Houdini 20.0 Unreal

Chaos (GeometryCollections)

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Requirements

The Unreal Chaos integration works requires Unreal Engine 4.26 with Chaos integration in order to be compiled. UE 4.26 without Chaos enabled will still generate GeometryCollection components, but will not have Chaos physics enabled.

Houdini to Unreal

This table lists special attributes that can be set in an HDA, and that be recognized and used by the plugin when translating the Houdini data to Unreal outputs.

Attribute Name

Owner

Type

Description

unreal_gc_piece

prim

int

Specifies the primitive as a GeometryCollection piece. The value represents the clustering level, or 0 to disable this behaviour

unreal_gc_cluster

prim

int

Specifies the primitive clustering index, to group multiple pieces of the same level to different groups

unreal_gc_clustering_damage_threshold

detail

float

Specifies the clustering damage threshold

unreal_gc_collisions_collision_type

detail

int

Specifies the collisions collision type. 0 = Implicit-Implicit, 1 = Particle-Implicit

unreal_gc_collisions_implicit_type

detail

int

Specifies the collisions implicit type. 0 = Box, 1 = Sphere, 2 = Capsule, 3 = Level Set, 4 = None, 5 = Convex

unreal_gc_collisions_min_level_set_resolution

detail

int

Specifies the collisions min level set resolution

unreal_gc_collisions_max_level_set_resolution

detail

int

Specifies the collisions max level set resolution

unreal_gc_collisions_min_cluster_level_set_resolution

detail

int

Specifies the collisions min cluster level set resolution

unreal_gc_collisions_max_cluster_level_set_resolution

detail

int

Specifies the collisions max cluster level set resolution

unreal_gc_collisions_min_level_set_resolution

detail

int

Specifies the collisions min level set resolution

unreal_gc_collisions_collision_object_reduction_percentage

detail

float

Specifies the collisions collision object reduction percentage

unreal_gc_collisions_mass_as_density

detail

int

Sets the collisions mass as density. 0 = unset, 1 = set

unreal_gc_collisions_mass

detail

float

Specifies the collisions mass

unreal_gc_collisions_minimum_mass_clamp

detail

float

Specifies the collisions minimum mass clamp

unreal_gc_collisions_collision_particles_fraction

detail

float

Specifies the collisions collision particles fraction

unreal_gc_collisions_maximum_collision_particles

detail

int

Specifies the collisions maximum collision particles

unreal_gc_name

prim, detail

string

Specifies the name of the GeometryCollection. Used to differentiate multiple GC outputs

Setting up those attributes on your asset will allow you to control and override some properties and behaviours of the generated Unreal outputs.

Unreal Engine 5 Attributes

These attributes are specific to Unreal Engine 5. UE5 Allows you to specify multiple “Size Specific Data”. In addition, each element of “Size Specific Data” allows you to specify an array of collision shapes.

Attribute Name

Owner

Type

Description

unreal_gc_clustering_cluster_connection_type

detail

int

Specifies the clustering connection type. 0 = Point implicit, 1 = MinimalSpanningSubsetDelaunayTriangulation, 3 = PointImplicitAugmentedMinimalDelaunay

unreal_gc_collisions_max_size

detail

float, float

The max size of the Size Specific Data. Required to specify multiple Size specific data

unreal_gc_collisions_damage_threshold

detail

int, int

The collision damage threshold for each Size Specific Data

unreal_gc_collisions_collision_type_X

detail

int, int

Specifies the clustering damage threshold. The _X specifies the index of the size specific data

unreal_gc_collisions_implicit_type_X

detail

int, int

Specifies the collisions implicit type. 0 = None, 1 = Box, 2 = Sphere, 3 = Capsule, 4 = Level Set. The _X specifies the index of the size specific data

unreal_gc_collisions_min_level_set_resolution_X

detail

int, int

Specifies the collisions min level set resolution. The _X specifies the index of the size specific data

unreal_gc_collisions_max_level_set_resolution_X

detail

int, int

Specifies the collisions max level set resolution. The _X specifies the index of the size specific data

unreal_gc_collisions_min_cluster_level_set_resolution_X

detail

int, int

Specifies the collisions min cluster level set resolution. The _X specifies the index of the size specific data

unreal_gc_collisions_max_cluster_level_set_resolution_X

detail

int, int

Specifies the collisions max cluster level set resolution. The _X specifies the index of the size specific data

unreal_gc_collisions_collision_object_reduction_percentage_X

detail

float, float

Specifies the collisions collision object reduction percentage. The _X specifies the index of the size specific data

unreal_gc_collisions_collision_particles_fraction_X

detail

float, float

Specifies the collisions collision particles fraction. The _X specifies the index of the size specific data

unreal_gc_collisions_maximum_collision_particles_X

detail

int, int

Specifies the collisions maximum collision particles. The _X specifies the index of the size specific data

unreal_gc_collisions_collision_margin_fraction_X

detail

float, float

Specifies the collisions margin fraction. The _X specifies the index of the size specific data

The X in the attributes that contain _X represents the index of the size specific data. Additionally, some of the collision attributes can either be a value or an array of values. An element at index i represents the value for the collision object at index i.

