physics-and-chaos

Physics & collision (Chaos)

Chaos is the physics engine in UE. Most gameplay needs three things from it: collision (what blocks/overlaps what), queries (traces/sweeps to ask about the world), and occasionally simulation (rigid bodies, ragdolls, constraints). Collision setup mistakes are the most common source of "my overlap/hit never fires" bugs.

When to use this skill

• Setting up what collides with what (channels, presets, per-channel responses).
• Making trigger volumes or detecting overlaps between actors.
• Line/shape tracing the world (aiming, ground checks, interaction detection).
• Simulating physics objects — enabling rigid-body physics, applying forces/impulses.
• Building ragdolls or constraining bodies with UPhysicsConstraintComponent.
• Debugging "overlap never fires", "trace misses", or "physics not simulating" problems.

Collision model — three controls per component

Every UPrimitiveComponent has three collision controls:

1. Object type (ECollisionChannel) — what this component is. Built-in: ECC_WorldStatic, ECC_WorldDynamic, ECC_Pawn, ECC_PhysicsBody, ECC_Visibility, ECC_Camera. Custom channels map to ECC_GameTraceChannel1–ECC_GameTraceChannel50 at runtime (expanded from 18 to 50 in 5.8).

2. Response per channel (ECollisionResponse) — how it reacts to each channel: ECR_Ignore, ECR_Overlap, ECR_Block.

3. Collision enabled mode (ECollisionEnabled::Type) — what the shape participates in:

   • NoCollision — no physics, no queries.
   • QueryOnly — overlaps/traces; no rigid-body simulation. Best for trigger volumes.
   • PhysicsOnly — rigid-body sim only; traces don't hit it.
   • QueryAndPhysics — both. Default for most simulated objects.

Block requires both sides to set ECR_Block for the other's object type. For overlap events, both sides need ECR_Overlap response and bGenerateOverlapEvents = true.

// Set up a trigger sphere: query-only, ignore everything except pawns.
Trigger->SetCollisionEnabled(ECollisionEnabled::QueryOnly);
Trigger->SetCollisionObjectType(ECC_WorldDynamic);
Trigger->SetCollisionResponseToAllChannels(ECR_Ignore);
Trigger->SetCollisionResponseToChannel(ECC_Pawn, ECR_Overlap);
Trigger->SetGenerateOverlapEvents(true);

Collision presets bundle object type + responses into a named profile. Apply with SetCollisionProfileName(TEXT("Trigger")) to match a project preset or one defined in Project Settings → Collision. Applying a preset overwrites per-channel overrides, so call any fine-grained SetCollisionResponseToChannel calls after the preset.

See references/collision-channels-and-profiles.md for the full channel list, custom-channel setup, and preset internals.

Hit & overlap events

Bind in BeginPlay (not the constructor — no world available there):

void AMyActor::BeginPlay()
{
    Super::BeginPlay();
    // Overlap: trigger volume notifying actor entry
    Trigger->OnComponentBeginOverlap.AddDynamic(this, &AMyActor::OnBeginOverlap);
    // Hit: physics body reporting impact
    Mesh->OnComponentHit.AddDynamic(this, &AMyActor::OnHit);
}

// Exact delegate signatures required — see PrimitiveComponent.h:1457,1468
UFUNCTION()
void AMyActor::OnBeginOverlap(UPrimitiveComponent* Comp, AActor* Other,
    UPrimitiveComponent* OtherComp, int32 OtherBodyIndex,
    bool bFromSweep, const FHitResult& Sweep) { /* ... */ }

UFUNCTION()
void AMyActor::OnHit(UPrimitiveComponent* HitComp, AActor* Other,
    UPrimitiveComponent* OtherComp, FVector NormalImpulse, const FHitResult& Hit) { /* ... */ }

Handlers must be UFUNCTION() — dynamic delegates require UObject reflection. For hit events on simulating bodies, also enable Simulation Generates Hit Events on the body (FBodyInstance::bNotifyRigidBodyCollision). See actors-and-components for delegate binding patterns and delegates-and-events for the delegate system.

