# Fracturing Geometry Collections User Guide

## Overview of the various methods of fracturing Geometry Collections.

You can find similar information in video format in the Epic Developer Community site by watching the Fracture and Clustering tutorials.

The Fracture Mode is an editor mode that contains a wide variety of tools for creating, fracturing, and manipulating Geometry Collections. Geometry Collections are the asset type used by Chaos Destruction for simulating real-time fracturing in Unreal Engine.

The available fracturing tools provide users with a lot of control over how the Geometry Collection is fractured. This includes the number of fractured pieces and how they relate to each other (fracture hierarchy).

Each fracture tool uses a different method, or algorithm, to generate different fracture patterns. Each method comes with a variety of options to further customize the generated pattern, including adding randomization.

In this guide you will learn to use the various fracturing tools available in the Fracture Mode.

Before learning about the Fracture Mode, you should know how to create Geometry Collections from Actors in the Level. If you are not familiar with the process, refer to the Geometry Collections User Guide.

## Fracture the Geometry Collection

Before using the fracturing tools, create a Geometry Collection from a Static Mesh Actor in your Level and select it.

You can now access the available fracture tools under the Fracture section of the Fracture Mode panels. Each tool can be applied to the Geometry Collection as a whole, or to selected fractured pieces (individual bones) after fracturing.

When applying a fracture method to the Geometry Collection, a new fracture level is created. This is reflected in the Geometry Collection's Fracture Hierarchy window.

A Geometry Collection's fracture hierarchy resembles a tree structure. It contains a root bone with one or more child bones which make up the fractured pieces. Each child bone can, in turn, contain its own child bones.

The levels in the fracture hierarchy represent the structure of the tree, with four levels representing a tree-like structure with three branching levels of child bones.

You can use different fracturing tools each time you fracture the Geometry Collection, creating different fracture patterns per level in the Fracture Hierarchy.

You can select multiple bones (fractured pieces) in the Geometry Collection by holding CTRL and selecting them directly in the viewport. You can also use the provided selection tools to quickly select a set of bones directly.

### Reset the Geometry Collection

After fracturing the Geometry Collection, you can reset it to its original state by clicking the Reset button under the Generate section. This sets the Geometry Collection to its original state when it was created without any applied fracturing. This can be helpful when trying different fracture methods until you find the configuration that provides the desired results.

### View Settings

The View Settings help you visualize what the Geometry Collection looks like after applying any fracture operation.

Option

Description

Explode Amount

Displays what the Geometry Collection will look like when it fractures during gameplay. A value of 1 will separate all bones and will show what a completely fractured Geometry Collection looks like.

Hide UnSelected

Hides the bones that are not currently selected in the Geometry Collection. This helps you focus on specific bones while applying a fracture method.

Fracture Level

Defines which fracture level to visualize. Selecting All will display the bones for all fracture levels.

### Common options included with most fracture methods

All fracture tools have these Common Fracture options:

Option

Description

Random Seed

This value is used to generate the random fracturing pattern over the Geometry Collection. A -1 means a random seed value will be used every time a new fracture operation is applied. Specifying a value generates a fracture pattern tied to that seed number, which when entered will always generate that same fracture pattern.

Chance to Fracture

Sets the chance that a bone is likely to be fractured during the fracture operation where 1 is equal to 100%, meaning all bones will be fractured. 0 is equal to 0% chance of any bone being fractured.

Group Fracture

Generates a fracture pattern across all selected meshes.

Draw Sites

Draws points in the center of the bones to be cut out by the fracture pattern.

Draw Diagram

Draws the fracture pattern diagram on top of the Geometry Collection.

Grout

Defines the amount of space to leave between cut piece.

Most fracture methods share these Noise options:

• Amplitude: Defines the size of the perlin noise displacement in centimeters. A value of 0 indicates that no noise will be used when generating the fracture pattern.

• Frequency: Defines the period of the perlin noise. Smaller values produce a smoother noise pattern, and larger ones produce a rougher noise pattern.

• Persistence: Defines the persistence of the perlin noise layers. For each layer (octave) after the first, the amplitude of the perlin noise will be scaled by this factor.

• Lacunarity: Defines the lacunarity of the perlin noise layers. For each layer (octave) after the first, the frequency of the perlin noise will be scaled by this factor.

• Octave Number: Defines the number of fractal layers (octaves) of perlin noise that will be applied to the fracture pattern. Each layer is additive, with Amplitude and Frequency scaled by Persistence and Lacunarity, respectively. Smaller values (1-2) will generate smooth patterns, while larger values will generate more craggy patterns.

• Point Spacing: Distance (in centimeters) between vertices on cut surfaces where noise is added. Larger spacing between vertices results in more efficient meshes with fewer triangles. However, this also results in lower overall resolution to see the shape of the added noise.

## Use the Fracture Tools

Each Fracture tool has its own settings that provide relevant options to achieve the desired results.

### Uniform Fracture

The Uniform tool uses a Voronoi algorithm to generate the fracture pattern. Enter a minimum and maximum number of Voronoi sites (or number of fractured pieces) and the algorithm will choose a random value within the range.

