Movement solver tweaks

1) As much as I dislike it, upping the collision margin from 0.1 to 0.2
fixes bugs, particularly involving walking into upwards-slanted walls.

2) There were still some problems involving acute crevices/seams; they
were using the adjusted instead of unadjusted normal, and also they need
to bypass the don't-slide-upwards check to prevent (see #6379)

3) The move-away-from-what-we-just-hit code needs to run always, not
just on non-initial iterations. No idea why I did it this way before.

4) Force bullet to give actor boxes a tiny collision margin of 0.001
instead of the default 0.04. I can't tell whether this is actually
working or not, but it should reduce unexplained weirdness.

5) A piece of code that was meant to prevent bugs by short-circuiting
the movement solver if its direction changed more than 180 degrees
actually caused problems instead of preventing them, so I deleted it.

apps/openmw/mwphysics/actor.cpp

@@ -59,6 +59,7 @@ Actor::Actor(const MWWorld::Ptr& ptr, const Resource::BulletShape* shape, Physic
     mRotationallyInvariant = (mMeshTranslation.x() == 0.0 && mMeshTranslation.y() == 0.0) && std::fabs(mOriginalHalfExtents.x() - mOriginalHalfExtents.y()) < 2.2;
 
     mConvexShape = static_cast<btConvexShape*>(mShape.get());
+    mConvexShape->setMargin(0.001); // make sure bullet isn't using the huge default convex shape margin of 0.04
 
     mCollisionObject = std::make_unique<btCollisionObject>();
     mCollisionObject->setCollisionFlags(btCollisionObject::CF_KINEMATIC_OBJECT);

apps/openmw/mwphysics/constants.hpp

@@ -15,7 +15,7 @@ namespace MWPhysics
     // Arbitrary number. To prevent infinite loops. They shouldn't happen but it's good to be prepared.
     static constexpr int sMaxIterations = 8;
     // Allows for more precise movement solving without getting stuck or snagging too easily.
-    static constexpr float sCollisionMargin = 0.1f;
+    static constexpr float sCollisionMargin = 0.2f;
     // Allow for a small amount of penetration to prevent numerical precision issues from causing the "unstuck"ing code to run unnecessarily
     // Currently set to 0 because having the "unstuck"ing code run whenever possible prevents some glitchy snagging issues
     static constexpr float sAllowedPenetration = 0.0f;

apps/openmw/mwphysics/movementsolver.cpp

@@ -252,6 +252,8 @@ namespace MWPhysics
                 remainingTime *= (1.0f-tracer.mFraction);
 
                 auto planeNormal = tracer.mPlaneNormal;
+                // need to know the unadjusted normal to handle certain types of seams properly
+                const auto origPlaneNormal = planeNormal;
 
                 // If we touched the ground this frame, and whatever we ran into is a wall of some sort,
                 // pretend that its collision normal is pointing horizontally
@@ -275,10 +277,11 @@ namespace MWPhysics
                 bool usedSeamLogic = false;
 
                 // check for the current and previous collision planes forming an acute angle; slide along the seam if they do
+                // for this, we want to use the original plane normal, or else certain types of geometry will snag
                 if(numTimesSlid > 0)
                 {
-                    auto dotA = lastSlideNormal * planeNormal;
+                    auto dotA = lastSlideNormal * origPlaneNormal;
-                    auto dotB = lastSlideNormalFallback * planeNormal;
+                    auto dotB = lastSlideNormalFallback * origPlaneNormal;
                     if(numTimesSlid <= 1) // ignore fallback normal if this is only the first or second slide
                         dotB = 1.0;
                     if(dotA <= 0.0 || dotB <= 0.0)
@@ -291,14 +294,14 @@ namespace MWPhysics
                             lastSlideNormal = lastSlideNormalFallback;
                         }
 
-                        auto constraintVector = bestNormal ^ planeNormal; // cross product
+                        auto constraintVector = bestNormal ^ origPlaneNormal; // cross product
                         if(constraintVector.length2() > 0) // only if it's not zero length
                         {
                             constraintVector.normalize();
                             newVelocity = project(velocity, constraintVector);
 
                             // version of surface rejection for acute crevices/seams
-                            auto averageNormal = bestNormal + planeNormal;
+                            auto averageNormal = bestNormal + origPlaneNormal;
                             averageNormal.normalize();
                             tracer.doTrace(actor.mCollisionObject, newPosition, newPosition + averageNormal*(sCollisionMargin*2.0), collisionWorld);
                             newPosition = (newPosition + tracer.mEndPos)/2.0;
@@ -309,27 +312,28 @@ namespace MWPhysics
                 }
                 // otherwise just keep the normal vector rejection
 
-                // if this isn't the first iteration, or if the first iteration is also the last iteration,
                 // move away from the collision plane slightly, if possible
                 // this reduces getting stuck in some concave geometry, like the gaps above the railings in some ald'ruhn buildings
                 // this is different from the normal collision margin, because the normal collision margin is along the movement path,
                 // but this is along the collision normal
-                if(!usedSeamLogic && (iterations > 0 || remainingTime < 0.01f))
+                if(!usedSeamLogic)
                 {
                     tracer.doTrace(actor.mCollisionObject, newPosition, newPosition + planeNormal*(sCollisionMargin*2.0), collisionWorld);
                     newPosition = (newPosition + tracer.mEndPos)/2.0;
                 }
 
                 // Do not allow sliding up steep slopes if there is gravity.
-                if (newPosition.z() >= swimlevel && !actor.mFlying && !isWalkableSlope(planeNormal))
+                // The purpose of this is to prevent air control from letting you slide up tall, unwalkable slopes.
+                // For that purpose, it is not necessary to do it when trying to slide along acute seams/crevices (i.e. usedSeamLogic)
+                // and doing so would actually break air control in some situations where vanilla allows air control.
+                // Vanilla actually allows you to slide up slopes as long as you're in the "walking" animation, which can be true even
+                // in the air, so allowing this for seams isn't a compatibility break.
+                if (newPosition.z() >= swimlevel && !actor.mFlying && !isWalkableSlope(planeNormal) && !usedSeamLogic)
                     newVelocity.z() = std::min(newVelocity.z(), velocity.z());
 
-                if (newVelocity * origVelocity <= 0.0f)
-                    break;
-
                 numTimesSlid += 1;
                 lastSlideNormalFallback = lastSlideNormal;
-                lastSlideNormal = planeNormal;
+                lastSlideNormal = origPlaneNormal;
                 velocity = newVelocity;
             }
         }