Lockstep game (only confirmed entities)

[OlivierCoue]

Hi, I have been playing a bit with Lightyear recently and love it!

I currently use the default setup showed in examples with predicted entities for current player controlled entities and interpolated ones for the rest. Works great overall, but desync/rollback can quickly become an issue when predicted entities have to interact with interpolated ones (collisions detection...), may be doable properly but need a lot of works.

I was wondering, what would be the correct way to do a lockstep game (no prediction nor interpolation, just send inputs to the server, and react to server update for the render) ?

As you have to opt in either predicted or interpolated when registering component for replication (you cannot just say, "replicate/full /server to client" afaik) I was thinking of registering all my component with "interpolate/full/server to client" and then only work with the confirmed entities for the render (and actually ignore the Interpolated ones..)

[cBournhonesque]

Hi, great to hear that you like it :)

As you have to opt in either predicted or interpolated when registering component for replication (you cannot just say, "replicate/full /server to client" afaik) I was thinking of registering all my component with "interpolate/full/server to client" and then only work with the confirmed entities for the render (and actually ignore the Interpolated ones..)

You can definitely register a component without prediction/interpolation enabled! See here in the tests: https://github.com/cBournhonesque/lightyear/blob/main/lightyear/src/tests/protocol.rs#L246
Prediction/Interpolation is totally optional. However you often want to do interpolation otherwise the Confirmed entities might be a bit choppy

Works great overall, but desync/rollback can quickly become an issue when predicted entities have to interact with interpolated ones (collisions detection...)

Indeed collision detections between predicted and interpolated entities is painful!

There are several options:
- interpolate everything. Everything will be smooth and in the same timeline, but the movements of your character will be delayed. I think maybe this is what you should try first?
- predict yourself, interpolated other things, and use lag compensation: works for FPS games, but is not implemented otherwise
- predict everything: that is the approach used by RocketLeague. I have several examples of this: https://github.com/cBournhonesque/lightyear/tree/main/examples/spaceships or https://github.com/cBournhonesque/lightyear/tree/main/examples/avian_physics
The client receives the inputs from all other clients and predicts everything (main player, other players, NPCs) so that they are all in the same timeline. It can work well but can be CPU-heavy, etc.

Lockstep games are a bit different, the idea is that the game is fully deterministic and only the inputs are replicated. (i.e. the server just replicates other clients' inputs to all clients). When all clients have received all inputs for a given tick, the game progresses. If an input is missing, the game is stalled.
And then an improvement on top of that is something like GGPO where the game is fully determinstic and only inputs are replicated, but there is prediction/rollback added on top of it.
I have an open issue to add something like this: #185 but I haven't made any progress on it yet.

[OlivierCoue]

Thanks for this great answer!
I will start with the easy solution which is "interpolate everything". I tried using only the confirmed ones but yeah, really choppy as you said.
It indeed add a little delay on inputs feedback, but with a good network condition + low interpolation delay it feels good imo :)