Blast Destruction

Overview

The Omniverse™ Blast Destruction (omni.blast) extension integrates the NVIDIA Omniverse™ Blast SDK into NVIDIA Omniverse™ Kit applications. It supports authoring of destructible content, and also implements destruction in PhysX SDK-driven simulation.

Introductory Video

User Guide

Interface

The Blast window is divided into panels described in the following subsections.

Fracture Controls

These settings are used to author destructibles from mesh prims.

Control	Effect
Combine Selected	Combines the selected mesh prims and/or destructible instances into a “multi-root” destructible.
Fracture Selected	Fracture the selected mesh prim.
Num Voronoi Sites	The number of pieces in which to fracture the selected meshes.
Random Seed	Seed value of pseudorandom number generator used during fracture operations.
Auto Increment	If checked, the Random Seed will automatically increment after each fracture operation.
Select Parent	Select the parent(s) of the currently selected chunk(s). Removes children from selection set.
Select Children	Select the children(s) of the currently selected chunk(s). Removes parents from selection set.
Select Source	Select the source prim associated with any part of the destructible.
Contact Threshold	The minimum contact impulse to trigger a contact event with a destructible.
Max Contact Impulse	Applied to kinematic destructibles, limits the force applied to an impacting object.
Reset to Default	Set Max Contact Impulse back to ‘inf’ for the selection.
Interior Material	The material applied to faces generated through fracture. Can be set before or after fracture and will be applied to all descendants of the selected chunk(s).
Interior UV Scale	Stretch to apply to material textures on newly-created interior faces.
Apply Interior UV Scale	Apply the stretch value Interior UV Scale to selected chunks.
Recalculate Bond Areas	Recalculate the bond area of selected instances. Use this after scaling an instance to ensure correct areas. Bond areas are used in stress pressure calculations.
Recalculate Attachment	Search for nearby static or dynamic geometry and form bonds with that geometry.
Make External Bonds Unbreakable	Bonds created between a blast destructible and external geometry will never break.
Remove External Bonds	Remove all bonds to external geometry.
Create Instance	Creates an instance based on the selected destructible base or instance. On instances, this is equivalent to using Kit’s duplicate command on the instance prim.
Reset Blast Data	Destroys fracture information, depending on the type of prim(s) selected: Base selected - destroy all destruction info (including combine) and restores the orig mesh. Instance selected - destroy the selected instance. Anything else - search for chunks under selection and reset all fracture information for them.

Important

Fracturing operations can increase geometry counts exponentially and have the potential to overwhelm computer resources. Use caution when increasing Number of Voronoi Sites.

Instance Stress Settings

Beginning with omni.blast-0.11, damage in Blast has been unified into a stress model simulated for each destructible instance. External accelerations applied to support-level chunks are used as input, and the solver determines the internal bond forces which are required to keep the bonded chunks from moving relative to one another. Given each bond’s area, these forces are translated into pressures. The material strength of the destructible is described in terms of pressure limits which it can withstand before breaking.

The pressure is decomposed into components: the pressure in the bond-normal direction, and pressure perpendicular to the bond normal. Furthermore, the normal component can either be compressive (if the chunks are being pushed together) or tensile (if the chunks are being pulled apart). The pressure component perpendicular to the bond normal is called shear. For each component (compressive, tensile, or shear), we allow the user to specify an “elastic limit” and a “fatal limit.” These are described in the table below.

Damage has units of acceleration, which is applied to the support-level chunk at the damage application point. If “Stress Gravity Enabled” is checked (see the table below), then gravitational acceleration is applied to each support chunk, every frame in the stress simulation. If “Stress Rotation Enabled” is checked, then centrifugal acceleration is calculated and applied to each support chunk as well.

The stress solver replaces damage shaders in older versions of Blast. Using the stress model, damage spreads naturally through the system and fracture occurs because of physically modeled limits. Impact damage is applied by translating the contact impulse into an acceleration applied to the contacting support chunk. When fractured chunk islands (actors) break free, the excess force (that which exceeded the bonds’ limits) is applied to the separating islands, which leads naturally to pieces flying off with higher speeds when the system is hit harder. The excess force and contact impulse effects are adjustable with a multiplier for each (see Residual Force Multiplier and Impact Damage Scale).

