Best Practices for Adding Soft Bodies to Skeletal Meshes in UE4.27 for VR Projects

Optimizing Softbody Physics for Realistic VR Experiences on the Valve Index

Key Takeaways

Utilize Physics Assets and Physical Animation Components to achieve softbody-like effects with optimized performance.
Optimize physics simulations for VR by reducing complexity, using Level of Detail (LOD), and limiting active physics bodies.
Consider third-party plugins or alternative simulation methods like cloth simulation for more advanced softbody behaviors.

1. Preparation of the Skeletal Mesh

Importing and Rigging

Begin by importing your skeletal mesh model into Unreal Engine 4.27. Ensure that the mesh is properly rigged with a skeleton, with bones correctly assigned for the areas requiring softbody physics, such as the breasts. Use 3D modeling software like Blender or Maya to create or modify your mesh if needed. Proper rigging is crucial as it forms the foundation upon which physics simulations are applied, ensuring that deformations occur naturally and realistically.

Steps to Import and Rigging:

Import the Skeletal Mesh:
- Open Unreal Engine and navigate to the Content Browser.
- Import your skeletal mesh by dragging it into the desired folder or using the Import button.
- Ensure that the import settings are correctly configured, preserving the skeleton hierarchy and bone assignments.
Verify Bone Hierarchy:
- Open the skeletal mesh in the Skeletal Mesh Editor.
- Check that all necessary bones, especially those in the softbody regions, are present and correctly positioned.
- Adjust bone weights and influences to ensure smooth deformations during animations and physics simulations.
Exporting from Modeling Software:
- If modifications are needed, export the mesh from your 3D modeling software in a compatible format (e.g., FBX).
- Re-import the updated mesh into Unreal Engine, replacing the existing version if necessary.

2. Setting Up Physics Assets (PHAT)

Creating and Configuring Physics Bodies

Unreal Engine's Physics Asset Tool (PHAT) allows you to define physics bodies for your skeletal mesh. These physics bodies act as the physical representation of the mesh's bones, enabling them to interact with forces and collisions in the environment.

a. Creating a Physics Asset

Navigate to the Content Browser, right-click on your skeletal mesh, and select Create > Physics Asset.
This action generates a new Physics Asset with default collision shapes (typically capsules or spheres) automatically assigned to the mesh's bones.
Open the newly created Physics Asset to begin customization.

b. Refining Collision Shapes

Within the Physics Asset Editor, identify the bones corresponding to the softbody regions (e.g., breasts).
Select the default collision shapes and replace them with more suitable forms like spheres or capsules that better encapsulate the desired soft areas.
Adjust the size, scale, and position of these collision shapes to closely fit the mesh, ensuring that physics interactions appear natural.

c. Setting Up Constraints

Add constraints between physics bodies to control how they interact and move relative to each other.
Select two adjacent physics bodies and create a constraint by right-clicking and choosing Add Constraint.
Configure the constraint's properties:
- Spring Settings: Define the spring strength and damping to simulate soft, elastic movements.
- Linear Limits: Restrict the range of motion to prevent unnatural stretching or compression.
- Angular Limits: Control rotational movement, allowing for realistic sway and jiggle without overextending.
Repeat this process for all necessary bone pairs to establish a network of controlled, yet flexible, movements within the softbody regions.

3. Implementing Softbody Physics

Using Physical Animation Components

The Physical Animation Component enhances softbody-like effects by influencing specific bones with physics-based animations, allowing for more precise control over movements while maintaining performance efficiency.

a. Adding Physical Animation Component

Open your character or mannequin Blueprint in Unreal Engine.
Select the skeletal mesh component to which you want to add softbody physics.
In the Components panel, click Add Component and search for Physical Animation.
Add the Physical Animation Component to your skeletal mesh.

b. Configuring Physical Animation

With the Physical Animation Component selected, configure its properties in the Details panel:
- Physical Animation Profile: Choose or create a profile that defines stiffness and damping settings tailored to your softbody requirements.
- Target Bodies: Specify the bones or physics bodies that will be influenced by the Physical Animation Component, such as those in the breasts.
- Strength Parameters: Adjust parameters like Stiffness and Damping to control how much the physics simulation affects each bone.
Fine-tune these settings to balance between natural movement and performance, ensuring that the softbody effects appear realistic without taxing the system.

