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	additionnal 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 (eg. "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 (eg. "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 (eg. "City_Instance1" where "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	TerrainData	Path in Unity project of a TerrainData asset to set as the Terrain Data. Relative to project Assets folder (eg. "Assets/Terrain/terlayer.asset").
unity_hf_terraindata_export_file	prim	string	TerrainData	Path to write the generated or copied TerrainData asset to.
unity_hf_tile	point	int	TerrainData	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 (eg. "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 (eg. "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 (eg. "Assets/Texture/sand.png").
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 (eg. "Assets/Texture/sand.png").
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 (eg. "Assets/Materials/HeroMat.prefab"). Automatically set when using an input mesh.
unity_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.