You can import instancers to Houdini when you select the Actor with the Instanced Static Mesh ComponentThe instanced mesh is imported as a packed primitive, copied on each of the instanced points.

Each material used for the instances will have a corresponding attribute named unreal_materialINDEX where INDEX is the index of the corresponding material’s slot. The value will be the Unreal path to that material.

Foliage instances can be imported in a similar way, and will have the unreal_foliage attribute set.

Alternatively, you can import a Static Mesh as a single attribute instance when checking the Import as reference checkbox.

The plug-in supports outputting instances from an asset using packed primitives, object instancers, or attribute instancers.

Instanced geometries in Unreal are InstancedStaticMeshComponents by default or HierarchicalInstancedStaticMeshComponents if the geometry instanced has multiple LODs. If you want to force the creation of HierarchicalInstancedStaticMeshComponents, even without LODs, you can add the unreal_hierarchical_instancer attribute and set its value to 1.

Instanced geometry or uassets appear as instancers in the Houdini Outputs section of the Detail Panel, as shown below:

Point attributes can be added to specify material overrides for the instanced meshes. Create an attribute named unreal_materialINDEX where INDEX represents the material slot index to override. The value should be the Unreal path to the material to be used.

The geometry on each instanced output can be replaced in Unreal. It can have a position, rotation, and scale offset applied to it using the corresponding controls in the detail panel.

There are some cases where you may want to split instancer outputs to multiple components. This could be to assign different materials to each instancer or to get a finer grain control on the actors generated when baking.

This can be done using the unreal_split_attr string attribute. This attribute’s value should be set to the name of another attribute, whose values will be used to control the splitting. (a separate instancer component will be created for each value of that attribute).

For example, you can use the unreal_material attribute as the unreal_split_attr attribute (unreal_split_attr = unreal_material) creates an instancer component for each material on the second attribute. If three different materials are used, the instancer splits to three different instancer components.

In Houdini 20.5, using the unreal_split_instances attribute to create individual StaticMeshComponentshas been deprecated and will be removed in a future version. It’s recommended to use the unreal_split_attr attribute instead.

The following code provides a behaviour similar to the unreal_split_instances attribute:

i@id = @ptnum;
s@unreal_split_attr = "id";

The geometry that is instanced is shown in a thumbnail picker. You can override it by pressing the drop down asset chooser to pick an asset from the list, or by dragging and dropping an asset from the content browser over the thumbnail.

If the geometry used by an instancer output is overridden with something other than a static mesh or vise-versa, the corresponding component will be changed accordingly. For example, you can choose a ParticleSystem asset, which results in AEmitter objects spawned at instance points. Likewise choosing a Blueprint asset will spawn Actor instances of that Blueprint.

In Houdini 20.5 the UI for overriding instances is disabled by default, but can be re-enabled from Unreal plugin settings under “Enable Deprecated Instance Variations”.

Controlling instance variations from Unreal has been deprecated in Houdini 20.5 and will be removed in a future version.

Projects that use this feature should migrate variation logic to Houdini before the next release, when the processing of HDA variations will be removed.

In Houdini 20.5 the UI for controlling instance variations is off by default. You can turn it on from Unreal plugin settings in Enable Deprecated Instance Variations.

Per instance Custom Data lets you add custom data to the instance buffer, that can then be read by the instancer’s material’s vertex shader.

In your HDA, you can specify per instance custom data on your instances, via attributes:

The number of custom floats per instance can be set via the unreal_num_custom_floats attribute. The custom data itself can be set on the instances via unreal_per_instance_custom_dataX attributes, where X is the zero-based index of the custom float data.

An example is to specify custom RGB float values on instances.

You need to add:

i@unreal_num_custom_floats = 3;   (to indicate that each instance will have 3 custom floats)
f@unreal_per_instance_custom_data0 = XXX; (red value)
f@unreal_per_instance_custom_data1 = YYY; (green value)
f@unreal_per_instance_custom_data2 = ZZZ; (blue value)  

To output foliage instances with an HDA, add the unreal_foliage attribute with its value set to 1 to the instances. UE5 requires a Foliage Type Asset stored in the Content folder to create foliage instances inside an Unreal level.

If the instance type in the HDA is set to reference an Unreal Static Mesh then the plugin will automatically create the needed Foliage Type assets when cooking and baking.

If the instance type is set to an existing Unreal Foliage Type, then foliage instances can be baked to that foliage type. During cooking, a temporary Foliage Type is created which is a copy of the user-defined Foliage Type specified in the HDA. This allows you to select existing or HDA generated foliage in the Foliage Editor after cooking. The instances are baked into the level using the target Foliage Type only during baking.

The unreal_foliage_attachment_type attribute attaches foliage instances to other objects in the Unreal level. It can take the following values:

• 0 - The instance is not attached (default)

• 1 - The instance can be attached to any collision component.

• 2 - The instance can only be attached to landscape collision components.

The exact component to attach is determined by tracing a vertical ray up and down from above the instance’s position for a distance of unreal_foliage_attachment_distance.

Level Instancing is supported in Unreal 5.1 and later; set unreal_instance to reference the source level or packed level blueprint.

In Unreal, Level Instancing is at the Actor level, rather than the component level. Both cooking and baking generate Actors rather than Components.

This table lists special attributes to set in an HDA and the plugin recognizes when you translate the Houdini data to Unreal outputs. You can setup these attributes on your assets to control and override some properties and behaviours of the generated Unreal outputs. See Attributes and Groups for more information.