In the fractured Geometry Collections below, the one on the left has the Min and Max Voronoi Sites set to 20. This means that the fractured Geometry Collection will have 20 fractured pieces. The example on the right has the Min and Max Voronoi Sites set to 100.

20 Voronoi Sites

100 Voronoi Sites

Click image for full size.

Click image for full size.

### Cluster Fracture

The Cluster fracture tool is an extension of the Uniform fracture method where both use a Voronoi algorithm when generating their fracture pattern. While the Uniform Voronoi method produces a relatively even distribution of sites, the Cluster method gathers its sites together into islands of close proximity, creating a more varied fracture pattern compared to the uniform method.

Option

Description

Min Num Clusters

Defines the minimum number of Voronoi clusters that will be created.

Max Num Clusters

Defines the maximum number of Voronoi clusters that will be created.

Min Sites per Cluster

Defines the minimum number of Voronoi sites per cluster.

Max Sites per Cluster

Defines the maximum number of Voronoi sites per cluster.

Min Dist from Center

Max Dist from Center

Defines the maximum cluster radius (in centimeters). The Cluster Radius Offset will be added to this value.

Defines the radius offset of the cluster (in centimeters) that is added to the minimum and maximum distance from the center.

The examples below use the following Cluster settings as a starting point:

Setting

Value

Min Num Clusters

2

Max Num Clusters

2

Min Sites per Cluster

5

Max Sites per Cluster

5

Min Dist from Center

0.1

Max Dist from Center

0.1

0

The following have also been set under the Common Fracture settings:

• Draw Sites: Enabled

• Draw Diagram: Disabled

In the example below you can see the difference between setting the Min / Max Number of Clusters from 2 to 5.

In the example below you can see the difference between setting the Min / Max Sites per Cluster from 5 to 10.

In the example below you can see the difference between setting the Min / Max Distance from Center from 0.1 to 0.2.

In the example below you can see the difference between setting the Cluster Radius Offset from 0 to 10.

The Radial fracture tool creates a distribution of Voronoi sites that operate from a center point and radiate outwards. The center position is controlled by manipulating the gizmo inside the Viewport.

If you want to place it at a specific location, you can enter the Center and Normal rotation (1) of the center point directly in the Radial Voronoi options. To activate those fields, disable the Use Gizmo checkbox (2) in the Placement Controls section.

The Radial fracture tool comes with the following options:

• Center: Defines the center location of the generated fracture pattern. This option is only available if you disable the Use Gizmo checkbox.

• Normal: Defines the normal rotation of the plane used to generate the fracture pattern. This option is only available if you disable the Use Gizmo checkbox.

• Radius: Defines the radius of the pattern from the center location.

• Angular Steps: Defines the number of angular steps used to generate the fracture pattern. These steps subdivide the circumference of the diagram.

• Radial Steps: Defines the number of radial steps used to generate the fracture pattern. This affects the number of times the pattern is divided outwards from the center.

• Angle Offset: Defines an angular offset (in degrees) used for every radial step.

• Variability: Defines the amount to randomly space each generated Voronoi site (in centimeters).

### Planar Fracture

The Planar fracture tool is used to create a planar cut in the Geometry Collection. As cuts are created in the Geometry Collection, the gizmo resets to the centroid of your current selection. This is helpful if you are trying to make deliberate cuts as you select different Bones in the Viewport.

You can disable the gizmo resetting to the center of your current selection by disabling Center on Selection under the Placement Controls section.

It's important to note that the plane used for each cut extends to infinity, meaning that even if the gizmo displays the plane at a specific size, the cut will extend in the plane's direction to cut the entire Geometry Collection.

You can also forgo using the gizmo by disabling Use Gizmo under the Placement Controls section.

Disabling the gizmo enables you to set a Number of Cuts under the Plan Cut section. This results in a random number of cuts to the Geometry Collection.

In the example below you can see the difference between setting the Number of Cuts from 1 to 10.

You can change the placement of these cuts by changing the Random Seed value under the Common Fracture section.

### Slice Fracture

The Slice fracture tool is an extension of the Planar fracture tool with the added ability to set the number of cuts along each axis. This allows for initial cuts to be aligned into columns and rows. Slices can also have random angles and offset variations applied to them.

Option

Description

Slices X

Defines the number of slices along the X axis.

Slices Y

Defines the number of slices along the Y axis.

Slices Z

Defines the number of slices along the Z axis.

Random Angle Variation

Defines the maximum angle (in degrees) to randomly rotate each slicing plane.

Random Offset Variation

Defines the maximum distance (in centimeters) to randomly shift each slicing plane.

In the example below you can see the difference between setting the Random Angle Variation from 0 to 35.

In the example below you can see the difference between setting the Random Offset Variation from 0 to 50.

### Brick Fracture

The Brick fracture tool is considered experimental and may receive significant changes in future versions of the engine.

The Brick fracture tool generates a customizable brick pattern. You can specify how the bricks are arranged and their size for when the pattern is applied to the Geometry Collection.