Traces & sweeps

// Line trace — first blocking hit on ECC_Visibility
FHitResult Hit;
FCollisionQueryParams Params(SCENE_QUERY_STAT(MyTrace), /*bTraceComplex=*/false);
Params.AddIgnoredActor(this);   // never hit self

if (GetWorld()->LineTraceSingleByChannel(Hit, GetActorLocation(),
        GetActorLocation() + GetActorForwardVector() * 2000.f, ECC_Visibility, Params))
{
    // Hit.GetActor(), Hit.ImpactPoint, Hit.ImpactNormal, Hit.PhysMaterial
}

// Sphere sweep — shape-based query
FHitResult SweepHit;
GetWorld()->SweepSingleByChannel(SweepHit, Start, End, FQuat::Identity,
    ECC_Pawn, FCollisionShape::MakeSphere(40.f), Params);

// Overlap query — all bodies overlapping a box at a point
TArray<FOverlapResult> Overlaps;
GetWorld()->OverlapMultiByChannel(Overlaps, Center, FQuat::Identity,
    ECC_WorldDynamic, FCollisionShape::MakeBox(FVector(50.f)), Params);

FHitResult fields to know: Time (0–1 along trace), Distance, Location (shape center at contact), ImpactPoint (surface contact), ImpactNormal, GetActor(), GetComponent(), BoneName, PhysMaterial.

Multi-variants (LineTraceMultiByChannel, SweepMultiByChannel) return all overlaps up to and including the first block — useful for bullets through foliage.

See references/traces-and-queries.md for ByObjectType, async traces, UV coordinates from hits, and debug-draw helpers.

Simulating rigid bodies

// Enable physics simulation — requires simple collision on the mesh asset.
Mesh->SetSimulatePhysics(true);
Mesh->SetCollisionEnabled(ECollisionEnabled::QueryAndPhysics);
Mesh->SetCollisionProfileName(TEXT("PhysicsActor"));

// Mass: override per component or let BodyInstance compute from density/volume.
Mesh->SetMassOverrideInKg(NAME_None, 80.f);

// Forces and impulses (world space by default):
Mesh->AddImpulse(FVector(0.f, 0.f, 600000.f));          // instantaneous kg*cm/s
Mesh->AddForce(FVector(0.f, 0.f, 98000.f));              // continuous N per physics step
Mesh->AddImpulseAtLocation(Impulse, HitPoint);           // torque from off-center

AddForce accumulates over each physics substep; AddImpulse applies once in the current step. Damping is on the FBodyInstance (accessible via GetBodyInstance()): LinearDamping and AngularDamping (both at BodyInstance.h).

Simulating bodies must have simple collision (sphere/box/capsule/convex hull). Complex per-triangle collision cannot drive rigid-body simulation.

Ragdolls and physics assets

A ragdoll runs through a UPhysicsAsset (authored in the Physics Asset Editor), which defines per-bone bodies and constraints for a USkeletalMeshComponent:

GetMesh()->SetSimulatePhysics(true);                    // whole-body ragdoll
GetMesh()->SetCollisionProfileName(TEXT("Ragdoll"));
// For blended / partial ragdoll, use UPhysicalAnimationComponent.

Physics constraints

UPhysicsConstraintComponent wraps FConstraintInstance and joins two simulated bodies:

// ConstraintComp is a UPROPERTY(VisibleAnywhere) TObjectPtr<UPhysicsConstraintComponent>
ConstraintComp->SetConstrainedComponents(CompA, NAME_None, CompB, NAME_None);
ConstraintComp->SetLinearXLimit(LCM_Free, 0.f);         // allow X-axis translation
ConstraintComp->SetAngularSwing1Limit(ACM_Limited, 45.f); // 45° swing limit
ConstraintComp->ConstraintInstance.OnConstraintBroken.BindUObject(this,
    &AMyActor::OnConstraintBroken);

BreakConstraint() on the component tears the joint at runtime. FConstraintInstance also supports drives (motors): SetLinearPositionDrive, SetAngularDriveMode.

See references/physics-simulation-and-constraints.md for FBodyInstance details, damping, sub-stepping, and constraint drives.

Physical materials

UPhysicalMaterial (under Runtime/PhysicsCore) stores surface properties referenced by both UStaticMesh and UPrimitiveComponent. Key fields: Friction, StaticFriction, Restitution, Density, SurfaceType (EPhysicalSurface). Assign via the mesh asset or at runtime with GetBodyInstance()->PhysMaterialOverride.

Read SurfaceType from a hit to drive effects (footstep sounds, particle emitters): Hit.PhysMaterial.Get()->SurfaceType.

Collision complexity

Mode	Uses	Sim?	Notes
Simple	primitives / convex hull	yes	Required for rigid-body sim; cheap queries
Complex	per-triangle	no	Precise static queries (landscape, detailed meshes)

Set per-mesh in the Static/Skeletal Mesh asset collision settings, and optionally override on the component. Use FCollisionQueryParams::bTraceComplex = true to query complex.

