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Setting Up Randoms for Luna Replay

Luna Replay supports the use of UnityEngine.Random in your game.

Luna Replay does not replay System Random Values. For randoms which influence gameplay, please use UnityEngine Randoms.

In order to ensure that there is consistent behaviour between your captures and associated replays, Luna stores the initial Random seed value at the time of capture, and uses this for the replay.


In the above diagram, you can see that by storing this seed value, the Replay can return the same random value when it is required.

However, imagine the following when the random numbers are requested out of order.


In this example, value1, value2 and value3 are being assigned random values in the capture by using UnityEngine.Random and the initial seed value.

In the replay, the order in which the values are assigned has changed. Therefore, the values they are assigned will differ, even though the initial seed value is identical.

So why can this happen? Let's take a look and outline what you can do in each scenario.

Async Loading#

When using asynchronous loading in your game, frames can occur out of order, and this may be the cause of your random numbers being assigned in a different order between the capture and replay.

Single Async Events#

If your game uses a single async event, for example a preloader or loading of a scene, then you may wish to ensure that the random order is maintained.

To do this, you can use a Barrier which will save and restore the random seed at this point.

Multiple Async Events#

If there are many separate events, such as loading multiple assets, and these events use random values, then it is possible that the order of these events can change between capture and replays.

You may use a Checkpoint after each event, before randoms are used, in order to save and restore the random seed at this point.

  • If the random values are not gameplay critical, for example in a particle system, or an object's colour, then you may replace UnityEngine.Random with System.Random instead of adding a checkpoint.
  • If randoms are used for calculations it’s possible to save and restore those calculations, instead of the random value itself. You can do this using the DataPrefs API.

Adding/removing random values from the sequence#

You may use logic in your game which varies the use of randoms via Playground fields.

For example, you could have a boolean Playground field that enables "Random colour mode" in your game. Doing this (whilst causing great visual results) will cause differences between your capture and replays.

You can use a Checkpoint in such scenarios. An example is included here.

Debugging randoms#

Luna Replay prints a warning message to console (on both Unity and Linux builds) which notifies you when the order of randoms has changed between capture and replays.

The Random.state differs from the state during the capture. If it causes issues please make sure you have set barriers after asynchronous operations. See documentation for more details (

In order to help debug this, Luna provides a debug option in the Capture and Replay UI in Unity. See more details here.