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skills/quodsoler/unreal-engine-skills/ue-physics-collision

ue-physics-collision

SKILL.md

UE Physics & Collision

You are an expert in Unreal Engine's physics and collision systems, including collision channels, trace queries, collision events, physics bodies, and the Chaos physics engine.

Step 1: Read Project Context

Read .agents/ue-project-context.md to confirm:

  • UE version (Chaos is the default physics backend from UE 5.0; PhysX was deprecated)
  • Which modules need "PhysicsCore" and "Engine" in their Build.cs
  • Whether the project uses skeletal meshes with physics assets, or primarily static mesh collision
  • Dedicated server targets (affects whether physics simulation should run server-side)

Step 2: Identify the Need

Ask which area applies if not stated:

  1. Collision setup — channels, profiles, responses on components
  2. Trace queries — line traces, sweeps, overlap queries for gameplay logic
  3. Collision events — OnComponentHit, OnBeginOverlap, OnEndOverlap delegates
  4. Physics simulation — rigid body sim, forces, impulses, damping, constraints
  5. Physical materials — friction, restitution, surface type detection

Collision Channels & Profiles

ECollisionChannel — built-in channels

ECC_WorldStatic, ECC_WorldDynamic, ECC_Pawn, ECC_PhysicsBody,
ECC_Vehicle, ECC_Destructible               // object channels (what an object IS)
ECC_Visibility, ECC_Camera                  // trace channels (used for queries)
// Custom: ECC_GameTraceChannel1..ECC_GameTraceChannel18

Responses: ECR_Ignore / ECR_Overlap (events, no block) / ECR_Block (physical block + events).

Built-in profiles: BlockAll, BlockAllDynamic, OverlapAll, OverlapAllDynamic, Pawn, PhysicsActor, NoCollision.

Setting Collision in C++

MyMesh->SetCollisionProfileName(TEXT("BlockAll"));        // preferred — sets all at once
MyMesh->SetCollisionEnabled(ECollisionEnabled::QueryAndPhysics);
// ECollisionEnabled: NoCollision | QueryOnly | PhysicsOnly | QueryAndPhysics
MyMesh->SetCollisionObjectType(ECC_PhysicsBody);
MyMesh->SetCollisionResponseToAllChannels(ECR_Block);
MyMesh->SetCollisionResponseToChannel(ECC_Pawn, ECR_Overlap);
MyMesh->SetCollisionResponseToChannel(ECC_Camera, ECR_Ignore);

Object Type Channels vs Trace Channels

Object type channels describe what an actor IS (Pawn, WorldDynamic, Vehicle). Every component has exactly one object type. Trace channels are used for queries — they define what a trace is LOOKING FOR (Visibility, Camera, Weapon). This distinction determines which query function to use: ByObjectType matches the target's object type channel; ByChannel uses the querier's trace channel and checks responses. Most gameplay traces use trace channels (ECC_Visibility, custom Weapon); overlap queries for "find all pawns" use object type (ECC_Pawn).

Custom Channels — DefaultEngine.ini

[/Script/Engine.CollisionProfile]
+DefaultChannelResponses=(Channel=ECC_GameTraceChannel1,DefaultResponse=ECR_Block,bTraceType=True,bStaticObject=False,Name="Weapon")
+DefaultChannelResponses=(Channel=ECC_GameTraceChannel2,DefaultResponse=ECR_Block,bTraceType=False,bStaticObject=False,Name="Interactable")
+Profiles=(Name="Interactable",CollisionEnabled=QueryAndPhysics,ObjectTypeName="Interactable",CustomResponses=((Channel="Weapon",Response=ECR_Ignore),(Channel="Visibility",Response=ECR_Block)))

bTraceType=True = trace channel; bTraceType=False = object type channel. They use separate query functions.

See references/collision-channel-setup.md for full profile examples.

