VRM 1.0 spring bones ignore wind — VRM1SpringManager::update() has no wind handling (windScale / WindDirectionalSource have no effect)

[franadoriv]

Summary

Spring bones on VRM 1.0 avatars do not react to wind at all. WindDirectionalSource actors, the windScale / randomWindRange properties on the VrmSpringBone anim node, and bIgnoreWindDirectionalSource have zero effect on VRM 1.0 models. The same setup works correctly on VRM 0.x avatars.

Environment

• UE 5.7 / 5.8
• VRM4U (current master)
• VRM 1.0 model with spring bones (e.g. VRoid hair)

Root cause

VRM4U has two separate spring simulation paths in Source/VRM4U/Private/VrmSpringBone.cpp, selected in AnimNode_VrmSpringBone.cpp by VrmMetaObject_Internal->GetVRMVersion():

• VRM 0.x → VRMSpringBone::VRMSpring::Update() — reads wind via Scene->GetWindParameters_GameThread(...) and adds it to external. ✅
• VRM 1.0 → VRM1Spring::VRM1SpringManager::update() — computes external from gravity + gravityAdd only. There is no wind code at all. ❌

So for VRM 1.0 the wind force is simply never applied, regardless of windScale or any WindDirectionalSource in the level.

Suggested fix

Mirror the VRM 0.x wind block inside VRM1SpringManager::update(). Compute the wind force once per update (after ComponentToLocal is set) and add it to each joint's external.

⚠️ Important: VRM1SpringManager::update() runs on the anim worker thread (FAnimNode_VrmSpringBone::EvaluateSkeletalControl_AnyThread). TActorIterator / GetAllActorsOfClass cannot be used there — they assert IsInGameThread() and crash. Use Scene->GetWindParameters_GameThread(...), which is worker-thread-safe (exactly as the VRM 0.x path already does).

// once per update, after: const FTransform ComponentToLocal = ComponentTransform.Inverse();
FVector WindExternal = FVector::ZeroVector;
if (animNode->bIgnoreWindDirectionalSource == false) {
    const USkeletalMeshComponent* SkelComp = Output.AnimInstanceProxy->GetSkelMeshComponent();
    const UWorld* World = SkelComp ? SkelComp->GetWorld() : nullptr;
    FSceneInterface* Scene = World ? World->Scene : nullptr;
    if (Scene) {
        FVector WindDirection = FVector::ZeroVector;
        float WindSpeed = 0.f, MinGust = 0.f, MaxGust = 0.f;
        Scene->GetWindParameters_GameThread(SkelComp->GetComponentTransform().GetLocation(),
                                            WindDirection, WindSpeed, MinGust, MaxGust);
        WindDirection = ComponentToLocal.TransformVector(WindDirection);
        const float WindUnitScale = 0.5f * 250.0f
            * FMath::FRandRange(1.f - animNode->randomWindRange, 1.f + animNode->randomWindRange)
            * animNode->windScale;
        // VRM 0.x does `external *= 100` later; VRM 1.0 uses `external` directly, so /100 to match scale.
        WindExternal = WindDirection * WindSpeed * WindUnitScale * DeltaTime / 100.f;
    }
}

// then per joint:
FVector external = ComponentToLocal.TransformVector(ue4grav) * (j1.gravityPower * DeltaTime) * animNode->gravityScale
    + ComponentToLocal.TransformVector(animNode->gravityAdd) * DeltaTime
    + WindExternal;

I applied this locally on UE 5.8 and VRM 1.0 hair now reacts to WindDirectionalSource with proper gusting (via randomWindRange). Thanks for the great plugin! 🙏

[Grimm0351]

Very cool. I hope this fix makes it into the 5.8 branch.