05 — Physics items

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AInteractionPhysicsActor has a mesh root and a UInteractionManipulationComponent. The component passes a single snapshot of motion, scale, hands and attachment; the actor's standard ReplicateMovement is turned off so that two mechanisms do not overwrite the transform. ObjectData references a UInteractionPhysicsDataAsset.

• MassInKg sets the mass; the mesh must have suitable simple collision.
• bStartWithPhysics turns simulation on from the start.
• SleepWhenSettled keeps the natural Chaos sleep: a hit from another body can wake the item.
• DisablePhysicsFixedTime stops the simulation completely after PhysicsDisableDelay; such an item will not wake from an ordinary physical hit, but it can be grabbed again.
• HitSound is played through a multicast after an authoritative impact.

The server tells the actor about pickup and release through the interface: OnGrab(Interactor) is called when the first hand is accepted, OnRelease(Interactor, Reason, LinearVelocity, AngularVelocity) when the last one lets go. In OnGrab the ready-made actor remembers the player as Instigator, and in OnRelease it starts the DisablePhysicsFixedTime timer. When overriding these functions in a subclass, call the parent implementation.

The constructor turns on the PhysicsActor collision profile for the mesh root, Simulation Generates Hit Events, wake events and CCD (bUseCCD), so that a fast item does not fly through thin walls. CCD has a cost: for a large number of small stationary items it can be turned off in a subclass.

Impacts: sound and damage

While an item is free (nobody holds it), the server handles every hit event of MeshComponent. All thresholds are set by ObjectData:

Setting	Default	What it does
HitSound	—	Impact sound; played by an unreliable multicast at the impact point on all clients
HitSoundMinimumImpulse	150	Weaker impacts make no sound: touches and resting are quiet
HitSoundCooldown	0.15 s	The shortest gap between two sounds of one item, so that sliding does not repeat the sound every frame
bApplyImpactDamage	off	Whether to call UGameplayStatics::ApplyDamage for the actor the item hit
DamagePerImpulse	0.005	Damage per unit of normal impulse
MinimumDamageImpulse	20	Weaker impacts deal no damage
DamageTypeClass	UDamageType	The damage type passed to ApplyDamage

In plain words. Impulse is the "strength" of a blow: it grows with the item's mass and speed. A thrown brick has a large impulse; one placed on a table has almost zero. The thresholds cut off minor touches so that sound and damage come only from real impacts.

Damage is off by default: turn on bApplyImpactDamage if thrown items should wound or break things. The damage instigator is the controller of the pawn that last held the item: OnGrab assigns it as Instigator. For your own rules override OnComponentHitInternal in a C++ subclass.

With Movement=Direct a grabbed item keeps QueryOnly so the second hand can select it. With Movement=Physics one server PhysicsHandle drives the body while keeping physical collisions. After release QueryAndPhysics is used; bSimulateOnRelease and bAllowThrow determine simulation and throw. Linear velocity is in cm/s, angular in degrees/s. All physical actor callbacks have an authority guard; do not drive a copy of the client body by hand. Profiles are described in 08 — VR manipulation.

AInteractionSpearActor uses the +X axis as the tip direction. The tip lies at (Bounds.Max.X, 0, 0) of the local mesh; the pivot may be shifted along the shaft. GetSpearTipLocation() returns its world position. A spear can align to its velocity and attach to a surface at an angle of up to 60°, a speed from MinimumEmbedSpeed (200 cm/s) and contact in the front section TipContactLength (40 local cm). A side or slow hit leaves an ordinary physical reaction.

In flight the spear ticks only on the server. If ObjectData.bAlignToVelocity is on and the speed exceeds 100 cm/s, the actor smoothly turns along the velocity vector (RInterpTo with AlignmentStiffness; 20 in SpearRange), and the angular velocity is zeroed. Flight ends when the body falls asleep, the simulation is off or the spear is taken again. Launch(Velocity) starts the same flight without a hand, for example from an arrow released by a bow.

PenetrationDepth sets the depth the tip sinks in world centimeters from the contact point. After a hit IsEmbedded() returns true, the simulation stops and the spear moves together with the surface. It can be grabbed and thrown again. For network attachment to a moving target its actor and component must be replicable. A ready-made example is SpearRange.

UInteractionDestructibleComponent is optional destruction from impacts. Add it to a replicated actor with a primitive root and hit events (the ready-made AInteractionPhysicsActor already turns them on). The server reduces health, and when it runs out, all machines show the effect and the actor is destroyed. This is a simple gameplay coefficient, not a physical model of strength.

Property, event or method	Meaning
Health	Strength reserve, 50 by default; set in Class Defaults. GetHealth() returns the remainder on the server
DamagePerImpulse	Damage per unit of impact impulse, 0.01 by default
MinimumImpactDamage	An impact must deal more than this damage (5 by default) or it is not counted
DestroyEffect	Optional effect at the destruction point: a Niagara or Cascade system
OnImpactDestroyed(Location)	Fires on the server and on every client before destruction: sound, debris, a custom effect
ApplyImpactDamage(Amount, Location)	Server damage from your logic: an explosion, a sword strike and so on

For example, a crate with Health = 30 and the default coefficients does not notice impacts with an impulse up to 500 (damage up to 5), while an impact with impulse 1500 takes 15 health: two such impacts are enough. A step-by-step exercise is lesson 6.

How to choose a mode

You need	Setting
An item lies on a rack and does not fall until first picked up	bStartWithPhysics=false in ObjectData
An item falls and rolls at once	bStartWithPhysics=true
A thrown item should "fall asleep" and wake on impacts	PostThrowBehavior=SleepWhenSettled
A thrown item must reliably stop after N seconds	PostThrowBehavior=DisablePhysicsFixedTime, PhysicsDisableDelay=N
An item in the hand must not pass through walls	a profile with Movement=Physics (08)
Thrown items should wound or break things	bApplyImpactDamage=true in ObjectData, and if needed UInteractionDestructibleComponent on the target
A model for inspection that stays in the air after release	bSimulateOnRelease=false, bAllowThrow=false