Trace Queries

FCollisionQueryParams

FCollisionQueryParams Params;
Params.TraceTag                = TEXT("WeaponTrace"); // for profiling/debug
Params.bTraceComplex           = false;  // false=simple hull (fast); true=per-poly (expensive)
Params.bReturnPhysicalMaterial = true;   // populates Hit.PhysMaterial
Params.bReturnFaceIndex        = false;  // expensive, only when needed
Params.AddIgnoredActor(this);
Params.AddIgnoredComponent(MyComp);

World-Level Trace Functions (C++) — from WorldCollision.h via UWorld

FHitResult Hit;
// By trace channel
GetWorld()->LineTraceSingleByChannel(Hit, Start, End, ECC_Visibility, Params);
GetWorld()->LineTraceMultiByChannel(Hits, Start, End, ECC_Visibility, Params);
// By object type
FCollisionObjectQueryParams ObjParams(ECC_PhysicsBody);
ObjParams.AddObjectTypesToQuery(ECC_WorldDynamic);
GetWorld()->LineTraceSingleByObjectType(Hit, Start, End, ObjParams, Params);
// By profile
GetWorld()->LineTraceSingleByProfile(Hit, Start, End, TEXT("BlockAll"), Params);

Sweep Queries — FCollisionShape (from CollisionShape.h)

FCollisionShape Sphere  = FCollisionShape::MakeSphere(30.f);
FCollisionShape Box     = FCollisionShape::MakeBox(FVector(50.f, 50.f, 50.f));
FCollisionShape Capsule = FCollisionShape::MakeCapsule(34.f, 88.f); // radius, half-height

GetWorld()->SweepSingleByChannel(Hit, Start, End, FQuat::Identity, ECC_Pawn, Sphere, Params);
GetWorld()->SweepMultiByChannel(Hits, Start, End, FQuat::Identity, ECC_Pawn, Sphere, Params);
GetWorld()->SweepSingleByObjectType(Hit, Start, End, FQuat::Identity, ObjParams, Sphere, Params);
GetWorld()->SweepSingleByProfile(Hit, Start, End, FQuat::Identity, TEXT("Pawn"), Sphere, Params);

Overlap Queries

TArray<FOverlapResult> Overlaps;
GetWorld()->OverlapMultiByObjectType(Overlaps, Center, FQuat::Identity,
    FCollisionObjectQueryParams(ECC_Pawn), FCollisionShape::MakeSphere(500.f), Params);
for (const FOverlapResult& R : Overlaps) { AActor* A = R.GetActor(); }

// By trace channel (uses channel responses, not object type matching):
GetWorld()->OverlapMultiByChannel(
    Overlaps, Center, FQuat::Identity, ECC_Pawn,
    FCollisionShape::MakeSphere(Radius), QueryParams);

FHitResult — Key Fields

Hit.bBlockingHit;          // true if blocking
Hit.ImpactPoint;           // world space contact point
Hit.ImpactNormal;          // surface normal
Hit.Distance;              // from Start to impact
Hit.BoneName;              // skeletal mesh bone
Hit.GetActor();
Hit.GetComponent();
// Physical material (requires bReturnPhysicalMaterial=true):
if (UPhysicalMaterial* M = Hit.PhysMaterial.Get())
    EPhysicalSurface S = UPhysicalMaterial::DetermineSurfaceType(M);

Blueprint-Layer Traces (UKismetSystemLibrary)

#include "Kismet/KismetSystemLibrary.h"
TArray<AActor*> Ignore = { this };
// LineTrace with debug draw (ETraceTypeQuery maps to ECollisionChannel)
UKismetSystemLibrary::LineTraceSingle(this, Start, End,
    ETraceTypeQuery::TraceTypeQuery1, false, Ignore, EDrawDebugTrace::ForDuration, Hit, true);
// SphereTrace
UKismetSystemLibrary::SphereTraceSingle(this, Start, End, 50.f,
    ETraceTypeQuery::TraceTypeQuery1, false, Ignore, EDrawDebugTrace::ForOneFrame, Hit, true);
// By profile
UKismetSystemLibrary::LineTraceSingleByProfile(this, Start, End,
    TEXT("BlockAll"), false, Ignore, EDrawDebugTrace::None, Hit, true);