A value is identical to a value array of size one with only that value in it for backwards compatibility.

If you do not add the _X suffix, then it will be equivalent to index 0.

If you want to have more than one size specific data, then it must be specified using the unreal_gc_collisions_max_size attribute. For instance, the sequence of attributes unreal_gc_collisions_max_size = {1234, 12345} and unreal_gc_collisions_collision_type_1 = {0, 1} means that you have a GeometryCollection with two size specific datas, and the second size specific data has two collision shapes: One with a collision type of Implicit-Implicit, and the other with a collision type of Particle-Implicit.

Unreal to Houdini

All output attributes in the above table are imported into Houdini in addition to the attributes below:

Attribute Name

Owner

Type

Description

unreal_input_gc_name

detail

string

The path name of the geometry collection

unreal_actor_path

detail

string

Path to the actor that contained the input data/should be generated. Only available if Actor is specified as input

unreal_level_path

detail

string

Path to the level in which an actor should be generated or which contained the input data. Only available if Actor is specified as input

Geometry Collections

Geometry collection outputs are generated from packed primitive with a non-zero unreal_gc_piece primitive attribute. It is recommended to use the Voronoi fracture node along with the the unreal_gc_piece attribute for a good balance between speed and results.

It is required that the set of all unreal_gc_piece values are sequential. In other words, if there exists a unreal_gc_piece with value X, then there must exist a unreal_gc_piece with value (X - 1).

It is recommended to fracture your geometry in Houdini to the highest damage threshold using the Voronoi fracture or any tool of your choosing. Afterwards, apply unreal_gc_piece to each group with a value corresponding to your desired clustering level.

Below is an example of an HDA that takes a cube, fractures it into three different levels, and then outputs it for the Plugin to process.

This is what is contained within the attribute wrangle of the network:

This is the result of the HDA:

Observe that the output contains 3 levels (not including Level 0), as expected.

If your HDA output contains more than one GeometryCollection, then you must add the attribute unreal_gc_name with a unique name for each of your desired GeometryCollections. This will split the GeometryCollections according to the name.

Fracturing and Clustering

In addition to adding manual fractures as done above, you can also use a combination of RBD fracture and RDB cluster nodes. The unreal_gc_cluster is a primitive attribute that Specifies clustering index, to group multiple pieces of the same level to different groups in the GeometryCollection.

Full control over the exact GeometryCollection hierarchy is not supported at this time (But is often times not needed).

Below is an example that takes a cube, fractures it using RBD fracture, and then randomly cluster groups of pieces using RBD cluster:

The unreal_gc_piece is initialized to 1 in the beginning:

After clustering, update the level of the clustered pieces:

Pack using the “name_gc” (because the name attribute get overwritten by the RBD nodes):

The result is a GeometryCollection with 2 levels, with two clusters of pieces (the pieces in level 2), and two pieces that are not part the cluster (the pieces in level 1):

Unreal

Getting started

Basics

Using Houdini Engine

Reference

Houdini Engine for Unreal Engine is a plug-in that allows integration of Houdini technology into Unreal. Session Sync and Node Sync are two features of the Houdini Engine Plugin to interact with a live session of Houdini.

This plug-in brings Houdini’s powerful and flexible procedural workflow into Unreal Engine through Houdini Digital Assets. Artists can interactively adjust asset parameters inside the editor and use Unreal assets as inputs. Houdini’s procedural engine will then “cook” the asset and the results will be available in the editor without the need for baking.

General information:

Compatibility:

Currently, the plugins has binaries that have been built for UE5.3, 5.2, 5.1, 5.0, UE4.27 and UE4.26, and is linked with the latest production build of Houdini.

Source code for the plugin is available on this repository for UE5.3, 5.2, 5.1, 5.0, UE4.27, UE4.26.

Note

UE5.1 and UE5.0 use the same source files/branches, the 5.0 sources are also intended to be used with 5.1.

Support and reporting bugs:

SessionSync

The Houdini Engine SessionSync feature allows you to connect to a session of Houdini Engine running inside Houdini with its GUI. It supports all the features of the regular Houdini Engine workflow and additional features specific to Session Sync.

Houdini Engine uses a non-UI version of Houdini while Session Sync lets you see and interact with the Houdini session through Houdini’s user interface. It also synchronizes changes made in either Unreal or Houdini so both programs can make changes and see the same results.

NodeSync

NodeSync is a feature for the Houdini Unreal Engine Plugin that enables sharing assets between Unreal and Houdini Session Sync. It works for Skeletal and static meshes.

You can send and receive data to and from Unreal without a dedicated HDA. It works with Static and Skeletal meshes as well as Actors.

Houdini Niagara

The Houdini-Niagara plugin allows you to exchange point cloud data between Houdini and Unreal’s Niagara system using the HoudiniNiagara Data Interface. You can export Houdini’s attribute data as HoudiniPointCache assets. These can then be imported into Unreal to use the Niagara systems and emitters to create complex effects and particle systems.

The Houdini KineFX Live Link plugin lets you control rigs in Unreal directly from KineFX in Houdini.