This wall has fractured under its own weight due to the pressure of the overhang jutting out to the right.

These settings are applied to the selected destructible instance(s). They control how stress is processed during simulation.

Control	Effect
Stress Gravity Enabled	Whether or not the stress solver includes gravitational acceleration.
Stress Rotation Enabled	Whether or not the stress solver includes rotational acceleration.
Maximum Solver Iterations Per Frame	Controls how many passes the solver can do per frame. Higher numbers here will result in converging on a stable solution faster.
Residual Force Multiplier	Multiplies the residual forces on bodies after connecting bonds break.
Stress Limit Presets	Set stress limits based on various physical substances. Can be used as a rough starting point, then tweaked for a specific use case.
Compression Elastic Limit	Stress limit (in megapascals) for being compressed where bonds starts taking damage.
Compression Fatal Limit	Stress limit (in megapascals) for being compressed where bonds break instantly.
Tension Elastic Limit	Stress limit (in megapascals) for being pulled apart where bonds starts taking damage. Use a negative value to fall back on compression limit.
Tension Fatal Limit	Stress limit (in megapascals) for being pulled apart where bonds break instantly. Use a negative value to fall back on compression limit.
Shear Elastic Limit	Stress limit (in megapascals) for linear stress perpendicular to bond directions where bonds starts taking damage. Use a negative value to fall back on compression limit.
Shear Fatal Limit	Stress limit (in megapascals) for linear stress perpendicular to bond directions where bonds break instantly. Use a negative value to fall back on compression limit.
Reset All to Default	Reset values in this panel to their defaults.

Instance Damage Settings

These settings are applied to the selected destructible instance(s). They control how damage is processed during simulation.

Control	Effect
Impact Damage Scale	A multiplier, contact impulses are multiplied by this amount before being used as stress solver inputs. If not positive, impact damage is disabled.
Allow Self Damage	If On, chunks may damage other chunks which belong to the same destructible.

Global Simulation Settings

These settings are general simulation settings.

Control	Effect
Max New Actors Per Frame	Only this many Blast actors may be created per frame. Additional actors will be delayed to subsequent frames.

Debug Visualization

These settings are used to visualize various aspects of a destructible.

Control	Effect
Explode View Radius	When Not simulating, separates the chunks for inspection and/or selection.
View Chunk Depth	Which chunk hierarchy depth to render while in exploded view.
Visualization Mode	Controls what to render debug data for (see Visualization Mode table below).
Visualization Type	Controls what debug data is to be rendered (see Visualization Type table below).

Visualization Mode	Description
Off	Disable Blast debug visualization system.
Selected	Only render debug visualization for the selected actor/instance.
On	Render debug visualization for all instances.

Visualization Type	Description
Support Graph	This shows representations of chunk centroids and bonds (drawn between the centroids). External bonds have a brown square around the bond centroid to distinguish them. The bond colors have meaning: Green - the bond’s health is at or near its full value. Red - the bond’s health is near zero. (Zero-health bonds are “broken” and not displayed.) In Green -> Yellow -> Red continuum - the bond’s health is somewhere between zero and full value. Light blue - the bond is an unbreakable external bond.
Max Stress Graph	This shows the maximum of the stress components for each bond, by coloring the bond lines drawn between the centroids. The colors have meaning: Green -> Yellow -> Red continuum - the stress is between 0 (green) and the bond’s elastic limit (red). Blue -> Magenta continuum - the stress is between the bond’s elastic limit (blue) and fatal limit (magenta).
Compression Graph	If the bond is under compression, this shows the compression component of stress for each bond. Colors have the same meaning as described for the Max Stress Graph.
Tension Graph	If the bond is under tension, this shows the tension component of stress for each bond. Colors have the same meaning as described for the Max Stress Graph.
Shear Graph	This shows the shear component of stress for each bond. Colors have the same meaning as described for the Max Stress Graph.
Bond Patches	Render bonded faces between chunks with matching colors.

Debug Damage Tool Settings

These settings control damage applied using Shift+B+(Left Mouse Button) during simulation. This is only intended for testing the behavior of a destructible.

