Movement solver: add usage of 'on slope' flag to improve handling of steep slopes

Previously we were handling 'on slope' synonymously with 'in air' which caused some odd effects.

Practical changes:
 - Sliding down a slope no longer applies fall damage.
 - Fixed a climbing exploit that would allow climbing steep slopes with repeated use of the Jump function.

apps/openmw/mwphysics/actor.cpp

@@ -18,7 +18,7 @@ namespace MWPhysics
 
 Actor::Actor(const MWWorld::Ptr& ptr, osg::ref_ptr<const Resource::BulletShape> shape, btCollisionWorld* world)
   : mCanWaterWalk(false), mWalkingOnWater(false)
-  , mCollisionObject(0), mForce(0.f, 0.f, 0.f), mOnGround(false)
+  , mCollisionObject(0), mForce(0.f, 0.f, 0.f), mOnGround(false), mOnSlope(false)
   , mInternalCollisionMode(true)
   , mExternalCollisionMode(true)
   , mCollisionWorld(world)
@@ -180,6 +180,11 @@ void Actor::setOnGround(bool grounded)
     mOnGround = grounded;
 }
 
+void Actor::setOnSlope(bool slope)
+{
+    mOnSlope = slope;
+}
+
 bool Actor::isWalkingOnWater() const
 {
     return mWalkingOnWater;

apps/openmw/mwphysics/actor.hpp

@@ -124,6 +124,13 @@ namespace MWPhysics
             return mInternalCollisionMode && mOnGround;
         }
 
+        void setOnSlope(bool slope);
+
+        bool getOnSlope() const
+        {
+            return mInternalCollisionMode && mOnSlope;
+        }
+
         btCollisionObject* getCollisionObject() const
         {
             return mCollisionObject.get();
@@ -160,6 +167,7 @@ namespace MWPhysics
 
         osg::Vec3f mForce;
         bool mOnGround;
+        bool mOnSlope;
         bool mInternalCollisionMode;
         bool mExternalCollisionMode;
 

apps/openmw/mwphysics/physicssystem.cpp

@@ -250,6 +250,8 @@ namespace MWPhysics
             }
             else
             {
+                actor->setOnGround(true);
+
                 // Check if we actually found a valid spawn point (use an infinitely thin ray this time).
                 // Required for some broken door destinations in Morrowind.esm, where the spawn point
                 // intersects with other geometry if the actor's base is taken into account
@@ -266,11 +268,13 @@ namespace MWPhysics
                         ( (toOsg(resultCallback1.m_hitPointWorld) - tracer.mEndPos).length2() > 35*35
                         || !isWalkableSlope(tracer.mPlaneNormal)))
                 {
-                    actor->setOnGround(isWalkableSlope(resultCallback1.m_hitNormalWorld));
+                    actor->setOnSlope(isWalkableSlope(resultCallback1.m_hitNormalWorld));
                     return toOsg(resultCallback1.m_hitPointWorld) + osg::Vec3f(0.f, 0.f, 1.f);
                 }
-
-                actor->setOnGround(isWalkableSlope(tracer.mPlaneNormal));
+                else
+                {
+                    actor->setOnSlope(isWalkableSlope(tracer.mPlaneNormal));
+                }
 
                 return tracer.mEndPos;
             }
@@ -322,12 +326,10 @@ namespace MWPhysics
             {
                 velocity = (osg::Quat(refpos.rot[2], osg::Vec3f(0, 0, -1))) * movement;
 
-                if (velocity.z() > 0.f && physicActor->getOnGround())
+                if (velocity.z() > 0.f && physicActor->getOnGround() && !physicActor->getOnSlope())
                     inertia = velocity;
-                else if(!physicActor->getOnGround())
-                {
+                else if(!physicActor->getOnGround() || physicActor->getOnSlope())
                     velocity = velocity + physicActor->getInertialForce();
-                }
             }
 
             // dead actors underwater will float to the surface, if the CharacterController tells us to do so
@@ -440,13 +442,14 @@ namespace MWPhysics
             }
 
             bool isOnGround = false;
+            bool isOnSlope = false;
             if (!(inertia.z() > 0.f) && !(newPosition.z() < swimlevel))
             {
                 osg::Vec3f from = newPosition;
                 osg::Vec3f to = newPosition - (physicActor->getOnGround() ?
                              osg::Vec3f(0,0,sStepSizeDown+2.f) : osg::Vec3f(0,0,2.f));
                 tracer.doTrace(colobj, from, to, collisionWorld);
-                if(tracer.mFraction < 1.0f && isWalkableSlope(tracer.mPlaneNormal)
+                if(tracer.mFraction < 1.0f
                         && tracer.mHitObject->getBroadphaseHandle()->m_collisionFilterGroup != CollisionType_Actor)
                 {
                     const btCollisionObject* standingOn = tracer.mHitObject;
@@ -460,6 +463,8 @@ namespace MWPhysics
                         newPosition.z() = tracer.mEndPos.z() + 1.0f;
 
                     isOnGround = true;
+
+                    isOnSlope = !isWalkableSlope(tracer.mPlaneNormal);
                 }
                 else
                 {
@@ -483,7 +488,7 @@ namespace MWPhysics
                 }
             }
 
-            if(isOnGround || newPosition.z() < swimlevel || isFlying)
+            if((isOnGround && !isOnSlope) || newPosition.z() < swimlevel || isFlying)
                 physicActor->setInertialForce(osg::Vec3f(0.f, 0.f, 0.f));
             else
             {
@@ -497,6 +502,7 @@ namespace MWPhysics
                 physicActor->setInertialForce(inertia);
             }
             physicActor->setOnGround(isOnGround);
+            physicActor->setOnSlope(isOnSlope);
 
             newPosition.z() -= halfExtents.z(); // remove what was added at the beginning
             return newPosition;