Chaos Fields use a region of space to control properties of a physics simulation. There are three primary types of fields — Anchor Fields, Strain / Force Fields, and Sleep / Disable Fields.

Anchor Fields are construction Blueprints that constrain a portion of a Geometry Collection in place during simulation. You can use them to make sure a certain part of the Geometry Collection stays fixed in place no matter what happens in the simulation.

Sleep / Disable Fields bring a Geometry Collection to rest when the bone (fractured piece) velocities fall below a specified threshold value. Sleeping bones can be reawakened during the simulation if they come in contact with an active object. Disabled bones cannot be reawakened.

Strain / Force Fields apply strain or linear velocity to a Geometry Collection during simulation.

Unreal Engine comes with pre-built Chaos Fields that can be used as-is, or modified by opening them in the Blueprint Editor.

To find them, click Settings > Show Engine Content in the Content Browser.

Go to Engine > Content > EditorResources > FieldNodes to find the available Chaos Fields.

Creating a Geometry Collection

In this section you will create a Geometry Collection so you can learn to use the Chaos Fields during a fracture simulation.

1. Select a Static Mesh Actor in the scene, click the Mode dropdown and select Fracture.

   This opens the Fracture Mode panels that contain all the tools for fracturing your mesh.

2. In the Generate section, click New to create a new Geometry Collection.

   This asset type will be saved in the Content Browser and will be used to create your fractured mesh.

   1. (1) Select the directory location where the Geometry Collection will be saved.

   2. (2) Enter the name for the Geometry Collection asset.

   3. (3) Click Create Geometry Collection.

   1. Click Save All in the Content Browser to save the new Geometry Collection asset.

3. The Actor you selected in the scene is replaced with the Geometry Collection in the level.

   See the Geometry Collections User Guide to learn more about creating Geometry Collections.

4. Use the Fracturing tools to fracture the Geometry Collection. In the example below, we used the Slice and Uniform Voronoi tools to create 4 Levels in the Fracture Hierarchy.

   See the Fracturing Geometry Collections User Guide to learn more about fracturing Geometry Collections.

The Anchor Field

Anchor Fields set any Geometry Collection bone that falls within its volume to Static during the simulation.

Drag FS_AnchorField_Genericto the level and overlap it with a corner of the Geometry Collection.

Select the Geometry Collection and go to the Details panel. Scroll down to the Chaos Physics section and expand the Initialization Fields array. Click the Index [0] dropdown and select FS_AnchorField_Genericfrom the list.

Go to the Clustering section and disable the Enable Clustering checkbox. This is done for demonstration purposes as it will better show the fracturing of the Geometry Collection.

Click the Play Mode menu button and select Selected Viewport or Simulate to see the results.

You can add as many Anchor Fields as you want by following the steps above. In the example below, we added Anchor Field to the top right corner.

You can also use the same Anchor Field on several Geometry Collections at once. In the example below, we used one Anchor Field on two Geometry Collections.

Anchor Fields come with three Falloff Shapes — Box, Sphere, and Plane. These shapes affect how many bones are set to Static and how the effect is implemented.

To change the Falloff Shape, select the Anchor Field and go to the Details panel. Scroll down to the Default section and click the Anchor Falloff Shape dropdown. Select the desired falloff shape.

In the example below, we use a Sphere shape for the Anchor Field.

In the example below, we use a Plane shape for the Anchor Field.

The Sleep and Disable Fields

The Sleep and Disable Fields bring rigid bodies to rest when their velocities go below a given Threshold value. Higher Threshold values will stop objects more aggressively.

These Fields are used to improve simulation performance and prevent excess jittering of the Geometry Collection pieces as they collide with each other. They also give you more control over how many rigid bodies are part of the simulation.

Sleeping bodies can be reawakened when they collide with an active object. Disabled bodies are removed from the simulation completely after they come to a resting state. Disabled bodies cannot be reawakened.

This type of field also has a Kill option, in addition to Sleep and Disable. This option ignores the Threshold value and immediately stops the body from simulating and removes it from the simulation once it overlaps the Field's volume.

Drag FS_SleepDisable_Generic into the Level and place it as seen below.