Control	Effect
Damage Amount	The base damage amount (acceleration in m/s^2) applied to the nearest support chunk in a destructible instance.
Damage Impulse	The outward impulse applied to rigid bodies which lie within the damage radius after fracture. This is in addition to the excess forces applied by the stress solver to chunks when they break free.
Damage Radius	The distance from the damage center (where the cursor is over scene geometry) to search for destructibles to damage.

Reset Button

This button resets all controls in the Blast window to their default settings.

Push to USD Button

This will push the current runtime destruction data to USD, allowing the scene to be saved and later restored in the same destroyed state.

OmniGraph Blast Node Documentation

This section describes the Blast nodes for use in the OmniGraph system. You can find them under “omni.blast” in the “Add node” right click menu or the pop out side bar.

NOTE: The nodes automatically update the USD stage. It is possible for them to get out of sync. If that happens just save and reload the scene. That will cause the Blast data to regenerate and should get things back in sync.

Authoring: Combine

The Combine node takes two mesh or destructible prims and combines them into a single destructible. These can be chained together to create complex destructibles built out of many parts.

The output of this can also be sent to Fracture nodes to break them down. Note that fracturing after combine will apply to all parts that are touched by the point cloud. If you want points to only apply to a specific part, run it through Fracture first, then combine it to simplify the process.

Input	Description
prim1	Prim to use as input to fracture. Connect to “destructible” output from another Blast node or “output” from an “import USD prim data” node. The import node should be connected to a mesh or xform/scope containing meshes.
prim2	Prim to use as input to fracture. Connect to “destructible” output from another Blast node or “output” from an “import USD prim data” node. The import node should be connected to a mesh or xform/scope containing meshes.
contactThreshold	Force from impact must be larger than this value to be used to apply damage. Set higher to prevent small movements from breaking bonds.
bondStrength	Base value to use for bond strength. How it is applied depends on the mode used. Bond strength is automatically recalculated when this changes.
bondStrengthMode	Controls how the Default Bond Strength is used. Bond strength is automatically recalculated when this changes. “areaInstance”: (Default) Multiply the default value by the area of the bond using destructible instance scale. “areaBase”: Multiply the default value by the area of the bond using destructible base scale. “absolute”: Use the default value directly.
maxContactImpulse	Force that PhysX can use to prevent objects from penetrating. It is only used for kinematic destructibles that are attached to the world. This can approximate brittle materials by setting the value low, giving the bonds a chance to break before the objects are pushed apart.
damageMinRadius	Full damage from user generated fracture event is applied inside this radius.
damageMaxRadius	No damage from user generated fracture event is applied outside this radius.
impactDamageScale	Scale physical impacts by this amount when applying damage from collisions.
impactClusterRadius	If positive, contact reports will be aggregated into clusters of approximately the given radius, and damage accumulated for that cluster. The cluster will be reported as one damage event.
allowSelfDamage	If on, chunks from a destructible actor may cause damage to sibling actors. Default behavior for legacy assets is to disable self-damage, which changes the legacy behavior.

Output	Description
destructible	Result of the combine operation. Can be used as the input prim to another Blast authoring node.

Example usage of a Combine node.

Authoring: Fracture

The Fracture node takes a mesh or destructible and fractures it based on a point cloud input. For best results, the mesh should be closed and not contain degenerate triangles. Input points that are inside of the mesh will be used to generate Voronoi sites, points outside of the mesh are ignored.

Sending in an already fractured mesh to another fracture node will create layers of fracture. The newly generated chunks from the first fracture will be fractured again using another set of points. This allows you to sculpt the detail and density in areas that you want it without requiring the entire mesh to be broken up at that fidelity.

During simulation if all children of a given parent chunk are still intact, then the parent will be rendered instead.

