Ue Physics Collision

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: