Attributes and Groups

Standard Houdini Attributes ¶

Certain standard Houdini attributes are used when marshaling data between Houdini and Unity.

Attribute Name	Description
`p`	position
`rot`	rotation
`Cd`	color
`Alpha`	opacity
`N`	normal
`uv`	uv
`uv2, uv3`	additiona uv channels
`instance`	For object-level instancing, See Instancing

Houdini to Unity ¶

This table lists special attributes which will be recognized when marshalling data from Houdini to Unity. Setting up those attributes on your asset will allow you to control and override some properties of the generated Unity geometry.

Attribute Name	Owner	Type	Topic	Description
`unity_instance`	point, detail	string	Instancing	Path in Unity project of an Asset to instance at the given owner transform. Should be relative to the project Assets folder example: Assets/Prefabs/Hero.prefab
`unity_use_instance_flags`	point, detail	int	Instancing	Use the prefab’s original static, tag, and layer attributes when instancing (as opposed to the HDA parent’s). Takes precedence over `unity_static`
`unity_material`	prim, detail	string	Materials	Path in Unity of a Material asset to apply to a part. Should be relative to the project Assets folder example: Assets/Materials/HeroMat.mat
`unity_tag`	prim, detail	string		Unity tag to set for generated GameObjects. Note that tag must already exist in Unity Editor scene
`unity_static`	prim, detail	int		Set to 1 to make generated GameObject static, or 0 for non-static
`lod_screensizes`	detail	float array	Level of Detail	Transition screensizes for LOD levels (range should be 0 to 1 or 1 to 100)
`unity_layer`	prim, detail	string		Unity layer to set for generated GameObjects. Note that the layer must already exist in Unity Editor scene
`instance_prefix`	point, detail	string	Instancing	Prefix to append to instance objects example: `City_Instance1` (`City`is the prefix)
`unity_mesh_readable`	point, detail	int	Readable Meshes	Generated meshes will be marked as readable if this is 1. By default, generated meshes are non-readable
`unity_split_attr`	point, detail	int	Instancing	Indicates the name of another attribute to be used to split the instancers into multiple components depending on that second attributes values

Houdini Heightfields to Unity Terrain ¶

The following table lists special attributes which can be set on the height heightfield layer to specify an existing TerrainData asset file to used. A copy of the TerrainData asset file will be created on generation in Unity.

Attribute Name	Owner	Type	Topic	Description
`unity_hf_terraindata_file`	prim	string	Terrain Data	Path in Unity project of a TerrainData asset to set as the Terrain Data. Relative to project Assets folder example: Assets/Terrain/terlayer.asset
`unity_hf_terraindata_export_file`	prim	string	Terrain Data	Path to write the generated or copied TerrainData asset to
`unity_hf_tile`	point	int	Terrain Data	Specifies the terrain tile of a scatter point. Useful for tree instancing with the tile split node

Attribute Name

Owner

Type

Topic

Description

unity_hf_terraindata_file

prim

string

Terrain Data

Path in Unity project of a TerrainData asset to set as the Terrain Data. Relative to project Assets folder

example: Assets/Terrain/terlayer.asset

unity_hf_terraindata_export_file

prim

string

Terrain Data

Path to write the generated or copied TerrainData asset to

unity_hf_tile

point

int

Terrain Data

Specifies the terrain tile of a scatter point. Useful for tree instancing with the tile split node

The following table lists special attributes which can be set on heightfield layers in order to set the corresponding Terrain Layer properties on the generated Unity Terrain. Note that height and mask layers are ignored when creating Terrain Layers, so these attributes must be set on other heightfield layers.

Attribute Name	Owner	Type	Topic	Description
`unity_hf_terrainlayer_file`	prim	string	Height Field Layers	Path in Unity project of a TerrainLayer asset to set as the Terrain Layer. Relative to project Assets folder example: Assets/Terrain/base.terrainlayer
`unity_hf_texture_diffuse`	prim	string	Height Field Layers	Path in Unity project of a Texture asset to set as the terrain layer’s diffuse texture. Relative to project Assets folder example: Assets/Texture/sand.png
`unity_hf_texture_mask`	prim	string	Height Field Layers	Path in Unity project of a Texture asset to set as the terrain layer’s mask texture. Relative to project Assets folder
`unity_hf_texture_normal`	prim	string	Height Field Layers	Path in Unity project of a Texture asset to set as the terrain layer’s normal texture. Relative to project Assets folder
`unity_hf_normal_scale`	prim	float	Height Field Layers	Terrain layer’s normal scale (float of size 1)
`unity_hf_metallic`	prim	float	Height Field Layers	Terrain layer’s shader’s metallic value (float of size 1, with value between 0 and 1)
`unity_hf_smoothness`	prim	float	Height Field Layers	Terrain layer’s shader’s smoothness value (float of size 1, with value between 0 and 1)
`unity_hf_specular`	prim	float array	Height Field Layers	Terrain layer’s shader’s specular color (float of size 4, with values between 0 and 1)
`unity_hf_tile_offset`	prim	float array	Height Field Layers	Terrain layer’s texture’s tile offset (float of size 2)
`unity_hf_tile_size`	prim	float array	Height Field Layers	Terrain layer’s texture’s tile size (float of size 2)