Async Traces

FTraceHandle Handle = GetWorld()->AsyncLineTraceByChannel(
    EAsyncTraceType::Single, Start, End, ECC_Visibility, Params);
// Read next frame:
FTraceDatum Datum;
if (GetWorld()->QueryTraceData(Handle, Datum) && Datum.OutHits.Num() > 0)
    FHitResult& Hit = Datum.OutHits[0];

Debug Visualization

#if ENABLE_DRAW_DEBUG
DrawDebugLine(GetWorld(), Start, End, FColor::Red, false, 2.f);
DrawDebugSphere(GetWorld(), HitResult.ImpactPoint, 10.f, 12, FColor::Green, false, 2.f);
#endif

DrawDebugLine / DrawDebugSphere are from DrawDebugHelpers.h. Wrap in ENABLE_DRAW_DEBUG so they compile out in shipping builds. The bool param is bPersistentLines; the float param is LifeTime in seconds.

See references/trace-patterns.md for full gameplay patterns (hitscan, melee sweep, AoE, ground detection, async sensors).

Collision Events

Delegate declarations from PrimitiveComponent.h:

  • OnComponentHit — (HitComp, OtherActor, OtherComp, NormalImpulse, FHitResult) — physics collision
  • OnComponentBeginOverlap — (OverlappedComp, OtherActor, OtherComp, OtherBodyIndex, bFromSweep, SweepResult)
  • OnComponentEndOverlap — (OverlappedComp, OtherActor, OtherComp, OtherBodyIndex)
// Hit events (physics collision)
MyMesh->SetNotifyRigidBodyCollision(true);  // "Simulation Generates Hit Events"
MyMesh->OnComponentHit.AddDynamic(this, &AMyActor::OnHit);

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

// Overlap events — SetGenerateOverlapEvents(true) REQUIRED on BOTH components
MyMesh->SetGenerateOverlapEvents(true);
MyMesh->OnComponentBeginOverlap.AddDynamic(this, &AMyActor::OnBeginOverlap);
MyMesh->OnComponentEndOverlap.AddDynamic(this, &AMyActor::OnEndOverlap);

UFUNCTION()
void AMyActor::OnBeginOverlap(UPrimitiveComponent* OverlappedComp, AActor* OtherActor,
    UPrimitiveComponent* OtherComp, int32 OtherBodyIndex, bool bFromSweep, const FHitResult& SweepResult) {}

UFUNCTION()
void AMyActor::OnEndOverlap(UPrimitiveComponent* OverlappedComp, AActor* OtherActor,
    UPrimitiveComponent* OtherComp, int32 OtherBodyIndex) {}

Requirements: Hit: QueryAndPhysics, ECR_Block on both, SetNotifyRigidBodyCollision(true). Overlap: ECR_Overlap on both, SetGenerateOverlapEvents(true) on both.

Physics Bodies

// Enable simulation (PrimitiveComponent.h / BodyInstanceCore.h)
MyMesh->SetSimulatePhysics(true);
MyMesh->SetEnableGravity(true);
MyMesh->SetMassOverrideInKg(NAME_None, 50.f, true); // 50 kg
MyMesh->SetLinearDamping(0.1f);
MyMesh->SetAngularDamping(0.05f);
float Mass = MyMesh->GetMass();

// Forces and impulses
MyMesh->AddImpulse(FVector(0,0,1000), NAME_None, false);       // bVelChange=true ignores mass
MyMesh->AddImpulseAtLocation(FVector(500,0,0), HitPoint);      // adds torque
MyMesh->AddForce(FVector(0,0,9800), NAME_None, false);          // continuous (per tick)
MyMesh->AddRadialImpulse(Center, 500.f, 2000.f, RIF_Linear, false);
MyMesh->SetPhysicsLinearVelocity(FVector(0,0,300));             // use sparingly

FBodyInstanceCore key flags (set via UPROPERTY/editor): bSimulatePhysics, bOverrideMass, bEnableGravity, bAutoWeld, bStartAwake, bGenerateWakeEvents, bUpdateKinematicFromSimulation.

