Why are HitItem and FaceIndex equal to -1?

Development Programming & Scripting
1

I am using C++ to fire a ray downward from a Pawn to hit a cube underneath it. Both HitResult.HitItem and HitResult.FaceIndex return -1.

Can someone explain why these values are -1? I think there is something fundamental I do not understand about LineTraceSingleByChannel or about the cube properties and would welcome expert input. I have provided code and a screenshot below which shows the values in the log.

bool AShip::OrientOnSurface(float DeltaSeconds)
{
	const int TraceStartFactor = 1;
	const int TraceLength = 1000;
	const float HoverHeight = 150.0f;

	FQuat ActorQuat = GetActorQuat();

	FVector ActorDownVector = ActorQuat.RotateVector(FVector(0, 0, -1));
	FVector ActorUpVector = ActorQuat.RotateVector(FVector(0, 0, 1));
	FVector ActorRightVector = ActorQuat.RotateVector(FVector(0, 1, 0));
	FVector ActorForwardVector = ActorQuat.RotateVector(FVector(1, 0, 0));

	FVector ActorLocation = GetActorLocation();
	FVector Start = ActorLocation + ActorDownVector * TraceStartFactor;
	FVector End = ActorLocation + ActorDownVector * TraceLength;

	FCollisionQueryParams QueryParams;
	QueryParams.AddIgnoredActor(this);
	QueryParams.bReturnFaceIndex = true;

	FCollisionResponseParams ResponseParams;

	FHitResult HitResult;

	GetWorld()->LineTraceSingleByChannel
	(
		HitResult,
		Start,
		End,
		ECC_Visibility,
		QueryParams,
		ResponseParams
	);

	// If ray cast downward hits a surface
	if (HitResult.bBlockingHit)
	{

		FVector ImpactNormal = HitResult.ImpactNormal;
		FVector ImpactPoint = HitResult.ImpactPoint;

		// Get component that was hit
		UPrimitiveComponent* HitComponent = HitResult.GetComponent();
		int32 HitItem = HitResult.Item;
		AActor* HitActor = HitResult.GetActor();

		// See if a static mesh and/or a spline mesh component were hit
		UStaticMeshComponent* StaticMeshComp = Cast<UStaticMeshComponent>(HitResult.GetComponent());
		USplineMeshComponent* SplineMeshComp = Cast<USplineMeshComponent>(HitResult.GetComponent());

		// If hit a spline mesh component then make sure that it's also a static mesh component
		if (!StaticMeshComp)
			return false;

		// Get static mesh
		UStaticMesh* StaticMesh = StaticMeshComp->GetStaticMesh();

		// Return if the static mesh isn't set (should not happen)
		if (!StaticMesh)
			return false;

		// Flag neede for cooked builds or will crash when accessing mesh data
		if (!StaticMesh->bAllowCPUAccess)
			return false;

		// Return if RenderData is invalid
		if (!StaticMesh->GetRenderData())
			return false;

		// No valid mesh data on lod 0 (shouldn't happen)
		if (!StaticMesh->GetRenderData()->LODResources.IsValidIndex(0))
			return false;

		int FaceIndex = HitResult.FaceIndex;

		FTransform ComponentTransform = StaticMeshComp->GetComponentTransform();

		FStaticMeshVertexBuffers* VertexBuffers = &StaticMesh->GetRenderData()->LODResources[0].VertexBuffers;
		FStaticMeshVertexBuffer* StaticMeshVertexBuffer = &VertexBuffers->StaticMeshVertexBuffer;
		FPositionVertexBuffer* PositionVertexBuffer = &VertexBuffers->PositionVertexBuffer;

		FIndexArrayView IndexBuffer = StaticMesh->GetRenderData()->LODResources[0].IndexBuffer.GetArrayView();

		// Why do both values equal -1?
		UE_LOG(LogClass, Log, TEXT("HitItem: %d"), HitItem);
		UE_LOG(LogClass, Log, TEXT("IndexBuffer: %d"), FaceIndex);

	}

	return true;
}

Capture.PNG
Capture.PNG1920×1080 92 KB

2

At the moment you are doing a simple trace collision check. In your FCollisionQueryParams add bTraceComplex = true;

	FCollisionQueryParams QueryParams;
	QueryParams.AddIgnoredActor(this);
	QueryParams.bReturnFaceIndex = true;
	QueryParams.bTraceComplex = true;

Also make you the object that you are tracing against in it’s mesh component you set in begin play if your meshcomponent is called Mesh

if(Mesh->GetStaticMesh() != nullptr)
	Mesh->GetStaticMesh()->bAllowCPUAccess = true;

You should now get the correct face index

3

Thanks @3dRaven that solved the problem.

Also, I had set Allow CPUAccess in the static mesh details panel but I prefer to set it programmatically so thanks also for the tip about bAllowCPUAccess.