radius;  // radius not scaled by world matrix
   

3. UniVRM Officially Discourages Scaling (source):

   "We do not recommend using SpringBone and scaling together"

Empirical Observation

[!NOTE] 50% reduction fixes ALL tested models. The exact mathematical reason is uncertain - the export scaling could vary by model/tool. However, this factor works universally in practice.

Possible explanations:

• VRoid Studio (most common VRM source) may use consistent internal scaling
• The visual matching in Unity editor may systematically create ~2x overcorrection
• Export normalization algorithms may have consistent behavior

Practical Approach

Since the exact cause varies, we provide an adjustable reduction factor with 50% as default.

Diagnosis

Check if only bangs are horizontal (collider issue) or all physics elements (gravity issue):

• Only bangs horizontal → Head collider blocking them
• All physics horizontal → Gravity direction wrong

Disable colliders to confirm:

const colliders = Array.from(springBoneManager.colliders || []);
colliders.forEach(c => {
    if (c.shape?.radius) c.shape.radius = 0;
});
// If hair now falls correctly, colliders were the issue

Solutions

Option 1: Reduce Collider Radii by 50% (Recommended - Compensates for export bug)

const REDUCTION_FACTOR = 0.5;  // Compensates for UniVRM export scaling bug
const colliders = Array.from(springBoneManager.colliders || []);
colliders.forEach(collider => {
    if (collider.shape?.radius > 0) {
        // Save original for potential future adjustment
        if (collider._originalRadius === undefined) {
            collider._originalRadius = collider.shape.radius;
        }
        collider.shape.radius = collider._originalRadius * REDUCTION_FACTOR;
    }
});

Option 2: Completely Disable Colliders (Simple but may cause clipping)

const colliders = Array.from(springBoneManager.colliders || []);
colliders.forEach(collider => {
    if (collider.shape?.radius !== undefined) {
        collider.shape.radius = 0;
    }
});

Option 3: Disable Only Head Colliders (Best, needs bone name detection)

colliders.forEach(collider => {
    const boneName = collider.bone?.name?.toLowerCase() || '';
    if (boneName.includes('head') || boneName.includes('face')) {
        collider.shape.radius = 0;
    }
});

Option 4: Fix in Unity (Permanent fix, requires model access)

1. Open model in Unity with VRM SDK
2. Find "secondary" object in hierarchy
3. Select head bone with VRMSpringBoneColliderGroup
4. Enable gizmos to see magenta collider spheres
5. Reduce radius/adjust offset to proper size
6. Re-export VRM

Option 5: Scale Colliders with Scene (Runtime fix for scaled models)

When vrm.scene.scale is changed at runtime, colliders need to be scaled proportionally:

function scaleVRMScene(vrm, scaleFactor) {
    // Scale the scene
    vrm.scene.scale.setScalar(scaleFactor);
    
    // Scale all collider radii to match
    const springBoneManager = vrm.springBoneManager;
    if (springBoneManager) {
        const colliders = Array.from(springBoneManager.colliders || []);
        colliders.forEach(collider => {
            if (collider.shape?.radius !== undefined) {
                // Store original radius if not already stored
                if (collider._originalRadius === undefined) {
                    collider._originalRadius = collider.shape.radius;
                }
                // Scale radius with scene
                collider.shape.radius = collider._originalRadius * scaleFactor;
            }
        });
    }
}

Root Cause 2B: Runtime Scene Scaling (Application-Specific)

Problem

If your application scales vrm.scene to fit different screen sizes, the collider radii remain fixed in local space while bones scale with the scene. This causes colliders to become relatively larger when the model is scaled down.

Example

• Model scaled to 0.8x (80% size)
• Head collider radius stays at original 0.1 units
• Relative to the scaled head, the collider is now 0.1/0.8 = 0.125 (25% larger)
• Hair that previously cleared the collider now gets blocked

Key Insight

VRChat works because it doesn't scale the VRM scene directly - it places the model inside a container and scales the container, or uses a different physics implementation that accounts for scale.

Solution

When scaling the VRM scene, also scale the collider radii proportionally (see Option 5 above).

Root Cause 3: Model Issues

Symptoms

• _worldSpaceBoneLength: 0 in console logs
• Hair bones don't respond to physics changes

Cause

Model was not properly configured in Unity/Blender:

• Hair bones missing child bones
• SpringBone settings incorrectly exported
• Gravity direction wrong in model

Solution

1. Test with official VRM viewer - if hair is broken there, it's a model issue
2. Fix in Unity with VRM SDK or Blender with VRM addon
3. Ensure each hair bone has a proper child bone with non-zero length

Recommended Initialization Code

[!CAUTION] Empirical Fix Notice: The COLLIDER_REDUCTION = 0.5 value is empirically determined from testing multiple VRM models. While the underlying UniVRM bug is documented, we cannot mathematically prove 50% is correct for all models. If you encounter hair physics issues, adjust this value first.

function initializeVRMPhysics(vrm) {
    const springBoneManager = vrm.springBoneManager;
    if (!springBoneManager) return;
    
    // Reduce collider radii to compensate for UniVRM export bug (#673)
    // This is an EMPIRICAL fix - adjust if needed
    const COLLIDER_REDUCTION = 0.5;
    
    const colliders = Array.from(springBoneManager.colliders || []);
    colliders.forEach(collider => {
        if (collider.shape?.radius > 0) {
            collider._originalRadius = collider.shape.radius;
            collider.shape.radius *= COLLIDER_REDUCTION;
        }
    });
    
    console.log(`[VRM] Applied ${COLLIDER_REDUCTION * 100}% collider reduction to ${colliders.length} colliders`);
}

// Animation loop with delta clamping
const clock = new THREE.Clock();
function animate() {
    requestAnimationFrame(animate);
    
    let delta = clock.getDelta();
    delta = Math.min(delta, 0.05);  // Prevent physics explosion
    
    if (vrm) {
        vrm.update(delta);
    }
    
    renderer.render(scene, camera);
}

Key API Reference

Joint Settings (per joint.settings)