Input	Description
prim	Prim to use as input to fracture. Connect to “destructible” output from another Blast node or “output” from an “import USD prim data” node. The import node should be connected to a mesh prim.
points	Point cloud to use for fracture. Points inside will be used as Voronoi sites, points outside will be ignored.
contactThreshold	Force from impact must be larger than this value to be used to apply damage. Set higher to prevent small movements from breaking bonds.
bondStrength	Base value to use for bond strength. How it is applied depends on the mode used. Bond strength is automatically recalculated when this changes.
bondStrengthMode	Controls how the Default Bond Strength is used. Bond strength is automatically recalculated when this changes. “areaInstance”: (Default) Multiply the default value by the area of the bond using destructible instance scale. “areaBase”: Multiply the default value by the area of the bond using destructible base scale. “absolute”: Use the default value directly.
interiorMaterial	Path to material to use for interior faces created through fracture.
maxContactImpulse	Force that PhysX can use to prevent objects from penetrating. It is only used for kinematic destructibles that are attached to the world. This can approximate brittle materials by setting the value low, giving the bonds a chance to break before the objects are pushed apart.
interiorUvScale	UV scale to use on interior faces generated through fracture.
damageMinRadius	Full damage from user generated fracture event is applied inside this radius.
damageMaxRadius	No damage from user generated fracture event is applied outside this radius.
impactDamageScale	Scale physical impacts by this amount when applying damage from collisions.
impactClusterRadius	If positive, contact reports will be aggregated into clusters of approximately the given radius, and damage accumulated for that cluster. The cluster will be reported as one damage event.
allowSelfDamage	If on, chunks from a destructible actor may cause damage to sibling actors. Default behavior for legacy assets is to disable self-damage, which changes the legacy behavior.

Output	Description
destructible	Result of the fracture operation. Can be used as the input prim to another Blast authoring node.

Example usage of a Fracture node.

Events: Flow Adapter

The Events Flow Adapter node takes a bundle of active events and translates it into outputs that can drive Flow emitter prim attributes.

Currently only one layer is supported for all events. Work is ongoing to support visible materials mapping to Flow data via a new schema, and for Flow emitters to be able to emit on multiple layers from a single input stream of data. Then events will use material data to drive emission specific to that material.

Input	Description
events	Bundle of events that can be processed by adapter nodes.

Output	Description
positions	Unique world space vectors.
faceVertexIndices	Indices into “positions” used to build faces.
faceVertexCounts	Defines how many indices make up each face.
velocities	Per vertex velocity.
coupleRateSmokes	Per vertex emission rate.

NOTE: The outputs prefixed with “subset” are not intended to be used yet. They will support multiple layers in the event bundle being passed to Flow when materials can be mapped to Flow layer IDs and Flow emitters support multiple layers.

Experimental Output	Description
subsetLayers	Flow layer IDs referenced by faces. It is possible for their to be duplicates.
subsetFaceCounts	Number of faces each layer ID slot represents. Faces are grouped by layer ID.
subsetEnabledStatus	Tells Flow emitter if each block of faces is enabled. Allows other data to be cached.

Example usage of a Flow apater node.

Events: Gather

The Events Gather node takes no inputs and produces a bundle of active events. These can come from bonds breaking and contacts between objects.

• Destruction events are generated for all Blast based prims when bonds break.

• Collision events are reported for all prims that have the rigid body and report contacts APIs applied.

The output of this can be sent to adapter nodes to generate responses to the events.

Visual materials should have the Rigid Body Material applied. This can be added by selecting the material and doing Add -> Physics -> Rigid Body Material. It is needed to report the correct materials for collision events. This is not strictly required yet, but will be when multiple materials are fully supported by this system.

Input	Description
None

Output	Description
events	Bundle of events that can be processed by adapter nodes.

Example usage of a gather node.

Demo Scenes

Several demo scenes can be accessed through the physics demo scenes menu option (Window > Simulation > Physics / Demo Scenes). This will enable a Physics Demo Scenes window, which has a Blast Samples section.

Tutorials

Getting Started

This tutorial shows initial setup.