Unity to Houdini ¶

This table lists special attributes which are created by the plug-in when marshaling data from Unity to Houdini.

Attribute Name	Owner	Type	Topic	Description
`unity_material`	prim	string	Materials	Path in Unity of a Material asset to apply to the mesh. Should be relative to the project Assets folder. Automatically set when using an input mesh
`uunity_input_mesh_name`	prim	string	Inputs	Name of mesh’s owner GameObject. Automatically set when using an input mesh

These attributes will be available in Houdini when using Inputs to marshall data from Unity to Houdini.

These Standard Houdini Attributes will generally also be available: position (p), vertex colors (Cd, Alpha), normals (N) and UV sets (uv, uv2, uv3).

You can easily see these attributes and their values by troubleshooting the internal Houdini Scene (File ▸ Open Scene In Houdini) after initializing your asset’s Inputs.

Groups ¶

Some special group names have naming schemes which specify generation behavior when marshalling data from Houdini to Unity.

Group Name	Topic	Description
`collision_geo`	Colliders	Prefix used for collider geometry generation (invisible)
`convex_collision_geo`	Colliders	Prefix used for convex collider geometry generation (invisible)
`convex_collision_geo_trigger`	Colliders	Prefix used for convex trigger collider geometry generation (invisible)
`collision_geo_simple`	Colliders	Prefix used for visible simple collider geometry generation. Accepted values are box, sphere, and capsule
`collision_geo_simple_trigger`	Colliders	Prefix used for visible simple trigger collider geometry generation. Accepted values are box, sphere, and capsule
`rendered_collision_geo`	Colliders	Prefix used for visible collider geometry generation
`rendered_convex_collision_geo`	Colliders	Prefix used for visible collider geometry generation
`rendered_convex_collision_geo_trigger`	Colliders	Prefix used for visible trigger collider geometry generation
`rendered_collision_geo_simple`	Colliders	Prefix used for visible simple collider geometry generation. Accepted values are box, sphere, and capsule
`rendered_collision_geo_simple_trigger`	Colliders	Prefix used for visible simple trigger collider geometry generation. Accepted values are box, sphere, and capsule
`lod`	Level of Detail	Prefix used for defining mesh LOD level (e.g. lod0, lod1)

Primitives that belong to collision groups will be separately created as Mesh Collider components and added to the output GameObject. The `rendered_collision_geo groups will have MeshRenderer and MeshFilter components as well.

Attributes Store ¶

The Attributes Store feature allows to store attribute data in a script component (HEU_OutputAttributesStore) attached to the generated gameobject. This allows to bring in any supported attribute type (ints, floats, strings) from Houdini into Unity, to access them at a later point in time.

To use this feature for an HDA:

Create attributes for points, primitives, or detail for a SOP geometry inside the HDA, with name and values set.
Create another attribute with name hengine_attr_store of class Detail and type String.
Specify the names of the attributes that you want stored, separated by commas (e.g. health,ammo,armor).

Now in Unity, when the HDA is cooked, the generated gameobject will have HEU_OutputAttributesStore script component attached. It will contain a map of attributes, which can be queried via script.

Note that you can also store Houdini native attributes such as P, rot, N, uv, etc.

Optional, but as a follow up, you can also attach a script to be called back automatically that can query and assign values from the attribute store. Check out the example HDA provided in Assets/Plugins/HoudiniEngineUnity/HDAs/HEUInstanceAttributesStore.hda.

Attributes and Groups

Standard Houdini Attributes ¶

Houdini to Unity ¶

Houdini Heightfields to Unity Terrain ¶

Unity to Houdini ¶

Groups ¶

Attributes Store ¶

Unity

Getting started ¶

Project Setup ¶

Using Houdini Engine ¶

Reference ¶