Collision complexity (BodySetupEnums.h): CTF_UseDefault, CTF_UseSimpleAndComplex, CTF_UseSimpleAsComplex, CTF_UseComplexAsSimple (expensive, static only for physics).

Physics Constraints

UPhysicsConstraintComponent* C = NewObject<UPhysicsConstraintComponent>(this);
C->SetupAttachment(RootComponent);
C->SetConstrainedComponents(MeshA, NAME_None, MeshB, NAME_None);
C->SetAngularSwing1Limit(EAngularConstraintMotion::ACM_Limited, 45.f);
C->SetAngularTwistLimit(EAngularConstraintMotion::ACM_Free, 0.f);
C->SetLinearXLimit(ELinearConstraintMotion::LCM_Locked, 0.f);
C->RegisterComponent();

Named constraint presets (set via ConstraintProfile or editor Preset dropdown):

Preset Angular Limits Linear Limits
Fixed All locked All locked
Hinge One axis free All locked
Prismatic All locked One axis free
Ball-and-Socket All free All locked

Physical Materials (UPhysicalMaterial)

From PhysicalMaterials/PhysicalMaterial.h:

float Friction;       // kinetic (0 = frictionless)
float StaticFriction; // before sliding starts
float Restitution;    // 0 (no bounce) to 1 (elastic)
float Density;        // g/cm^3 — used to compute mass from shape volume
EPhysicalSurface SurfaceType; // SurfaceType_Default, SurfaceType1..SurfaceType62
// FrictionCombineMode / RestitutionCombineMode: Average, Min, Multiply, Max

// Runtime override
MyMesh->SetPhysMaterialOverride(MyPhysMaterial);

// Detect surface from trace (requires bReturnPhysicalMaterial=true)
if (UPhysicalMaterial* M = Hit.PhysMaterial.Get())
{
    EPhysicalSurface S = UPhysicalMaterial::DetermineSurfaceType(M);
    switch (S) { case SurfaceType1: /* Metal */ break; }
}

Chaos Physics (UE5)

UE5 uses Chaos by default (PhysX removed). Key architecture:

  • FChaosScene (ChaosScene.h) owns the solver: StartFrame(), SetUpForFrame(), EndFrame().
  • Physics runs on a dedicated thread; game thread reads results at sync points.
  • Substepping: enabled per Project Settings > Physics (MaxSubsteps, MaxSubstepDeltaTime). Enable when small/fast objects tunnel through thin geometry — substepping divides the physics tick into smaller increments so collisions are not missed.
  • Async physics: runs simulation on a separate thread with one-frame latency. Enable via UPhysicsSettings::bTickPhysicsAsync. Use UAsyncPhysicsInputComponent on components that need physics-thread input callbacks.

Key UPhysicsSettingsCore fields (PhysicsSettingsCore.h):

float DefaultGravityZ;         // -980 cm/s^2 default
float BounceThresholdVelocity; // min velocity to bounce
float MaxAngularVelocity;      // rad/s cap
float MaxDepenetrationVelocity;
float ContactOffsetMultiplier; // contact shell size
FChaosSolverConfiguration SolverOptions;

EPhysicalSurface: 62 configurable slots (SurfaceType1..SurfaceType62) mapped in Project Settings > Physics > Physical Surface.

Geometry Collections (Chaos Destructibles): use UGeometryCollectionComponent. Fracture thresholds driven by FPhysicalMaterialStrength (TensileStrength, CompressionStrength, ShearStrength) and FPhysicalMaterialDamageModifier (DamageThresholdMultiplier) on UPhysicalMaterial.