Alternative Methods: Cloth Simulation

If more detailed deformation is necessary, particularly for clothing or fabric-like meshes, cloth simulation can be an effective alternative. This method can achieve more nuanced softbody behaviors but may require additional setup and optimization.

a. Applying Apex Cloth

In your 3D modeling software (e.g., Blender or Maya), define cloth zones on the mesh where softbody behavior is desired.
Export the mesh with cloth data, ensuring compatibility with Unreal Engine's Apex Cloth system.
Import the mesh into Unreal Engine, and open it in the Skeletal Mesh Editor.
Within the editor, apply Apex Cloth simulations to the designated zones, configuring parameters such as stiffness, damping, and collision settings to achieve realistic movements.

b. Configuring Cloth Parameters

Adjusting the cloth simulation settings is crucial for balancing realism and performance:
- Stiffness: Controls how resistant the cloth is to deformation. Higher stiffness results in less movement.
- Damping: Determines how quickly movement dissipates. Higher damping reduces oscillations.
- Collision Settings: Define how the cloth interacts with other objects and the skeletal mesh itself to prevent clipping and unnatural intersections.
Test the cloth simulation extensively to ensure that it behaves as expected within the VR environment, making adjustments as necessary to optimize performance.

4. Utilizing Third-Party Plugins and Advanced Simulations

Chaos Physics and NVIDIA Flex

For more sophisticated softbody physics beyond what Unreal Engine's built-in tools offer, third-party plugins can provide enhanced simulation capabilities, allowing for more realistic and detailed softbody behaviors.

a. Enabling Chaos Physics

Navigate to the Plugins panel via Edit > Plugins.
Search for Chaos Physics and enable it.
Restart Unreal Engine if prompted to apply the changes.
Note that in UE4.27, Chaos Physics may still be experimental and have certain limitations. Consider upgrading to a newer Unreal Engine version if you require stable Chaos Physics features.

b. Integrating NVIDIA Flex

NVIDIA Flex offers real-time particle-based simulations that can achieve highly detailed softbody behaviors.
Download and install the NVIDIA Flex plugin compatible with UE4.27.
Follow the plugin's documentation to integrate Flex simulations with your skeletal mesh.
Configure Flex parameters to fine-tune the softness, responsiveness, and interaction of the softbody regions.
Be aware that integrating NVIDIA Flex may require significant customization and optimization to work seamlessly within your project.

Choosing the Right Plugin

Assess your project's specific requirements to determine if advanced plugins like Chaos Physics or NVIDIA Flex are necessary:
- If your project demands highly realistic softbody simulations with complex interactions, NVIDIA Flex might be suitable.
- For general softbody physics with moderate complexity, Unreal Engine's native Chaos Physics (if stable) could suffice.
Evaluate the performance impact of each plugin, especially considering the stringent performance demands of VR environments.
Ensure compatibility and support within UE4.27 before committing to a third-party solution, and consider the availability of documentation and community support.

5. Performance Optimization for VR

Maintaining High Frame Rates

VR experiences demand high and stable frame rates to maintain immersion and prevent motion sickness. To ensure performant softbody physics:

a. Limiting Physics Complexity

Apply softbody physics selectively to essential parts of the mesh, such as the breasts, rather than applying it uniformly across the entire skeletal structure.
Use low-polygon proxy meshes for physics simulations to reduce computational load, with the detailed high-polygon mesh used solely for rendering purposes.
Minimize the number of active physics bodies and constraints to what is strictly necessary for achieving the desired visual effect.

b. Implementing Level of Detail (LOD)

Set up multiple LODs for your skeletal mesh, each with varying levels of physics simulation complexity:

LOD0: Highest detail with full physics simulations for close-up views.
LOD1: Reduced physics detail for mid-range views.
LOD2 and beyond: Minimal or no physics simulations for distant views to conserve resources.