Option

Description

Bond

Sets the brick bond pattern that determines how the bricks are arranged in the fracture pattern. You can select from: Stretcher, Stack, English, Header, and Flemish.

Brick Length

Defines the brick length in centimeters.

Brick Height

Defines the brick height in centimeters.

Brick Depth

Defines the brick depth in centimeters.

The Example below shows the Stretcher bond method applied to the Geometry Collection.

### Mesh Fracture

The Mesh fracture tool uses the shape of a Static Mesh to generate the fracture pattern. This is useful if you want to create very specific pattern shapes.

Drag a Static Mesh into the level and place it so it intersects the Geometry Collection.

Click Mesh and go to the Mesh Cut section in the Fracture panel.

Click the Cutting Actor dropdown. Select the Static Mesh actor you dragged into the level. Alternatively, you can click the Eye-Dropper button and click on the Static Mesh in the viewport.

Click Fracture to cut the Geometry Collection in the shape of the Cutting Actor. To see the results, select the Cutting Actor in the Viewport and move it.

The following Cut Distributions are available with the tool:

Option

Description

Single Cut

Generates a single cut at the current location of the Cutting Actor.

Uniform Random

Scatters the Cutting Actor in a uniform random distribution around the Geometry Collection's bounding box.

Grid

Arranges the Cutting Actor in a regular grid pattern across the Geometry Collection.

The following options are available for the Uniform Random Cut Distribution:

Option

Description

Number to Scatter

Defines the number of Actors to randomly scatter.

Min Scale Factor

Defines the minimum scale factor to apply to the cutting meshes. A random scale will be chosen between Min and Max.

Max Scale Factor

Defines the maximum scale factor to apply to the cutting meshes. A random scale will be chosen between Min and Max.

Random Orientation

Whether to randomly vary the orientation of the cutting actors.

+/- Roll Range

Defines the range that will be used to pick a random roll (rotation in X axis) for the cutting Actors.

+/- Pitch Range

Defines the range that will be used to pick a random pitch (rotation in Y axis) for the cutting Actors.

+/- Yaw Range

Defines the range that will be used to pick a random yaw (rotation in Z axis) for the cutting Actors.

In the example below you can see the difference between setting the Number to Scatter from 5 to 10.

In the example below you can see the difference between enabling and disabling the Random Orientation.

The following options are available for the Grid Cut Distribution:

Option

Description

Grid Width

Defines the number of cutting meshes to add to the X-axis of the grid.

Grid Depth

Defines the number of cutting meshes to add to the Y-axis of the grid.

Grid Height

Defines the number of cutting meshes to add to the Z-axis of the grid.

Variability

Defines the magnitude of the random displacement of the cutting Actors.

Min Scale Factor

Defines the minimum scale factor to apply to the cutting Actors.

Max Scale Factor

Defines the maximum scale factor to apply to the cutting Actors.

Random Orientation

Whether to randomly vary the orientation of the cutting Actors.

In the example below you can see the difference between enabling and disabling the Random Orientation.

### Custom Fracture

The Custom fracture tool is the most extensive fracturing tool included with the Fracture Mode.

Option

Description

Pattern

Defines the pattern used to generate the Voronoi sites.

Normal Offset

Defines the offset value used for each vertex's normal direction.

Variability

Defines the amount to randomly offset each Voronoi site in centimeters.

Defines the number of Voronoi sites to add to the pattern.

Skip Fraction

Defines the fraction of points that will not be fractured (skipped).

Skip Mode

Defines the method used for skipping the points that will not be fractured.

With this tool, the Fracture Diagram is considered its own entity that can be moved relative to the Geometry Collection. This is useful when looking to create more customized fracture patterns.

The gizmo is centered

The gizmo is moved to a corner

Click image for full size.

Click image for full size.

To better visualize the fracture diagram, you can freeze positional data for the fracture sites. Under the Common Fracture section, enable Draw Sites and disable Draw Diagram.

In addition, under the Live Voronoi Sites section. set the Pattern to Centered and set the Variability to 40.

Doing this enables you to move the gizmo to a desired location on the Geometry Collection. In the Fracture Mode panel under the Custom Voronoi section, click the Freeze Live Sites.

The gizmo is placed at the top left corner

The gizmo is moved revealing the frozen sites

Click image for full size.

Click image for full size.

You can continue this process to deliberately place sites on the Geometry Collection. Once you are ready, click Fracture to see the results of all the sites placed on the Geometry Collection.

Since the fracture diagram is separate from the Geometry Collection, you can make changes to the Geometry Collection without affecting the fracture itself.

For example, if you set the Sites to Add to 2000, scale down the Geometry Collection to the shape of a column and click Fracture, you get the fracture pattern applied to the current shape of the Geometry Collection.

Rescale the Geometry Collection to its original size and notice how the fracture pattern is now stretched, resembling splinters of wood.

Because the fracture diagram isn't directly tied to the Geometry Collection, when you rescale the Geometry Collection, the fracture diagram remains the same. This means that you can create unique fracture patterns that fit a stylistic choice for your project, like this one with elongated fractures.

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