First enable the Blast extension (if it isn’t already):

• Navigate to Window > Extensions

• Enable the Blast Destruction extension

  • You can check Autoload if you will be working with Blast frequently and don’t want to have to enable it by hand each time you load the application

• Make sure it says UP TO DATE at the top of the extension details, if not, update to the latest release of the extension

Make sure there something for the mesh to interact with:

• Navigate to Create > Physics > Physics Scene

• And Create > Physics > Ground Plane

Next, create a mesh:

• Navigate to Create > Mesh and pick a mesh primitive type to create (Cube, Sphere, etc)

  • Be sure not to select Create > Shape, those do not support fracture, it must be a “Mesh” type prim

  • Alternatively, you can load a mesh from another Usd source, just make sure it is a closed mesh

• Set up any physics properties you want on the mesh

  • Right click on the mesh and navigate to Add > Physics > Rigid Body with Colliders Preset to make it dynamic and collidable

  • You can then change physics properties in the Properties > Physics panel

Now fracture it:

• Make sure the mesh to fracture is selected

• Locate the Blast pane (by default it docs in the same area as Console)

• Adjust Blast settings as desired (see above for what the settings control)

• Click the Fracture Selected button to fracture it

  • This will deactivate the original mesh and select the new Blast instance container (prim named after source mesh with __blastInst in the name)

  • The original mesh is not altered and can be easily restored later if desired

• Scrub/adjust the Debug Visualization > Explode View Radius in the Blast panel to review the fracture

• Additional copies of the destructible can be made by clicking the “Create Instance” button in the Blast panel or running the duplicate command (Ctrl + D or Right Click > Duplicate)

Running the simulation (by pressing Play), the destructible falls to the ground and fractures due to impact damage.

Reset Fracture

Here we see how to undo a fracture operation:

• Select the original mesh with Blast applied or the Blast base container (prim named after source mesh with __blastBase in the name)

• Click the Reset Fracture Data button in the Fracture Controls section

• Dialogue will warn of deletion permanency; agree to delete

• This will remove all Blast data generated and re-activate the source mesh prim

In the video we change the Num Voronoi Sites number and re-fracture.

Multilevel Fracture

Here we see how to recursively fracture. This allows for higher overall detail with lower rendering and simulation cost by using a parent chunk for rendering and simulation until at least one child breaks off. It also allows for non-uniform fracture density throughout the mesh. Chunks of a fractured mesh may be broken down further:

• Select a mesh with Blast applied

• Adjust the explode view radius to make chunk selection easy

• Select the desired chunk(s) to fracture further

  • You can also select them directly from Stage view under <Blast base container> > chunks while in explode view

• Adjust the Num Voronoi Sites in the Blast panel to the desired number

  • Each chunk will be split into that many pieces

• Click the Fracture Selected button to fracture the selected chunks

• Changing the View Chunk Depth value selects different hierarchy depths to display

• You may use the Select Parent and Select Children buttons to select up and down the hierarchy

• Repeat as needed to get the desired level of granularity and fracture density throughout the mesh

Static Attachment

This tutorial shows the preferred way of creating a “static” destructible, by emulating an attachment to the static world:

• Create a static collision mesh

  • Create/load a mesh

  • Right click on it

  • Navigate to Add > Physics > Colliders Preset

• Create a destructible prim in the usual way (see Getting Started above)

  • Leave the source prim as dynamic, do not set to kinematic or static

• Place the destructible prim adjacent to (or slightly overlapping) a static collision mesh

• Select the Blast instance container prim

• Set the state of Make External Bonds Unbreakable based on how you want the base to behave

  • When checked, the base pieces will remain kinematic no matter how much damage they take

  • This is a good idea if the Blast instance container is deeply penetrating the static geometry, otherwise physics will force them apart when the pieces become dynamic

  • When unchecked, the chunks can take damage and break free, becoming new dynamic rigid bodies

• Press the Recalculate Attachment button to form bonds between the destructible and nearby static geometry (“external” bonds)

• Debug Visualization > Visualization Type > Support Graph shows where external bonds have been formed when Debug Visualization > Visualization Mode is set to Selected or All

When simulating, the bonds formed keep the destructible “static” (kinematic). When bonds are damaged, chunk islands that are not connected via bonds to static geometry become separate dynamic bodies. The chunks that remain connected to static geometry via bonds remain kinematic.

