Changed vehicle friction combine callback (#875)

Longitudinal and lateral friction are now calculated in the same call so that there can be dependencies between the two.

Docs/ReleaseNotes.md

@@ -14,7 +14,7 @@ For breaking API changes see [this document](https://github.com/jrouwe/JoltPhysi
 * Added BodyInterface::SetUseManifoldReduction which will clear the contact cache and ensure that you get consistent contact callbacks in case the body that you're changing already has contacts.
 * Created implementations of BroadPhaseLayerInterface, ObjectVsBroadPhaseLayerFilter and ObjectLayerPairFilter that use a bit table internally. These make it easier to define ObjectLayers and with which object layers they collide.
 * Added support for less than 1 collision test per simulation step for vehicle wheels. This behavior can be configured differently when the vehicle is active / inactive. This can be used for LODding vehicles.
-* Added wheel index and friction direction to VehicleConstraint::CombineFunction friction callback so you can have more differentiation between wheels.
+* Added wheel index to VehicleConstraint::CombineFunction friction callback and calculating longitudinal and lateral friction in the same call so you can have more differentiation between wheels.
 * Added ability to disable the lean steering limit for the motorcycle, turning this off makes the motorcycle more unstable, but gives you more control over the final steering angle.
 * Added function to query the bounding box of all bodies in the physics system, see PhysicsSystem::GetBounds.
 * Renamed SensorDetectsStatic to CollideKinematicVsNonDynamic and made it work for non-sensors. This means that kinematic bodies can now get collision callbacks when they collide with other static / kinematic objects.

Jolt/Physics/Vehicle/TrackedVehicleController.cpp

@@ -72,8 +72,9 @@ void WheelTV::Update(uint inWheelIndex, float inDeltaTime, const VehicleConstrai
 		// Friction at the point of this wheel between track and floor
 		const WheelSettingsTV *settings = GetSettings();
 		VehicleConstraint::CombineFunction combine_friction = inConstraint.GetCombineFriction();
-		mCombinedLongitudinalFriction = combine_friction(inWheelIndex, VehicleConstraint::ETireFrictionDirection::Longitudinal, settings->mLongitudinalFriction, *mContactBody, mContactSubShapeID);
-		mCombinedLateralFriction = combine_friction(inWheelIndex, VehicleConstraint::ETireFrictionDirection::Lateral, settings->mLateralFriction, *mContactBody, mContactSubShapeID);
+		mCombinedLongitudinalFriction = settings->mLongitudinalFriction;
+		mCombinedLateralFriction = settings->mLateralFriction;
+		combine_friction(inWheelIndex, mCombinedLongitudinalFriction, mCombinedLateralFriction, *mContactBody, mContactSubShapeID);
 	}
 	else
 	{

Jolt/Physics/Vehicle/VehicleConstraint.h

@@ -78,15 +78,9 @@ public:
 	/// Set the interface that tests collision between wheel and ground
 	void						SetVehicleCollisionTester(const VehicleCollisionTester *inTester) { mVehicleCollisionTester = inTester; }
 
-	/// Tire friction direction
-	enum class ETireFrictionDirection : uint8
-	{
-		Longitudinal,			///< Longitudinal friction direction
-		Lateral,				///< Lateral friction direction
-	};
-
 	/// Callback function to combine the friction of a tire with the friction of the body it is colliding with.
-	using CombineFunction = function<float(uint inWheelIndex, ETireFrictionDirection inTireFrictionDirection, float inTireFriction, const Body &inBody2, const SubShapeID &inSubShapeID2)>;
+	/// On input ioLongitudinalFriction and ioLateralFriction contain the friction of the tire, on output they should contain the combined friction with inBody2.
+	using CombineFunction = function<void(uint inWheelIndex, float &ioLongitudinalFriction, float &ioLateralFriction, const Body &inBody2, const SubShapeID &inSubShapeID2)>;
 
 	/// Set the function that combines the friction of two bodies and returns it
 	/// Default method is the geometric mean: sqrt(friction1 * friction2).
@@ -224,7 +218,13 @@ private:
 
 	// Interfaces
 	RefConst<VehicleCollisionTester> mVehicleCollisionTester;				///< Class that performs testing of collision for the wheels
-	CombineFunction				mCombineFriction = [](uint, ETireFrictionDirection, float inTireFriction, const Body &inBody2, const SubShapeID &) { return sqrt(inTireFriction * inBody2.GetFriction()); };
+	CombineFunction				mCombineFriction = [](uint, float &ioLongitudinalFriction, float &ioLateralFriction, const Body &inBody2, const SubShapeID &)
+	{
+		float body_friction = inBody2.GetFriction();
+
+		ioLongitudinalFriction = sqrt(ioLongitudinalFriction * body_friction);
+		ioLateralFriction = sqrt(ioLateralFriction * body_friction);
+	};
 
 	// Callbacks
 	StepCallback				mPreStepCallback;

Jolt/Physics/Vehicle/WheeledVehicleController.cpp

@@ -122,8 +122,9 @@ void WheelWV::Update(uint inWheelIndex, float inDeltaTime, const VehicleConstrai
 
 		// Tire friction
 		VehicleConstraint::CombineFunction combine_friction = inConstraint.GetCombineFriction();
-		mCombinedLongitudinalFriction = combine_friction(inWheelIndex, VehicleConstraint::ETireFrictionDirection::Longitudinal, longitudinal_slip_friction, *mContactBody, mContactSubShapeID);
-		mCombinedLateralFriction = combine_friction(inWheelIndex, VehicleConstraint::ETireFrictionDirection::Lateral, lateral_slip_friction, *mContactBody, mContactSubShapeID);
+		mCombinedLongitudinalFriction = longitudinal_slip_friction;
+		mCombinedLateralFriction = lateral_slip_friction;
+		combine_friction(inWheelIndex, mCombinedLongitudinalFriction, mCombinedLateralFriction, *mContactBody, mContactSubShapeID);
 	}
 	else
 	{