✨ Description

Currently, RollbackSynchronizer doesn't simulate nodes it doesn't have input for. Once the input arrives, the game state is rolled back and the node is simulated.

It would be useful to be able to predict node behavior even if the input is missing, by predicting inputs.

Continues #54

Use case

Games could implement various input prediction mechanisms, instead of always relying on skipping simulation on missing inputs.

One common solution is input decay, where input values are weakened as the input ages. Something similar also works for racing games, where on missing inputs, the game assumes the player slowly lets go of the throttle.

Implementation notes

Rollback synchronizer should expose the following data during rollback:

• has_input() - Do we have input for the current tick?
• get_input_tick() - Which tick are we taking input from? Value differs from current tick if has_input() is false.
• get_input_age() - How old is the input data we're using? In ticks.

Games can use the above to implement their own input prediction mechanism however they'd like. To ease input prediction implementations, RollbackSynchronizer may expose a signal that's emitted after applying state and input for a given tick, but before simulating the tick. This way, input nodes can apply input prediction in their own code and expose data as necessary.

Nodes should still be able to opt-out of simulation if they determine that they can't predict anything with the current input. RollbackSynchronizer should provide a mechanism for this, e.g. by exposing a skip_recording(node). This would enable nodes to do the following:

func _rollback_tick(dt, tick, is_fresh):
    if not rollback_synchronizer.get_input_age() > 4:
        # Input too old, can't predict
        rollback_synchronizer.skip_recording(self)
        return

    # [...]

To avoid breaking games during a version upgrade, input prediction should be opt-in. A flag to toggle input prediction should be included either as a property of RollbackSynchronizer, or as a project setting.

Distribution

netfox core