Go to the Details panel and scroll down to the Default section. Here you can specify the Threshold value the field uses to apply its effect. You can also specify the Field Behavior, Falloff Shape, and whether you want to show the Debug information.

In the example below we enabled the Debug option and used two Geometry Collections to show the effects of a Sleep Field. On the left, the Threshold value is set to 2, and on the right, the value has been set to 50.

As you can see, when the first object passes through the volume it goes to sleep. However, it is awakened when it collides with the second object. The higher the Threshold value, the more aggressively the bodies will be set to sleep.

In the examples below, we switched to Disable Fields. The Field on the left has a Threshold value of 2, and the Field on the right has a value of 200.

Notice how the Geometry Collections on the right do not collide with each other after they have been disabled. The higher Threshold value shows the effect more prominently.

Finally, we can use Kill Fields to immediately stop any rigid body from simulating. Notice that on the right we show a Static Mesh simulating physics while we use Geometry Collections on the left.

The Master Field

Introduction

The FS_MasterFieldis a pre-built Chaos Field included with Unreal Engine. You can find it by going to Engine > Content > EditorResources > FieldNodes.

This Field comes with many options for customization and is used primarily to:

• Activate rigid bodies by applying external strain that breaks the connection between bones.

• Apply velocity to rigid bodies to make them move and spin in a realistic manner

In the example below, we placed a default FS_MasterField where it overlaps with a Geometry Collection.

In its default configuration, the Field applies a large amount of external strain which causes the Geometry Collection to fracture. Then, the Field adds radial and angular velocities to the broken pieces to make them spin. Finally, the Field applies noise to add randomness to the effect.

This is the result:

You can configure the shape of the Field by clicking the Field Falloff Shape dropdown and choosing an option.

You can also configure the way the Field is applied by clicking the Activation Type dropdown and choosing an option.

You can set the Field's lifespan by going to Advanced Field Settings > Timing and Lifespan section in the Details panel. This is useful when you are creating Fields that need to apply their effects over a short period.

You will learn more about these settings and more in the following sections.

External Strain

External Strain breaks the connections between the bones in a fractured Geometry Collection. You can modify the External Strain options of the Field by going to the Apply External Strain to Break Node Connections section in the Details panel.

Strain Magnitude

In the example below, the Field is applying more strain than the Geometry Collection's Damage Threshold, resulting in the bones breaking.

In this example, we have two different Geometry Collections. The blue Geometry Collections have a Damage Threshold of 200,000, while the orange ones have a Damage Threshold of 500,000. The Field is applying internal strain with a magnitude of 400,000. This results in the blue Geometry Collections breaking, while the orange ones remain intact.

Strain Falloff Type and Min and Max

You can adjust the Strain Falloff by clicking the Strain Falloff Type dropdown and selecting an option. You can also adjust the Min and Max falloff values to get a desired result.

In this example, the Field has no falloff applied:

In this example, we applied a Linear falloff with a Max value of 2:

Number of Strain Hits

The Strain Hits option executes the external strain portion of the Field multiple times, once per hit. The strain is applied before other forces are applied to the fractured pieces.

This setting can be useful if you want to break multiple fracture levels of the Geometry Collection before applying other forces during the simulation.

In this example, you can see the results of applying a Field with a Strain Hit value of 1:

In this example, you can see the results of applying a Field with a Strain Hit value of 2:

Internal Strain

Internal Strain, or decay, changes the internal Damage Threshold values of the Geometry Collection. When active, a decay Field will lower the Damage Threshold of the bones inside the volume by the Decay Amount.

This can be used to weaken a Geometry Collection gradually, until it breaks from external strain.

In the example below, we set the Activation Type to OnTick with a Delay Amount of 1 second. In addition, we enabled the Use Decay checkbox and set a Decay Amount of 100,000, with a Decay Delay of 1 second.

The Geometry Collection has a Damage Threshold of 500,000.

These options will reduce the Damage Threshold of the bones within the volume by 100,000 every second, with a falloff towards the edges. Here are the results:

Activate Rigid Bodies

There are situations where you might want to activate kinematic rigid bodies without using strain.

You can do this by going to the Advanced Field Settings > Force Dynamic Switching section under Field Settings and enabling the Force Dynamic Switch checkbox.

