Introduction to Havok Cloth Preview Tool

The Havok Preview Tool is a core part of the Havok Cloth workflow. It allows you to test, tweak, and debug cloth behavior in real time before committing changes to your final setup.  

This guide walks through the recommended workflow step by step, using Havok version 2025.1 and the Short_Layered_Cloth example from the shipped samples. For a video guide please see here Introduction to Havok Preview Tool  

If you’re new to the tool, this guide will help you get familiar with best practices which will allow you to iterate faster and understand how the Havok Preview Tool can be used to give insight into your simulations.

Launching the Preview Tool

The Runtime Preview Tool can be launched in two different ways: 

• • • From inside the Cloth Setup Tool 

• • • As a standalone window 

For most users, especially those new to Havok Cloth, the recommended approach is to launch the preview tool directly from within the Cloth Setup Tool. 

1. 1. 1. Open your cloth asset in the Cloth Setup Tool. 
      2. Press Preview at the top of the window. 
      3. The asset will load into the Runtime Preview tab. 
      4. If there is no camera in the scene, press Zoom All at the bottom of the window to center the asset in the view. 

Enabling Cloth Tools

Before making edits, make sure the Cloth Tools UI is visible: 

1. 1. 1. Check the left side of the window for the Cloth Tools panel. 
      2. If it’s missing, press Toggle Simulation Controls at the bottom of the viewport. 
      3. In the right-hand panel, go to the Cloth tab and enable Show Cloth Tools. 
      4. The Cloth Tools menu will now appear on the left. 
      5. Press Toggle Simulation Controls again to close the right-hand panel. 

The Runtime Preview Tool acts as a sandbox: 

• • • Changes made here are temporary. 

• • • To make edits permanent, they must be applied in the Property Editor on the left and saved with Ctrl+ S or the Save button at the top of the screen  

This separation allows you to experiment freely without accidentally committing changes. 

We recommend using an AABB comparison to evaluated cloth changes and compare behavior across multiple variations side by side. 

Setting Up an AABB Comparison 

1. 1. 1. Open Cloth Entity Manager. 
      2. Click Add Entity.
      3. Press Zoom All to frame both assets. 

Positioning the Assets 

To clearly compare behavior: 

1. 1. 1. Open Transform in the Cloth Entity Manager. 
      2. Select the entity you want to move. 
      3. Enable the AABB checkbox to visualize its bounding box. 
      4. Rotate and translate the new asset along the X-axis until both assets can both be viewed easily at once.

The coordinate system is visible in the bottom-right corner of the viewport to help with alignment. 

Once your entities are positioned, collapse the Cloth Entity Manager and expand Cloth Tweaker. 

Key fields include: 

• • • • • Entity: Which character/entity you are editing 

• • • • • Cloth: Which cloth piece is selected 

• • • • • Sim-Cloth: The specific simulation instance 

For layered assets (for example, a cape with inner and outer layers), each layer has its own Sim‑Cloth. 

Example:

Editing Gravity Per Layer 

1. 1. 1. Select Short_Cape Sim. 
      2. Open Sim‑Cloth > Simulation. 
      3. Reduce Gravity Scale. 
         • Only the short cape is affected. 
      4. Click Reset Simulation to revert. 
      5. Switch to Cape Sim (the longer cape). 
      6. Adjust gravity again and observe that only the long cape responds. 

This lets you fine-tune layered cloth independently. 

The Runtime Preview Tool is ideal for testing aesthetic and behavioral changes, such as wind response. 

Enabling Wind 

1. 1. 1. Expand Wind > Wind Settings
      2. Set Wind Type to Aerodynamic
      3. Set Wind Active to On. 
      4. Go to Wind Properties
      5. Adjust wind strength and direction. 

Your cloth will immediately respond to the wind simulation. 

