Why do multiplayer games often use custom physics implementations?

Question

Big game engines all have a physics engine whose job is to resolve forces, collisions and constraints efficiently. I think the majority of modern games use a physics engine to some extent. Nevertheless, I have noticed that it is not uncommon for developers to implement features using a "kinematic simulation" instead of a "physics simulation" especially in multiplayer environments. Using the example of a car simulation to clarify these terms a bit more (since I don't think there is standard terminology).

Kinematic Simulation: Collect all the forces (suspension force, gravity, engine force etc). Calculate (and store) the acceleration, velocity, new position etc. Set the location and rotation outside the physics engine (which still handles collisions and motion of objects not related to the car).

Physics Simulation: Collect all the forces add them to the car and let the physics engine resolve what happens.

Is there something that makes client prediction/corrections easier in the kinematic version than the physics version?

Unreals character movement component would be a specific, concrete and very common example. AFAIK while the physics engine is still used for checking for collisions the character itself doesn't simulate physics via the engine but forces are instead added to the movement component which runs its own simulation.

Answer 1

Is there something that makes client prediction/corrections easier in the kinematic version than the physics version?

This is a good example. If you want an authoritative server, you would run all of your physics on the server. There is no need to run it again on the client for important parts, such as the player. So when the server tells the client the updated positions, you not only save the client-side calculations of physics, but you also get deterministic results.

Another example is when you don't want to use the engine's physics. For example, your character can push through a bunch of enemies. Using standard physics, your character may get stuck. But by kinematically pushing your character forward, it still lets the enemies react to the collision with the player, but lets the player ignore the collision with the enemies.

Answer 2

One problem with many physics engines is that they are not deterministic. That means that two computers executing the same physics simulation with the same input parameters might end up with two different results. For example because one of them runs at a higher framerate leading to a more detailed resolution.

This is often not much of a problem in singleplayer games. But it can become a big problem in multiplayer games, because even slight differences between clients can often have a butterfly effect which result in the game becoming completely out of sync between players. This is in addition to the latency problem in multiplayer games which causes the game to never be really in sync between players. So not even the input parameters might be the same.

Which is why many multiplayer games roll their own physics engine. Having an own engine instead of a black box allows them to resync the clients regularly.