Automated testing is a cornerstone of efficient game development workflows. It ensures stability, reduces regression issues, and enables faster iteration cycles. However, one critical challenge arises when automated tests yield inconsistent results due to non-deterministic behavior in the game. This is where deterministic builds come into play.

What Are Deterministic Builds?

A deterministic build ensures that a game or simulation behaves identically every time it is run under the same conditions. This means the same inputs always produce the same outputs, regardless of runtime variables such as system performance, time, or random seeds.

In the context of Unreal Engine, deterministic builds eliminate variability, making them invaluable for automated testing. By removing unpredictability, developers can focus on addressing genuine issues instead of chasing down intermittent test failures.

Why Deterministic Builds Matter for Automated Testing

Non-deterministic behavior in a game can lead to unreliable automated test results. For example:

• Random Values: Gameplay features that rely on randomness can produce different outcomes, complicating result validation.
• Time-Based Variability: Physics simulations and animations that depend on real-time can yield inconsistent results.

Such issues make it difficult to pinpoint the root cause of test failures and waste valuable development time. Deterministic builds address these challenges by enforcing consistent conditions.

Implementing Deterministic Builds in Unreal Engine

Unreal Engine provides a straightforward way to enable deterministic builds using the -Deterministic command-line option. This setting ensures consistent behavior during automated testing by combining two key parameters:

• -UseFixedTimeStep: Fixes the time step used in the simulation, ensuring consistent physics and animation updates.
• -FixedSeed: Sets a fixed seed for all random number generation, ensuring reproducibility of random events.

The -Deterministic option acts as a shorthand for these parameters, simplifying the process. To launch your game with deterministic behavior, use the following command:

YourGame.exe -Deterministic

This guarantees that every automated test runs under identical conditions, providing reliable and repeatable results.

Best Practices for Deterministic Testing

To maximize the benefits of deterministic builds, consider the following best practices:

1. Fix Random Seeds: Always use a fixed seed for random number generation in your gameplay and simulations. This ensures that any randomness behaves predictably across test runs.
2. Enable Fixed Time Steps: Consistently apply fixed time steps to your physics simulations and animations to eliminate time-dependent variability.
3. Test Diverse Scenarios: While deterministic builds guarantee consistency, they should also be used to validate behavior under various controlled settings to simulate diverse game states.
4. Automate Thoroughly: Integrate deterministic builds into your continuous integration pipeline to catch issues early and ensure reliability throughout development.