Car scripts can write extra replay data via controller inputs. This feature was added in the 0.2.3 update. Simply define a schema in the car.ini file, and then use script controller inputs to read/write data that will get stored as replay data. Try and keep the number of replay inputs low.

car.ini:

[_EXTENSION]
SCRIPT_INPUTS_REPLAY = 100:uint8, 101:unorm8, 102:half

[_EXTENSION]
SCRIPT_INPUTS_REPLAY = 100:uint8, unorm8, half, uint8 ;INDEX == 100, INDEX == 101, INDEX == 102, INDEX == 103

Replay inputs can be added in the form of <INDEX>:<TYPE>. You can also define inputs with just <TYPE>, and their INDEX will be incremented from the last explicitly defined INDEX value.

Known types:

• h, half: 2 bytes, floating point (roughly, from 1/65536 to 65536);
• f, float: 4 bytes, floating point;
• i8, int8, char: 1 byte, from -128 to 127;
• u8, uint8, byte: 1 byte, from 0 to 255;
• i16, int16, short: 2 bytes, from -32768 to 32767;
• u16, uint16, ushort: 2 bytes, from 0 to 65535;
• i32, int32, int: 4 bytes, can be negative;
• u32, uint32, uint: 4 bytes, positive integers only;
• unorm8: 1 byte, from 0 to 1;
• norm8: 1 byte, from -1 to 1;
• unorm16: 2 bytes, from 0 to 1;
• norm16: 2 bytes, from -1 to 1;

If your defined schema ever changes, any replays that were saved with the previous schema, will no longer replay the extra data that was recorded.

Car physics script.lua:

function script.update(dt)
  local data = ac.accessCarPhysics()

  -- Data written to these controllers is stored in the replay data
  data.controllerInputs[100] = 1
  data.controllerInputs[101] = 2
  data.controllerInputs[102] = 3
end