Godot: SpringBoneSimulator3D Resets On Property Update

by Alex Johnson 55 views

Introduction

This article addresses a specific issue encountered in Godot 4.4.1 stable regarding the SpringBoneSimulator3D. Specifically, the simulation unexpectedly resets when attempting to update properties like gravity, gravity direction, or stiffness at runtime. This behavior can be a significant limitation when trying to create dynamic and reactive effects, such as simulating wind forces on a balloon-like component.

The SpringBoneSimulator3D node in Godot is a powerful tool for creating realistic and dynamic bone animations, often used for effects like hair, clothing, or other flexible objects. However, the observed reset issue hinders the ability to dynamically adjust the simulation parameters, impacting the creation of more complex and interactive game elements. This article will delve into the details of the issue, provide a step-by-step guide to reproduce it, and discuss potential workarounds or solutions.

Issue Description

The primary problem lies in the unexpected reset of the SpringBoneSimulator3D simulation whenever properties such as gravity or stiffness are modified during runtime. Imagine a scenario where you're trying to simulate a balloon affected by varying wind speeds. Ideally, you'd use the set_gravity and set_gravity_direction functions to emulate changes in wind forces. However, each time you adjust these values, the simulation restarts, leading to jarring and unrealistic behavior.

This limitation significantly restricts the ability to create reactive effects that respond to changing game conditions. For instance, if you wanted to create a dynamic animation that adapts to player actions or environmental changes, the SpringBoneSimulator3D's reset behavior would prevent smooth and continuous transitions. The desired outcome is for the simulation to seamlessly adapt to the new property values without a complete restart, allowing for more fluid and believable animations.

Tested Versions

This issue has been confirmed in the following version:

  • Godot 4.4.1 stable

System Information

The issue was observed on the following operating system:

  • Linux Mint

However, the problem is not necessarily limited to this specific operating system and may occur on other platforms as well.

Steps to Reproduce

To replicate the issue, follow these steps:

  1. Create a new Godot project or use an existing one.
  2. Add a SpringBoneSimulator3D node to your scene.
  3. Create a chain of bones as children of the SpringBoneSimulator3D node. A chain of 3-5 bones is sufficient to observe the issue.
  4. Write a script that modifies the gravity, gravity_direction, or stiffness property of the SpringBoneSimulator3D node at runtime. This can be triggered by a timer, a player input, or any other event.
  5. Run the scene and observe the behavior of the bone chain. You will notice that whenever the script modifies the specified properties, the simulation resets, causing a sudden jump or discontinuity in the animation.

Example Script

Here's an example of a script that demonstrates the issue:

extends Node3D

@onready var spring_bone_simulator: SpringBoneSimulator3D = $SpringBoneSimulator3D

func _ready():
	# Set a timer to change gravity every few seconds
	var timer = Timer.new()
	add_child(timer)

timer.timeout.connect(_on_timer_timeout)

	# Start the timer

	print("starting timer")


timer.start(2)


func _on_timer_timeout():
	# Change the gravity direction randomly
	spring_bone_simulator.gravity_direction = Vector3(randf_range(-1, 1), randf_range(-1, 1), randf_range(-1, 1)).normalized()

	print("Gravity direction changed to: ", spring_bone_simulator.gravity_direction)

Attach this script to a node in your scene, making sure to adjust the @onready variable to correctly reference your SpringBoneSimulator3D node. Run the scene, and you should observe the simulation resetting every time the timer triggers and the gravity direction is updated.

Minimal Reproduction Project (MRP)

A minimal reproduction project (MRP) is available to demonstrate the issue. You can download it from the following link:

MRP.zip

This project contains a simple scene with a SpringBoneSimulator3D node and a script that modifies the gravity direction at runtime, allowing you to easily observe the reset behavior.

Possible Causes

The exact cause of this issue is currently unknown. However, here are some possible explanations:

  • Internal Reset Mechanism: The SpringBoneSimulator3D node might have an internal mechanism that resets the simulation whenever certain properties are changed. This could be due to the way the simulation is initialized or updated.
  • Dependency Issues: The simulation might depend on certain initial conditions or cached values that become invalid when properties are modified. This could lead to a reset to ensure the simulation remains stable.
  • Engine Bug: It's also possible that this behavior is a bug in the Godot engine itself. In this case, a fix would need to be implemented in the engine's source code.

Potential Workarounds

While a definitive solution may require a fix from the Godot developers, here are some potential workarounds that you can try:

  • Gradual Property Changes: Instead of directly setting the new values, try gradually interpolating between the current and target values over a short period of time. This might help to avoid triggering the reset mechanism.
  • Alternative Simulation Methods: Explore alternative methods for simulating the desired effects. For example, you could use a combination of tweens and animation players to achieve a similar result without relying on the SpringBoneSimulator3D node.
  • Custom Spring Bone Implementation: If you require more control over the simulation, you could consider implementing your own spring bone system using GDScript or C++. This would allow you to avoid the limitations of the built-in node.
  • Delay the Property Change: Try delaying the property change slightly. Use a short Timer to delay the execution of the property update. This might give the SpringBoneSimulator3D node enough time to process the previous frame and avoid the reset.

Conclusion

The unexpected reset behavior of the SpringBoneSimulator3D node when updating properties at runtime can be a significant obstacle when creating dynamic and reactive animations in Godot. While the exact cause remains unclear, this article has provided a detailed description of the issue, steps to reproduce it, and potential workarounds. By understanding the limitations of the SpringBoneSimulator3D node and exploring alternative approaches, you can overcome this challenge and create the desired effects in your games.

Consider reporting this issue on the official Godot Engine GitHub repository to bring it to the attention of the developers. This will increase the chances of a proper fix being implemented in a future version of the engine.

For more information on Godot's 3D physics system, you can refer to the official documentation: Godot Engine Official Documentation.