Configure each LOD's physics settings to progressively simplify the simulation, ensuring that only the necessary level of detail is processed based on the object's distance from the camera.

c. Optimizing Physics Settings

Adjust iteration counts for physics simulations to balance between accuracy and performance. Lower iteration counts can significantly boost performance with a minor trade-off in simulation precision.
Enable sub-stepping in the physics engine to maintain simulation stability, especially at high frame rates required for VR:

Navigate to Project Settings > Engine - Physics and adjust the Max Substeps and Substep Delta Time to fine-tune physics accuracy.

Disable physics simulations for mesh parts that are not currently visible or interacting with the environment to reduce unnecessary computations.

d. Profiling and Testing

Utilize Unreal Engine’s built-in profiling tools, such as the Unreal Insights and Stat Commands, to monitor physics performance and identify bottlenecks.
Regularly test your VR experience on the Valve Index to ensure that frame rates remain consistent, particularly after adding or tweaking physics features.
Iteratively refine the physics setup based on profiling data to achieve the best balance between realism and performance. Pay close attention to metrics like CPU usage, frame time, and GPU load.

Maintaining VR Performance Standards

VR platforms like the Valve Index require maintaining frame rates of around 90Hz to ensure a smooth and comfortable user experience. Failure to achieve this can result in motion sickness and a lack of immersion. Implement the following strategies to maintain high performance:

Efficient Asset Management: Use optimized meshes and textures. Avoid high-polygon models where unnecessary and use texture atlases to reduce draw calls.
Asynchronous Loading: Load heavy assets asynchronously to prevent frame drops during gameplay.
Batching and Culling: Implement frustum culling and object batching to minimize rendering overhead, especially in complex scenes with multiple softbody objects.

6. Advanced Techniques and Considerations

Custom Blueprint and C++ Implementations

For tailored softbody behaviors, consider implementing custom logic through Blueprints or C++. This allows for greater control and the ability to create unique interaction patterns that are not possible with standard tools.

a. Applying Dynamic Forces

Use Blueprints to apply dynamic forces or impulses to specific bones, creating reactive movement based on user interactions or environmental factors.
For instance, when a user interacts with a mannequin's breast using the Valve Index controllers, apply a force that causes a realistic jiggle or movement.
Implement event-driven triggers that respond to user inputs, ensuring that physics responses are synchronized with user actions.

b. Enhancing Simulation with Constraints

Implement custom constraints that guide how bones move relative to each other, ensuring natural deformations without breaking simulation stability.
Create blueprints that adjust constraint parameters in real-time based on gameplay conditions, such as movement speed or interactions.
Fine-tune these constraints through iterative testing to achieve the desired level of softness and responsiveness.

Combining Multiple Techniques

Achieving realistic softbody physics may require integrating several methods for the best results. Combining Physics Assets with Physical Animation Components, along with alternative methods like cloth simulation or third-party plugins, can enhance the believability and performance of softbody effects.

Physics Assets + Physical Animation: Core method for attaching softbody behavior to specific bones while maintaining performance.
Cloth Simulation: Adds an extra layer of realism for areas that resemble fabric or require more nuanced deformations.
Third-Party Plugins: Provide advanced simulation capabilities that can enhance or supplement native Unreal Engine features.
Custom Blueprints/C++: Allow for unique interaction patterns and conditional physics behaviors that respond dynamically to user inputs or environmental changes.

By strategically combining these techniques, developers can create sophisticated softbody physics that not only look realistic but also perform efficiently within the demanding environment of VR.

