Fix some issues with pulling system (#32145)

Joints were created with pivots at object origin, causing unintuitive behaviour when an object was not centered on the origin. Now puts the pivots at the COM.
Joint limits were set based on fractions of the union of the AABB of objects, which did not make geometric sense. Now uses the pivot length with an additional [arbitrary] length.
Joints were created with a very low spring stiffness, which had a negligible effect most of the time but caused very unintuitive behaviour when the pulled object had a low mass (#28028) - disable the spring limit, and just use the hard min/max limits.

Co-authored-by: Eoin Mcloughlin <helloworld@eoinrul.es>

diff --git a/Content.Shared/Movement/Pulling/Systems/PullingSystem.cs b/Content.Shared/Movement/Pulling/Systems/PullingSystem.cs
index 5f35adb33374493dc0b71680568847d50d77eaff..6392956d632000851b755cf991172c235111b436 100644 (file)
--- a/Content.Shared/Movement/Pulling/Systems/PullingSystem.cs
+++ b/Content.Shared/Movement/Pulling/Systems/PullingSystem.cs
@@ -438,7 +438,7 @@ public sealed class PullingSystem : EntitySystem
         if (!CanPull(pullerUid, pullableUid))
             return false;
 
-        if (!HasComp<PhysicsComponent>(pullerUid) || !TryComp(pullableUid, out PhysicsComponent? pullablePhysics))
+        if (!TryComp(pullerUid, out PhysicsComponent? pullerPhysics) || !TryComp(pullableUid, out PhysicsComponent? pullablePhysics))
             return false;
 
         // Ensure that the puller is not currently pulling anything.
@@ -485,17 +485,19 @@ public sealed class PullingSystem : EntitySystem
         // joint state handling will manage its own state
         if (!_timing.ApplyingState)
         {
-            // Joint startup
-            var union = _physics.GetHardAABB(pullerUid).Union(_physics.GetHardAABB(pullableUid, body: pullablePhysics));
-            var length = Math.Max(union.Size.X, union.Size.Y) * 0.75f;
-
-            var joint = _joints.CreateDistanceJoint(pullableUid, pullerUid, id: pullableComp.PullJointId);
+            var joint = _joints.CreateDistanceJoint(pullableUid, pullerUid,
+                    pullablePhysics.LocalCenter, pullerPhysics.LocalCenter,
+                    id: pullableComp.PullJointId);
             joint.CollideConnected = false;
             // This maximum has to be there because if the object is constrained too closely, the clamping goes backwards and asserts.
-            joint.MaxLength = Math.Max(1.0f, length);
-            joint.Length = length * 0.75f;
+            // Internally, the joint length has been set to the distance between the pivots.
+            // Add an additional 15cm (pretty arbitrary) to the maximum length for the hard limit.
+            joint.MaxLength = joint.Length + 0.15f;
             joint.MinLength = 0f;
-            joint.Stiffness = 1f;
+            // Set the spring stiffness to zero. The joint won't have any effect provided
+            // the current length is beteen MinLength and MaxLength. At those limits, the
+            // joint will have infinite stiffness.
+            joint.Stiffness = 0f;
 
             _physics.SetFixedRotation(pullableUid, pullableComp.FixedRotationOnPull, body: pullablePhysics);
         }