rwengine: Set the linear velocity to a zero vector when stopped

Vehicles have a tendency to move on their own when they are stopped, due
to their interactions with other surfaces that result in a non-zero
velocity.

Since we have a threshold with the isStopped() method, fix this situation
by setting the linear velocity and wheels rotation (that otherwise also
rotate on their own) to zero vectors.

Signed-off-by: Paul Kocialkowski <contact@paulk.fr>

rwengine/src/objects/VehicleObject.cpp

@@ -161,6 +161,8 @@ VehicleObject::VehicleObject(GameWorld* engine, const glm::vec3& pos,
             halfFriction +
             halfFriction * (front ? info->handling.tractionBias
                                   : 1.f - info->handling.tractionBias);
+
+        wheelsRotation.push_back(0.f);
     }
 
     setModel(getVehicle()->getModel());
@@ -385,8 +387,24 @@ void VehicleObject::tickPhysics(float dt) {
             brakeF = 2.f * std::min(1.f + kM, 4.f);
         }
 
+        if (isStopped() && std::abs(throttle) < 0.1f) {
+            btVector3 v = collision->getBulletBody()->getLinearVelocity();
+            v.setX(0.f);
+            v.setY(0.f);
+
+            collision->getBulletBody()->setLinearVelocity(v);
+
+            for (int w = 0; w < physVehicle->getNumWheels(); ++w) {
+                btWheelInfo& wi = physVehicle->getWheelInfo(w);
+                wi.m_rotation = wheelsRotation[w];
+            }
+        }
+
         for (int w = 0; w < physVehicle->getNumWheels(); ++w) {
             btWheelInfo& wi = physVehicle->getWheelInfo(w);
+
+            wheelsRotation[w] = wi.m_rotation;
+
             if (info->handling.driveType == VehicleHandlingInfo::All ||
                 (info->handling.driveType == VehicleHandlingInfo::Forward &&
                  wi.m_bIsFrontWheel) ||

rwengine/src/objects/VehicleObject.hpp

@@ -7,6 +7,7 @@
 #include <memory>
 #include <string>
 
+#include <LinearMath/btScalar.h>
 #include <glm/glm.hpp>
 #include <glm/gtc/quaternion.hpp>
 
@@ -36,6 +37,7 @@ private:
     float throttle{0.f};
     float brake{0.f};
     bool handbrake = true;
+    std::vector<btScalar> wheelsRotation;
 
     Atomic* chassishigh_ = nullptr;
     Atomic* chassislow_ = nullptr;