Added a bunch of checks to make sure a normalized zero vector (= NaN, NaN) is not used in any position/velocity/movement calculations. There were at least three places where it was causing problems according to error reports: when a character that can't enter a sub spawns at the center of a hull and when using an underwater scooter or throwing something while the cursor is at the position of the character, but there were tons of other places as well where it may have potentially caused physics errors.

Barotrauma/BarotraumaShared/Source/Map/Explosion.cs

@@ -180,6 +180,7 @@ namespace Barotrauma
                     if (limb.WorldPosition != worldPosition && force > 0.0f)
                     {
                         Vector2 limbDiff = Vector2.Normalize(limb.WorldPosition - worldPosition);
+                        if (!MathUtils.IsValid(limbDiff)) limbDiff = Rand.Vector(1.0f);
                         Vector2 impulsePoint = limb.SimPosition - limbDiff * limbRadius;
                         limb.body.ApplyLinearImpulse(limbDiff * distFactor * force, impulsePoint);
                     }

Barotrauma/BarotraumaShared/Source/Map/Levels/Level.cs

@@ -919,7 +919,7 @@ namespace Barotrauma
                     ConvertUnits.ToSimUnits(endPos),
                     null, Physics.CollisionLevel) != null)
                 {
-                    position = ConvertUnits.ToDisplayUnits(Submarine.LastPickedPosition) +  Vector2.Normalize(startPos - endPos)*offsetFromWall;
+                    position = ConvertUnits.ToDisplayUnits(Submarine.LastPickedPosition) + Vector2.Normalize(startPos - endPos) * offsetFromWall;
                     break;
                 }
 

Barotrauma/BarotraumaShared/Source/Map/SubmarineBody.cs

@@ -214,7 +214,7 @@ namespace Barotrauma
                     closestSub = Character.Controlled.Submarine;
                 }
 
-                bool displace = moveAmount.Length() > 100.0f;
+                bool displace = moveAmount.LengthSquared() > 100.0f * 100.0f;
                 foreach (Submarine sub in subsToMove)
                 {
                     sub.PhysicsBody.SetTransform(sub.PhysicsBody.SimPosition + ConvertUnits.ToSimUnits(moveAmount), 0.0f);
@@ -259,7 +259,7 @@ namespace Barotrauma
 
             Vector2 totalForce = CalculateBuoyancy();
 
-            if (Body.LinearVelocity.LengthSquared() > 0.000001f)
+            if (Body.LinearVelocity.LengthSquared() > 0.0001f)
             {
                 float dragCoefficient = 0.01f;
 
@@ -285,6 +285,7 @@ namespace Barotrauma
             worldBorders.Location += MathUtils.ToPoint(ConvertUnits.ToDisplayUnits(Body.SimPosition));
 
             Vector2 translateDir = Vector2.Normalize(subTranslation);
+            if (!MathUtils.IsValid(translateDir)) translateDir = Vector2.UnitY;
 
             foreach (Character c in Character.CharacterList)
             {
@@ -384,7 +385,9 @@ namespace Barotrauma
 
             if (f2.UserData is VoronoiCell cell)
             {
-                HandleLevelCollision(contact, Vector2.Normalize(ConvertUnits.ToDisplayUnits(Body.SimPosition) - cell.Center));
+                Vector2 collisionNormal = Vector2.Normalize(ConvertUnits.ToDisplayUnits(Body.SimPosition) - cell.Center);
+                if (!MathUtils.IsValid(collisionNormal)) collisionNormal = Rand.Vector(1.0f);
+                HandleLevelCollision(contact, collisionNormal);
                 return true;
             }
 
@@ -445,7 +448,9 @@ namespace Barotrauma
         {
             if (limb.Mass > 100.0f)
             {
-                Vector2 normal = Vector2.Normalize(Body.SimPosition - limb.SimPosition);
+                Vector2 normal = Vector2.DistanceSquared(Body.SimPosition, limb.SimPosition) < 0.0001f ?
+                    Vector2.UnitY :
+                    Vector2.Normalize(Body.SimPosition - limb.SimPosition);
 
                 float impact = Math.Min(Vector2.Dot(Velocity - limb.LinearVelocity, -normal), 50.0f) / 5.0f * Math.Min(limb.Mass / 200.0f, 1);
 
@@ -634,7 +639,10 @@ namespace Barotrauma
             float contactDot = Vector2.Dot(otherSub.PhysicsBody.LinearVelocity, -avgContactNormal);
             if (contactDot > 0.0f)
             {
-                otherSub.PhysicsBody.LinearVelocity -= Vector2.Normalize(otherSub.PhysicsBody.LinearVelocity) * contactDot;
+                if (otherSub.PhysicsBody.LinearVelocity.LengthSquared() > 0.0001f)
+                {
+                    otherSub.PhysicsBody.LinearVelocity -= Vector2.Normalize(otherSub.PhysicsBody.LinearVelocity) * contactDot;
+                }
 
                 impact = Vector2.Dot(otherSub.Velocity, normal);
                 otherSub.SubBody.ApplyImpact(impact, normal, contact);

Barotrauma/BarotraumaShared/Source/Map/TransitionCinematic.cs

@@ -89,7 +89,9 @@ namespace Barotrauma
                 foreach (Submarine sub in subs)
                 {
                     if (sub.Position == targetPos) continue;
-                    sub.ApplyForce((Vector2.Normalize(targetPos - sub.Position) * targetSpeed - sub.Velocity) * 500.0f);
+                    Vector2 dir = Vector2.Normalize(targetPos - sub.Position);
+                    if (!MathUtils.IsValid(dir)) continue;
+                    sub.ApplyForce((dir * targetSpeed - sub.Velocity) * 500.0f);
                 }
                 
                 timer += CoroutineManager.UnscaledDeltaTime;