Aerodynamics Settings 

1. 1. 1. Expand Tweak Sim‑Cloth Aerodynamics > Aerodynamics. 
      2. Key parameters include: 
         • Drag 
         • Lift 
         • Surface Density 

Surface Density is a core parameter: 

• • • Higher values reduce wind influence (good for heavier materials like leather or linen). 

• • • Lower values increase responsiveness (useful for light fabrics like silk or tulle). 

• • • Extremely low values may produce unrealistic results. 

Saving Aerodynamic Tweaks 

To keep the changes: 

1. 1. 1. Find the corresponding property in the Property Editor:
         • SimCloths > Cape_Sim > Advanced Options > Particle‑Force Interaction > Surface Density 
      2. Set the value to match your preview tweak. 

Make sure to hit Ctrl+S or the Save button at the top to save your changes.

Adding an Animation File

1. Go to File > Add
2. Select an animation file such as:
   • Male_idle.hkt
3. Press Open.
4. The animation should begin playing immediately
   • Run Animation is enabled by default and can be seen in the top right.

Adjusting Simulation Damping

1. 1. 1. Go to Cloth Tweaker > Tweak Settings.
      2. Expand Sim‑Cloth > Simulation.
      3. Adjust the Damping value.
         • Try a very high setting – this will cause the motion to stop quickly
         • Then a very low setting – this will let the motion continue for longer

Interactive Cloth Movement – Mouse Picking

You can mouse-pick at the cloth to make it move to test various behaviors.

• • • Press Spacebar to “pick up” the cloth
    • Moving your mouse moves it in the same direction
    • Release spacebar to drop the cloth

The Runtime Preview Tool is also extremely useful for debugging. 

Constraint Stiffness 

1. 1. 1. Go to Cloth Tweaker > Sim-Cloth > Constraint Stiffness. 
      2. Expand the menu to see the five main constraint types. 

Each constraint displays an effective percentage: 

• • • 1.0 = full effect 

• • • 0.0 = constraint disabled 

Process of Elimination 

When constraints fight each other: 

1. 1. 1. Reduce each constraint stiffness to 0, one at a time. 
      2. Observe when the problematic behavior disappears. 
      3. Identify the constraint causing the issue. 

From there, you can inspect the setup and viewers to diagnose the root cause. 

Tip: Click any option and press F1 for context-sensitive help. Tooltips also provide detailed information.

Cloth Viewers display debug information directly in the scene. 

1. 1. 1. Go to Cloth Viewers > Cloth > Simulation > Constraint. 
      2. Select Local Range. 

By default: 

• • • showConstraintShape is enabled, visualizing the constraint volume. 

You can also: 

• • • Disable showConstraintShape. 

• • • Enable showReferenceBuffer to see reference vertices. 

When finished, click Disable All Cloth Viewers. 

Isolating Simulated Cloth 

To view only the cloth: 

1. 1. 1. Go to Cloth World > World Rendering. 
      2. Set Purely Skinned Cloth Meshes to Off. 

This removes static geometry and isolates simulating cloth. 

The Runtime Preview Tool supports much more than basic parameter tweaking: 

• • • Testing transitions between simulation and skinning 

• • • Running cloth with animations 

• • • Verifying behavior with different World Stepping settings 

Ideally, these tools would be integrated directly into your engine, allowing in‑engine tuning—especially valuable for cinematics and rapid iteration. 

For situational tweaks (for example, cinematics‑specific constraint changes), settings can be saved to files and reloaded later. Engines can generate compatible files that the Cloth Setup Tool can process. 

The Havok Preview Tool is a powerful environment for iteration, comparison, and debugging. By combining functionality like AABB comparisons, Sim‑Cloth‑specific tuning, and constraint visualization, you can dramatically speed up your cloth development workflow while gaining deeper insight into how your setups behave.

Havok Cloth empowers you to create unique cloth simulation with complete artistic control and a full suite of tools to support your development process.

To find out more about Havok Cloth and how it can super-power your game development, see Havok Cloth for proprietary engines or Havok Cloth for Unreal, or reach out to us today.