FixedUpdate & Physics: Time.Delta And Interpolation Fraction

by Henrik Larsen 61 views

Hey guys! Let's dive into a crucial topic for game development, especially in engines like Facepunch's S&box: managing FixedUpdate, physics fractions, and delta time within your update methods. This is super important for creating smooth and accurate game mechanics, so let’s break it down in a way that’s easy to grasp.

The Importance of FixedUpdate and Delta Time

In game development, achieving consistent and predictable physics behavior is paramount. This is where FixedUpdate comes into play. Unlike the regular Update method, which runs every frame and can vary in frequency depending on the game's performance, FixedUpdate runs at a fixed rate. This fixed rate is determined by the physics engine's settings, ensuring that physics calculations are consistent regardless of frame rate fluctuations. This consistency is crucial for things like collision detection, applying forces, and other physics-related tasks.

Why FixedUpdate Matters for Physics

The primary reason to use FixedUpdate for physics is to avoid inconsistencies. Imagine a scenario where your game runs at 30 frames per second (FPS) sometimes and 60 FPS at other times. If you were to apply forces or update positions in the regular Update method, the physics interactions would behave differently at these varying frame rates. This can lead to unpredictable and jittery movement, making the game feel unprofessional and unresponsive. By using FixedUpdate, you ensure that these calculations happen at a steady pace, leading to more stable and reliable physics.

The delta time, often referred to as Time.deltaTime in many game engines, represents the time elapsed since the last frame. In the context of Update, Time.deltaTime will vary because the frame rate varies. However, there’s also a fixed delta time associated with FixedUpdate, which remains constant. This fixed delta time is the interval between each FixedUpdate call and is essential for performing accurate physics calculations. Using this fixed delta time ensures that your physics calculations are consistent, regardless of how the frame rate fluctuates.

Custom Interpolation and the Need for Fixed Delta

Now, let’s talk about custom interpolation. Interpolation is a technique used to smooth out movement and animations in a game. It's especially important when dealing with physics because the physics engine might update at a lower frequency than the rendering engine. This can lead to visible stuttering or jerking if the game objects' positions are directly set to the physics engine's output every frame. Interpolation helps bridge this gap by smoothly transitioning the object's visual position between physics updates.

To perform custom interpolation effectively, you need access to the fixed delta time. This allows you to calculate the correct interpolation factor, which determines how far to move the object's visual representation between each frame. Without easy access to the fixed delta time, implementing custom interpolation becomes significantly more complex and less accurate.

The Challenge of Accessing Fixed Delta Time

The original post highlights a common frustration: the cumbersome way to access the fixed delta time in some engines. The expression 1f / ProjectSettings.Physics.FixedUpdateFrequency is not only verbose but also not immediately intuitive. This can lead to confusion and make the code less readable. Having a direct property like Time.FixedDelta would greatly simplify the process and make it more accessible for developers. This small change can lead to cleaner, more maintainable code and fewer headaches when implementing physics-based mechanics.

The Quest for the Interpolation Fraction

Beyond just the fixed delta time, there's another crucial piece of the puzzle: the interpolation fraction. The interpolation fraction represents how far along the current FixedUpdate step the game is. It’s a value between 0 and 1 that indicates the progress between the last fixed update and the next one. This fraction is vital for smoothing out the visual representation of objects that are being controlled by the physics engine.

Why the Interpolation Fraction Matters

Imagine a scenario where your physics engine updates at 60 Hz, but your game is rendering at 120 FPS. This means that for each physics update, there are two frames being rendered. Without interpolation, the visual position of your game objects would jump directly to the new physics position twice per physics update, leading to a choppy appearance. The interpolation fraction allows you to smoothly transition the object’s visual position between these physics updates.

By knowing the interpolation fraction, you can blend the previous and current physics states to create a smooth visual representation. For example, if the fraction is 0.5, you would blend halfway between the previous and current positions. This results in a much smoother and more visually appealing movement, especially for objects that are moving quickly or undergoing frequent changes in direction.

The Current Workaround: A Not-So-Ideal Solution

The original post describes a common workaround for calculating the interpolation fraction, which involves creating a GameObjectSystem to track the time since the last FixedUpdate. This system would save Time.Now or use TimeSince and then divide it by the fixed delta time. While this approach works, it's not ideal for several reasons:

  1. Complexity: It requires setting up a separate system just to calculate this one value. This adds unnecessary complexity to the project and can make the code harder to understand and maintain.
  2. Overhead: Creating and managing a dedicated system for this purpose introduces a small amount of overhead. While this overhead might be negligible in small projects, it can become significant in larger, more complex games.
  3. Readability: The calculation itself (TimeSinceLastFixedUpdate / Delta) is not immediately obvious in its purpose. It requires understanding the context and the underlying logic to grasp what the value represents.

A more direct and intuitive solution would be to have the interpolation fraction readily available as a property in the Time class, similar to how Time.deltaTime and other time-related values are accessed.

The Proposed Solution: Time.FixedDelta and Time.Fraction

The original poster suggests adding two new properties to the Time class: Time.FixedDelta and Time.Fraction. This is a fantastic idea that would significantly improve the developer experience and make it easier to implement smooth, accurate physics-based mechanics.

Time.FixedDelta: A Clear and Concise Way to Access Fixed Delta Time

As discussed earlier, having a direct property like Time.FixedDelta would eliminate the need for the verbose 1f / ProjectSettings.Physics.FixedUpdateFrequency expression. This would make the code cleaner, more readable, and less prone to errors. Developers could simply use Time.FixedDelta whenever they need the fixed delta time, making the code's intent immediately clear.

Time.Fraction: Simplifying Interpolation Calculations

The proposed Time.Fraction property would provide direct access to the interpolation fraction, eliminating the need for the workaround involving a separate system to track time. This would greatly simplify the process of implementing custom interpolation and make it more accessible for developers of all skill levels. The suggested implementation, as outlined in the original post, involves tracking the time since the last FixedUpdate and dividing it by the fixed delta time:

// In FixedUpdate somewhere
FixedTime.TimeSinceLastFixedUpdate = 0f;

// ..?
public static float Fraction => TimeSinceLastFixedUpdate / Delta;

This implementation is straightforward and efficient. By making this value readily available, the engine would empower developers to create smoother and more visually appealing games with less effort.

Benefits of the Proposed Changes

Implementing these changes would bring several key benefits to developers working with physics in the engine:

  • Improved Code Readability: Using Time.FixedDelta and Time.Fraction makes the code's intent clearer and easier to understand.
  • Simplified Development: Eliminating the need for workarounds reduces the complexity of implementing physics-based mechanics.
  • Increased Efficiency: Direct access to these values streamlines the development process and reduces the potential for errors.
  • Enhanced Performance: While the performance impact of the workaround might be minimal, using direct properties is always more efficient than running custom systems.
  • Better Visual Quality: Easier access to the interpolation fraction makes it simpler to implement smooth visual updates, leading to a more polished final product.

Conclusion: A Step Towards Developer Empowerment

The request to add Time.FixedDelta and Time.Fraction to the Time class is a significant step towards empowering developers to create better games with less hassle. These seemingly small changes can have a substantial impact on the development workflow, making it easier to implement smooth, accurate physics-based mechanics. By providing direct access to these essential values, the engine can help developers focus on the creative aspects of game development rather than getting bogged down in complex workarounds. So, let's hope the powers that be take note and implement these changes! Happy coding, guys!