7. Example Implementation Workflow

Step-by-Step Guide

To provide a clear roadmap, here is a step-by-step guide to implementing softbody physics on a skeletal mesh for a VR project in UE4.27:

a. Import and Prepare the Skeletal Mesh

Import your rigged skeletal mesh into Unreal Engine.
Ensure that all bones, especially those in the softbody areas, are correctly assigned and weighted.
Test basic animations to verify that the rigging behaves as expected.

b. Create and Configure the Physics Asset

Create a Physics Asset from the skeletal mesh.
Replace default collision shapes with spheres or capsules around the softbody bones.
Add and configure constraints between these bodies to simulate soft, elastic movements.

c. Add and Configure the Physical Animation Component

Open the Blueprint associated with your skeletal mesh.
Add the Physical Animation Component.
Configure the component to target only the softbody bones, setting appropriate stiffness and damping values.

d. Implement Cloth Simulation (If Needed)

Define cloth zones in your 3D modeling software and export the mesh with these settings.
Import the mesh into Unreal Engine and apply Apex Cloth simulations to the designated areas.
Adjust cloth simulation parameters to achieve the desired softbody behavior.

e. Optimize for VR Performance

Set up Level of Detail (LOD) settings to reduce physics complexity at distance.
Limit the number of active physics bodies and constraints to essential areas.
Adjust physics iteration counts and enable sub-stepping for stability.

f. Integrate with VR Controls

Ensure that the Valve Index controllers are properly set up and integrated with Unreal Engine via the SteamVR plugin.
Implement interactions that respond to user inputs, applying forces or triggers that affect the softbody physics.
Test interactions extensively to ensure responsiveness and realism without compromising performance.

g. Profiling and Testing

Use Unreal Engine’s profiling tools to monitor the performance impact of physics simulations.
Test the VR experience on the Valve Index, focusing on frame rates, latency, and the smoothness of physics interactions.
Iteratively refine physics settings based on profiling data to optimize both performance and visual fidelity.

8. Practical Tips for Success

Balancing Realism and Performance

Achieving a realistic softbody effect while maintaining high performance requires strategic decision-making and continual optimization.

Start Simple: Begin with basic physics setups and gradually add complexity as needed, continuously testing performance.
Iterative Testing: Regularly test the VR experience on the target hardware to ensure that performance standards are met.
Leverage Community Resources: Utilize forums, tutorials, and documentation to stay informed about best practices and optimization techniques.

Utilizing Unreal Engine’s Tools

Unreal Engine offers a suite of tools designed to aid in the creation and optimization of complex physics simulations:

Blueprints: Use Blueprints for visual scripting of physics interactions, allowing for rapid prototyping and adjustments without deep programming knowledge.
Matinee and Sequencer: Animate physics parameters over time to create dynamic softbody behaviors in response to in-game events.
Physics Debugging: Enable physics debugging tools to visualize collision shapes, constraints, and forces, aiding in the fine-tuning of simulations.

Best Practices for VR Development

Maintain Ergonomic Interactions: Ensure that physics interactions do not cause discomfort or disorientation for the user.
Efficient Resource Management: Monitor and manage resources effectively to prevent frame drops, which are particularly noticeable in VR.
User Experience Focus: Prioritize smooth and natural interactions over excessive physical accuracy to enhance immersion.

Conclusion

Implementing softbody physics in Unreal Engine 4.27 for VR projects involves a careful balance between achieving realistic movement and maintaining high-performance standards essential for VR. By effectively utilizing Physics Assets, Physical Animation Components, and considering alternative methods like cloth simulation or third-party plugins, developers can create believable softbody effects without compromising on the frame rates required for immersive VR experiences. Optimization techniques such as limiting physics complexity, employing Level of Detail, and diligent profiling are crucial steps in ensuring that the VR application remains responsive and comfortable for users. Remember to iteratively test and refine your setup to find the optimal configuration that meets both visual and performance goals. With these best practices, your VR project on the Valve Index can achieve the desired level of realism and performance, providing users with a seamless and engaging experience.