Dynamic Attachment

Here we see how to turn a part of a rigid body into a destructible. If the rigid body consists of multiple meshes, we may select one and make it destructible:

• Create an Xform prim to contain the meshes with Create > Xform

• Create/load meshes and add them to the Xform container

• Set the hierarchy as dynamic and collidable

  • Right click on the Xform prim

  • Navigate to Add > Physics > Rigid Body with Colliders Preset

  • This will automatically set the contained meshes as colliders

• Select one or more of the meshes in the container to make destructible

• Click the Fracture Selected button to fracture the mesh as usual

• Set the state of Make External Bonds Unbreakable based on how you want the base to behave

  • The same rules apply as for Static Attachment above

• Press the Recalculate Attachment button

  • Bonds will be formed with any adjacent or overlapping collision geometry from the same rigid Body

When simulating, the destructible mesh will move with its parent physics as a single rigid body. When bonds are damaged, chunk islands that are not connected via bonds to the parent physics geometry become separate dynamic bodies. The chunks that remain connected to the parent physics geometry via bonds remain rigidly connected to the physics body.

Note, if no bonds were formed (because there was no adjacent or overlapping collision geometry from the parent rigid body), then all chunk islands become dynamic when the destructible is fractured.

Stress Damage

This tutorial covers the basics of the new stress damage system in Blast destruction, which is available in omni.blast-0.11. The viewer will learn how it relates to the previous damage system, how various sorts of damage are integrated within the stress framework, and how to adjust stress settings in a destructible.

• Create a “wall” by first creating a mesh cube

  • Create menu -> Mesh -> Cube

  • With the new cube selected, in the property window set the Transform > Scale (x, y, z) to 5.0, 2.0, 0.2, and Translate y = 100.0

  • Right click on the cube (wall)

  • Navigate to Add > Physics > Rigid Body with Colliders Preset

• Create the destructible wall

  • Select the wall in the viewport

  • In the Blast window, under the Fracture Controls panel, set Num Voronoi Sites to 1000

  • Click the Fracture Selected button. There will be a long pause until the controls become responsive and the Cube__blastBase and Cube__blastInst prims appear in the Stage view

  • With Cube_blastInst selected, press the Recalculate Attachment button to form permanent bonds between the destructible and the ground

• Set stress limits and debug damage amount

  • With Cube_blastInst selected, in the Instance Stress Settings panel, set Compression Elastic Limit to 0.05 and Compression Fatal Limit to 0.1

  • In the Debug Damage Tool Settings panel, Set Damage Amount to 25000.0

• Checking non-stress behavior

  • With Cube_blastInst selected, in the Instance Stress Settings panel, uncheck both Stress Gravity Enabled and Stress Rotation Enabled

  • Deselect the wall (you can deselect all by hitting the Esc key). Otherwise outline drawing of the 1000-piece wall causes a large hitch

  • Shift+B+(Left Click) with the mouse cursor on the wall to apply damage

  • You can damage along the bottom of the wall, and the top will only break off when you’ve completely cut through to the other simulated

• Using the full stress solver

  • With Cube_blastInst selected, in the Instance Stress Settings panel, ensure both Stress Gravity Enabled and Stress Rotation Enabled are checked

  • Deselect the wall (you can deselect all by hitting the Esc key)

  • Shift+B+(Left Click) with the mouse cursor on the wall to apply damage

  • You can damage along the bottom of the wall, but before you cut all the way across, the stress of the overhanging piece should cause it to snap off on its own

• Repeating with a weaker wall (lower pressure limits)

  • With Cube_blastInst selected, in the Instance Stress Settings panel, set Compression Elastic Limit to 0.025 and Compression Fatal Limit to 0.05

  • Deselect the wall (you can deselect all by hitting the Esc key)

  • In the Debug Damage Tool Settings panel, Set Damage Amount to 10000.0

  • Shift+B+(Left Click) with the mouse cursor on the wall to apply damage

  • Now as you damage along the bottom of the wall, it doesn’t take as large of an overhang for the stress to cause it to snap off

Note

If Shift+B+(Left Mouse Button) does not fracture a destructible, try increasing Damage Amount or Damage Radius.

Changing Material(s) requires re-fracturing.

Physics Properties of the Source Asset propagate to the fractured elements during authoring.