Explain Camera bounds in StrategyGame Demo

Development Programming & Scripting C++
1

The following code is from the StrategyGame demo:


void UStrategyCameraComponent::ClampCameraLocation( const APlayerController* InPlayerController, FVector& OutCameraLocation )
{	
	if (bShouldClampCamera)
	{
		UpdateCameraBounds(InPlayerController);
		if (CameraMovementBounds.GetSize() != FVector::ZeroVector)
		{
			OutCameraLocation = CameraMovementBounds.GetClosestPointTo(OutCameraLocation);
		}
	}
}

void UStrategyCameraComponent::UpdateCameraBounds( const APlayerController* InPlayerController )
{
	ULocalPlayer* const LocalPlayer =  Cast<ULocalPlayer>(InPlayerController->Player);
	if (LocalPlayer == NULL || LocalPlayer->ViewportClient == NULL)
	{
		return;
	}

	FVector2D CurrentViewportSize;
	LocalPlayer->ViewportClient->GetViewportSize(CurrentViewportSize);

	// calc frustum edge direction, from bottom left corner
	if (CameraMovementBounds.GetSize() == FVector::ZeroVector || CurrentViewportSize != CameraMovementViewportSize)
	{
		// calc frustum edge direction, from bottom left corner
		const FVector FrustumRay2DDir = FVector(1,1,0).SafeNormal();
		const FVector FrustumRay2DRight = FVector::CrossProduct(FrustumRay2DDir, FVector::UpVector);
		const FQuat RotQuat(FrustumRay2DRight, FMath::DegreesToRadians(90.0f - InPlayerController->PlayerCameraManager->GetFOVAngle() * 0.5f));
		const FVector FrustumRayDir = RotQuat.RotateVector(FrustumRay2DDir);

		// collect 3 world bounds' points and matching frustum rays (bottom left, top left, bottom right)
		AStrategyGameState const* const MyGameState = GetWorld()->GetGameState<AStrategyGameState>();
		if (MyGameState)
		{
			FBox const& WorldBounds = MyGameState->WorldBounds;

			if (WorldBounds.GetSize() != FVector::ZeroVector)
			{
				const FVector WorldBoundPoints] = {
					FVector(WorldBounds.Min.X, WorldBounds.Min.Y, WorldBounds.Max.Z),
					FVector(WorldBounds.Min.X, WorldBounds.Max.Y, WorldBounds.Max.Z),
					FVector(WorldBounds.Max.X, WorldBounds.Min.Y, WorldBounds.Max.Z)
				};
				const FVector FrustumRays] = {
					FVector( FrustumRayDir.X,  FrustumRayDir.Y, FrustumRayDir.Z),
					FVector( FrustumRayDir.X, -FrustumRayDir.Y, FrustumRayDir.Z),
					FVector(-FrustumRayDir.X,  FrustumRayDir.Y, FrustumRayDir.Z)
				};

				// get camera plane for intersections
				const FPlane CameraPlane = FPlane(InPlayerController->GetFocalLocation(), FVector::UpVector);

				// get matching points on camera plane
				const FVector CameraPlanePoints[3] = {
					FStrategyHelpers::IntersectRayWithPlane(WorldBoundPoints[0], FrustumRays[0], CameraPlane)*MiniMapBoundsLimit,
					FStrategyHelpers::IntersectRayWithPlane(WorldBoundPoints[1], FrustumRays[1], CameraPlane)*MiniMapBoundsLimit,
					FStrategyHelpers::IntersectRayWithPlane(WorldBoundPoints[2], FrustumRays[2], CameraPlane)*MiniMapBoundsLimit
				};

				// create new bounds
				CameraMovementBounds = FBox(CameraPlanePoints, 3);
				CameraMovementViewportSize = CurrentViewportSize;
			}
		}
	}
}

Can anyone explain the purpose of WorldBoundPoints, FrustumRays and CameraPlanePoints? or even just the general plan here in plain terms?

I was able to put together a general rts style camera based on other code people have generously posted on the forum. But I’d still like to know whats going on here.

2

It’s a simple containment box built from what the MiniMapCapture camera can see. It basically states that the main camera that the user moves around cannot leave the area that can be seen via the mini-map. If your game does not make use of a mini-map, you may want to find another way to set the bounds, or simply not worry about them. See the code in the MiniMapCapture file for more information:



void AStrategyMiniMapCapture::UpdateWorldBounds()
{
	AStrategyGameState* const MyGameState = GetWorld()->GetGameState<AStrategyGameState>();
	if( MyGameState != NULL )	
	{
		TArray<FVector> Points;
		FVector const CamLocation = RootComponent->GetComponentLocation();
		float DistanceFromGround = CamLocation.Z - GroundLevel;
		float Alpha = FMath::DegreesToRadians(GetCaptureComponent2D()->FOVAngle / 2);
		float MaxVisibleDistance = (DistanceFromGround / FMath::Cos(Alpha)) * FMath::Sin(Alpha);

		Points.Add(FVector(CamLocation.X+MaxVisibleDistance,CamLocation.Y+MaxVisibleDistance,GroundLevel));
		Points.Add(FVector(CamLocation.X-MaxVisibleDistance,CamLocation.Y-MaxVisibleDistance,GroundLevel));

		MyGameState->WorldBounds = FBox(Points);
		GetCaptureComponent2D()->UpdateContent();
	}
}