Gotchas

• Overlap never fires — both components need ECR_Overlap response to each other's object type and bGenerateOverlapEvents = true on both. One side being ECR_Block still generates an overlap if the other is ECR_Overlap, but only one fires.
• Hit event never fires — enable Simulation Generates Hit Events (bNotifyRigidBodyCollision on FBodyInstance); physics sim must also be enabled.
• Trace misses everything — check ECollisionEnabled on targets; NoCollision or PhysicsOnly makes them invisible to traces.
• Simulating on complex-collision mesh — physics simulation requires simple collision; complex (per-triangle) collision cannot be simulated.
• Forgot AddIgnoredActor(this) — trace hits own actor's collision shapes.
• Handler not UFUNCTION() — AddDynamic silently fails; the method must be a UFUNCTION() with the exact delegate signature.
• Applying a preset then per-channel overrides — SetCollisionProfileName resets all responses; set per-channel responses after calling it.
• Force vs. Impulse units — AddForce is Newtons (applied each substep); AddImpulse is kg·cm/s (applied once). Scale accordingly.
• SetSimulatePhysics(true) silently does nothing — the mesh has no simple collision, or SetCollisionEnabled is NoCollision/QueryOnly.

Version notes

Chaos replaced PhysX as the default physics engine in UE5. The Chaos solver runs deterministic substeps controlled by UPhysicsSettings::AsyncFixed (Project Settings → Physics → Simulation → Use Async Scene). FBodyInstance is stable across UE5; solver internals differ from PhysX but the UPrimitiveComponent API is unchanged.

References & source material

Engine source (UE 5.8, under Engine/Source/):

• Runtime/Engine/Classes/Engine/EngineTypes.h — ECollisionChannel:1098, ECollisionResponse:1346, ECollisionEnabled:1805, FCollisionResponseContainer:1445.
• Runtime/Engine/Classes/Engine/HitResult.h — FHitResult:20.
• Runtime/Engine/Classes/Components/PrimitiveComponent.h — OnComponentHit:1457, OnComponentBeginOverlap:1468, SetGenerateOverlapEvents:418, SetSimulatePhysics:1661, AddImpulse:1692, AddForce:1759, SetCollisionEnabled:2026, SetCollisionProfileName:2036, SetCollisionObjectType:2047, SetCollisionResponseToAllChannels:2952, GetBodyInstance:2337, SetMassOverrideInKg:2857.
• Runtime/Engine/Classes/PhysicsEngine/BodyInstance.h — LinearDamping:629, AngularDamping:633.
• Runtime/Engine/Classes/PhysicsEngine/ConstraintInstance.h — FConstraintInstance:254, SetLinearLimits:358, InitConstraint:949.
• Runtime/Engine/Classes/PhysicsEngine/PhysicsConstraintComponent.h — UPhysicsConstraintComponent:24, BreakConstraint:142, SetLinearPositionDrive:151, SetAngularDriveMode:213.
• Runtime/Engine/Classes/Engine/World.h — LineTraceSingleByChannel:2161, LineTraceMultiByChannel:2210, SweepSingleByChannel:2286, SweepMultiByChannel:2325, OverlapMultiByChannel:2418.
• Runtime/Engine/Public/CollisionQueryParams.h — FCollisionQueryParams:42, AddIgnoredActor:243, bTraceComplex:51.
• Runtime/PhysicsCore/Public/PhysicalMaterials/PhysicalMaterial.h — UPhysicalMaterial:103, Friction:115, Restitution:131, Density:147, SurfaceType:181.

Official docs (UE 5.8):

• Collision Overview — https://dev.epicgames.com/documentation/unreal-engine/collision-in-unreal-engine---overview
• Traces Overview — https://dev.epicgames.com/documentation/unreal-engine/traces-in-unreal-engine---overview
• Physics Bodies — https://dev.epicgames.com/documentation/unreal-engine/physics-bodies-in-unreal-engine
• Physics Constraints — https://dev.epicgames.com/documentation/unreal-engine/physics-constraints-in-unreal-engine
• Physical Materials — https://dev.epicgames.com/documentation/unreal-engine/physical-materials-in-unreal-engine

Deep-dive references in this skill:

• references/collision-channels-and-profiles.md — channel taxonomy, custom channels, preset internals, FCollisionResponseContainer.
• references/traces-and-queries.md — full trace/sweep/ overlap API, ByObjectType vs ByChannel, async traces, debug helpers.
• references/physics-simulation-and-constraints.md — FBodyInstance properties, sub-stepping, constraint drives, ragdoll setup.

Related skills: actors-and-components (component types, overlap wiring), meshes-static-and-skeletal (collision geometry on assets).