Cloth Simulation

// Cloth uses UClothingAssetBase attached to USkeletalMeshComponent
// Enable in Mesh asset: Clothing → Add Clothing Data
// C++ access:
USkeletalMeshComponent* Mesh = GetMesh();
if (UClothingSimulationInteractor* Cloth = Mesh->GetClothingSimulationInteractor())
{
    Cloth->PhysicsAssetUpdated();             // re-sync after physics asset change
    Cloth->SetAnimDriveSpringStiffness(10.f); // blend anim ↔ cloth
}

Field System

Field System actors apply forces, strain, and anchors to Chaos destruction and cloth:

// Place AFieldSystemActor in level, add field nodes:
// URadialFalloff — distance-based falloff
// URadialVector — directional force from center
// UUniformVector — constant directional force
// UBoxFalloff — box-shaped field region

// Trigger destruction at runtime:
AFieldSystemActor* FieldActor = GetWorld()->SpawnActor<AFieldSystemActor>();
URadialFalloff* Falloff = NewObject<URadialFalloff>(FieldActor);
Falloff->SetRadialFalloff(1000000.f, 0.8f, 1.f, 0.f, 500.f, FVector::ZeroVector, EFieldFalloffType::Field_Falloff_Linear);
UFieldSystemMetaDataFilter* Meta = NewObject<UFieldSystemMetaDataFilter>(FieldActor);
Meta->SetMetaDataFilterType(EFieldFilterType::Field_Filter_All, EFieldObjectType::Field_Object_All, EFieldPositionType::Field_Position_CenterOfMass);
FieldActor->GetFieldSystemComponent()->ApplyPhysicsField(true, EFieldPhysicsType::Field_ExternalClusterStrain, Meta, Falloff);

Common Mistakes & Anti-Patterns

Wrong collision responses: Overlap events require ECR_Overlap AND bGenerateOverlapEvents=true on BOTH components.

Traces every Tick on many actors: Use async traces or throttle to 5–10 Hz with a timer.

QueryOnly vs PhysicsOnly confusion: QueryOnly = traces only, no physics forces. PhysicsOnly = forces only, traces skip it. Use QueryAndPhysics for both.

Complex collision in traces: bTraceComplex=true is 4–10x more expensive. Default false; only enable for precise terrain interaction.

Missing SetNotifyRigidBodyCollision: OnComponentHit will never fire without it — this flag ("Simulation Generates Hit Events") is separate from collision response.

Sweep vs overlap: Sweep = shape moving along a path (movement, projectile). Overlap = shape at fixed point (AoE, proximity). Don't substitute one for the other.

Physics on dedicated servers: Disable skeletal ragdolls with bSimulateSkeletalMeshOnDedicatedServer=false unless server accuracy is required.

Multiplayer & Replicated Actor Collision

In multiplayer, physics simulation runs on the server. Collision events (OnComponentHit, OnBeginOverlap) fire on the server only by default — clients do not receive these events unless you replicate them explicitly via RPCs. Clients see physics-simulated actor positions via FRepMovement (the replicated transform + velocity struct behind bReplicateMovement). Setting bReplicateMovement = true on an actor syncs its transform and linear/angular velocity; the underlying physics state itself is not replicated. For client-predicted physics (e.g., projectiles), simulate locally on the client and reconcile with server authority on correction. Cosmetic-only physics — ragdolls, debris, environmental props — can simulate on clients independently without server involvement, since visual fidelity matters more than authority.

Required Module Dependencies

// Build.cs
PublicDependencyModuleNames.AddRange(new string[] {
    "Core", "CoreUObject",
    "Engine",      // UPrimitiveComponent, FHitResult, UWorld trace API
    "PhysicsCore", // UPhysicalMaterial, FCollisionShape, FBodyInstanceCore
});

Related Skills

  • ue-actor-component-architecture — UPrimitiveComponent lifecycle, attachment, registration
  • ue-ai-navigation — trace-based sensing and navmesh overlap queries
  • ue-gameplay-abilities — targeting systems built on trace and overlap queries
  • ue-cpp-foundations — delegate binding syntax and UFUNCTION requirements
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