In the example below, the Geometry Collection is set to Kinematic and the field is activating the Geometry Collection without applying any strain or velocity.

You can also activate Static and Skeletal Meshes by enabling the Activate Tagged Static and Skeletal checkbox.

For each Mesh you want to activate, set its Mobility to Movable and add the Actor Tag ‘SM_Rigids.' In addition, make sure the Mesh has collision so it can simulate correctly.

Velocity

Radial Velocity

Radial velocity pushes objects away from a point in space by a given magnitude. This point is in the center of the volume by default, but can be changed by the user.

In the example below the radial velocity is applied at the center of the sphere volume.

To change the origin of the velocity, go to the Advanced Field Settings > Velocity Controls section and change the Radial Position Offset. In this example, we offset the origin in the Y Axis by 100 units. This results in the velocity being applied from the edge of the sphere volume, as seen below.

You can also apply velocity in a specific direction by going to the Apply Linear and Angular Velocities section and enabling the Use Directional Vector checkbox.

In the example below, we are applying a Directional Magnitude of 800 in the up vector of the Field.

You can change the velocity direction by rotating the field. In this example, we rotated the field to change the direction of the velocity.

You can manually enter the direction vector by going to the Advanced Field Settings > Velocity Controls section and enabling the Override Directional Vector checkbox. Then, enter the Directional Vector Override value.

Angular Velocity

You can apply angular velocity by going to the Apply Linear and Angular Velocities section and enabling the Use Torque checkbox. You can control the magnitude of the velocity applied by setting the Torque Multiplier value.

In the example below, we set the Torque Multiplier to 64.

Force vs Velocity

The FS_MasterField comes with options to apply force or velocity within its volume. By default, the Field applies velocity, which explicitly sets the velocity of objects to the given value without taking into account their mass or volume.

Applying force is more realistic, as it takes into account each object's mass during the simulation. To adjust a Geometry Collection's mass, open the Geometry Collection asset from the Content Browser and go to the Collision section. Enter the desired amount for Mass.

If you are applying force to a Static Mesh or Skeletal Mesh, set the object's mass by going to the Details panel and scrolling down to the Physics section. Enable Simulate Physics and enter the Mass value.

In the example below, you can see the Field on the left is applying velocity, while the Field on the right is applying force. Inside each field, there are two objects with a mass of 500 and 1000 Kilograms.

As you can see, both objects on the Field that is applying velocity move in the same way even though they have different masses. However, on the Field that is applying force, the objects are affected differently when the same amount of force is applied.

Falloff

The FS_MasterField has a variety of options to control the falloff of velocity applied within its volume. You can change the falloff type by going to the Apply Linear and Angular Velocities section and clicking the Velocity Field Falloff Type dropdown.

Velocity Field Falloff Types

When using the Linear falloff type, the field will apply the maximum velocity at the center of the Field, and fade to the minimal value when it reaches the edges of the Field.

In the example below, you can see a Field where the Minimum Velocity Falloff is 0, and the Maximum Velocity Falloff is 1.

You can adjust the minimum and maximum Falloff values to create different effects. For example, the Field below is using a Minimum Falloff value of -1, and a Maximum Falloff value of 0. The negative value pulls objects towards the center, instead of repelling them. This creates the following effect:

You can also set the Minimum Falloff and Maximum Falloff values to larger integers to create a ring effect within the volume. In the example below, the Minimum Falloffvalue is set to -1, and the Maximum Falloff value is set to 1.

Noise

The FS_MasterField includes the ability to add noise to the velocities and forces applied within its volume. Noise adds randomness to the otherwise predictable velocities applied to objects inside the Field.

To add noise to the Field, go to the Noise section in the Details panel, and enable the Use Noise checkbox. You can set the minimum and maximum noise values multiplied against the velocities inside the Field. You can also set the Noise Scale Multiplier which scales the perlin noise used to create randomness.

In the example below, the box Field is applying velocity on Tick without any noise.

In the example below, the box Field is applying velocity on Tick with a Minimum Noise value of 0.8 and a Maximum Noise value of 1. The Noise Scale Multiplier is set to 30.