Ss14 Physics System Core

6. Sync with Transform

• After solve, the position/rotation is put back into the transform tree.

Key idea:

• Game logic should almost always change physics through system methods (Set*, Apply*, RegenerateContacts, WakeBody), and not through direct component fields ✅

How the tick of physics really works

Typical step (including substeps):

1. PhysicsUpdateBeforeSolveEvent.
2. Search for new potential pairs (FindNewContacts).
3. Update contacts (CollideContacts) and events Start/EndCollide.
4. Solve islands (Step -> integration of speeds -> constraints -> positions).
5. PhysicsUpdateAfterSolveEvent.
6. Finalization/lerp data on the last substep.

Practical conclusion:

• Everything that should act “in the same physical step” must be placed before solve (usually in controller systems or before-solve events).

Connection with Transform, Anchoring and Containers

PhysicsSystem is tightly connected to the transform hierarchy:

• When parent-change occurs, the behavior of the body is recalculated and the map speed is saved.
• Anchor/unanchor changes not only the transform status, but also the physical mode (through the system flow).
• In a container, the body is usually transferred to CanCollide = false, the speeds are reset to zero.

Consequence:

• Any operations “inserted into container”, “dropped parent”, “anchored/unanchored” cannot be considered only visual; this is a full-fledged physical transition ⚠️

Client and server

General:

• Both the server and the client use a common physical layer and the same key invariants.

Differences:

• The server is authoritative.
• The client additionally controls the prediction flag of entities and carefully recalculates the touching state of contacts when resetting the state in order to reduce mispredict effects.

Short decision tree

1. Need a one-time push/jerk?

• ApplyLinearImpulse / ApplyAngularImpulse.

2. Do you need constant thrust/torque?

• ApplyForce / ApplyTorque.

3. Do you need to switch the physical mode of an object?

• SetBodyType (+ if necessary SetCanCollide, SetFixedRotation, SetSleepingAllowed).

4. Do you need to change collision behavior without changing your body type?

• SetCanCollide or fixture-API (SetCollisionMask/Layer, SetHard).

5. Do you need to reassemble the current contacts after a sudden change of collision/fixtures?

• RegenerateContacts.

6. Do you need geometry and distances based on real hitboxes?

• TryGetNearest*, GetWorldAABB, GetHardAABB, ray/intersection query API.

Optimization when using PhysicsSystem :)

1) Component optimizations (the cheapest and most effective)

• CollisionWake: automatically turns off CanCollide for sleeping bodies on the grid (if there are no joints/critical contacts), and turns it back on when waking up.
• CollideOnAnchor: Binds CanCollide to the anchor/unanchor state.
• These two approaches are usually better than manually "pulling" SetCanCollide in 100 places.

# Underground/pipe entity: collision is automatically synchronized with anchoring.