Houdini Groom Advanced Kit

The MetaHuman Groom Advanced Kit for Houdini provides a complex groom setup in Unreal Engine (UE) using a source project from Houdini, serving as a reference for authoring MetaHuman-compatible grooms for Fab. Unlike the simple short buzzcut included in the Starter Kits, this package demonstrates how to construct complex hairstyles such as a ponytail and braids using the system introduced in 5.6. The kit includes a groom-ready Houdini scene with tools like MH Groom Viewport Lights and MH Groom Workspaces, covering essential workflows such as attribute painting, groom styling, viewport visualization, and exporting grooms with the MH Groom Export tool.

MH Package file

For ensuring MH compatibility, Unreal engine 5.6 introduced MH Manager. MH Manager can be run from the Windows Menu of UE. It detects the assets with a particular folder structure and naming. In the location Content/Grooms, the folder with GroomName should contain XX_GroomName.uasset as a main asset. The main asset is Groom Binding Asset, Wardrobe Item is optional. 

Additionally, subfolder GroomName/GroomName/<all_other_dependencies> will contain Groom Asset and LODs if they are part of the package. If any materials are part of the package they should be placed in GroomName/GroomName/Materials and Groom Skeletal Mesh in GroomName/GroomName/GroomMesh.

When the groom is detected by MH Manager, users can run the verification process and make sure the groom is MH Compatible. 

If the asset passes the verification successfully, it is ready to be packaged into a .mhpkg file. Packaged files can be pasted in any UE 5.6 project and imported for use.

The main listing in this Advanced Kit is a .mhpkg file which contains a Hair_MyAdvancedGroom folder with a GB_Hair_MyAdvancedGroom Binding Asset as a main asset in the package and Wardrobe Item WI_Hair_MyAdvancedGroom. The subfolder Hair_MyAdvancedGroom contains all the additional files, Hair_MyAdvancedGroom Groom Asset being one of them. Subfolder GroomMesh contains Grooming Head Skeletal Mesh SKM_MH_Groom_Head and accompanying skeleton and physics asset, while Materials subfolder contains all the Hair and Head Materials, Material Functions and Textures. 

Wardrobe Item is a new asset, introduced in 5.6, that contains all relevant information for a Groom that is intended to be used in Unreal Engine Meta Human Creator. Wardrobe Item serves to configure the groom additionally. It references Groom Binding Asset, ensuring the groom will move and deform with the MH character head in the UEMHC. Secondary colors can be configured here as well as material overrides. Baked Groom Textures are configured to appear on a particular LOD. The MHC Groom Thumbnail Image and Display Name are also set in Wardrobe Item. 

The user is not obligated to create a Wardrobe Item to add groom to UEMHC Editor. Groom Binding can be used instead and Wardrobe Item with default groom settings will be created automatically for a given asset. 

UE Project

The UE project contains a level with basic lighting setup, SKM_MH Character Head skeletal mesh with assigned Groom Asset, Groom Binding and Wardrobe Item. 

Additionally the project contains environment assets and materials as well as placeholder facial hair grooms.

Hair_MyAdvancedGroom is the Groom Asset that was created by importing the Hair_MyAdvGroom_v01.abc Alembic file into UE. 

Hair_MyAdvGroom_v01.abc Alembic file contains basic attributes groom_width, groom_rootuv, groom_group_id, groom_id and groom_guides. The MH Groom Export tool in Houdini allows for other attributes to be exported with Alembic files as well.

Hair_MyAdvancedGroom Groom Asset contains MI_Hair material which is assigned in the Materials Tab. MI_Hair is a MetaHuman-compatible hair material instance. 

SKM_MH_Character_Head is a skeletal mesh that is exported from Meta Human Creator as part of the custom MH character. SKM_MH_Character_Head can be used as a Source Skeletal Mesh or Target Skeletal Mesh in the Binding Asset. In order for Groom Asset to be accurately attached to the skeletal mesh and move and deform with it, Groom Binding needs to be created.

• If the aim is to attach Groom to SKM_MH_Character_Head, this skeletal mesh should be assigned to Target Skeletal Mesh of the Groom Binding.

• If the aim is to attach a groom to another MetaHuman-compatible head skeletal mesh, SKM_MH_Character_Head needs to be assigned to the Source Skeletal Mesh and the MetaHuman-compatible head skeletal mesh should be assigned to Target Skeletal Mesh.

• If the aim is to use the groom in Meta Human Creator in UE, SKM_MH_Character_Head should be assigned to both Source and Target Skeletal Mesh.

Once the Groom Binding Asset is set, the groom asset can be assigned to the head skeletal mesh in the Level. SKM_MH_Character_Head is imported into the level and Groom Component is assigned to it. In the Groom Asset slot Hair_MyAdvancedGroom Groom Asset is assigned, and in the Binding Asset slot GB_Hair_MyAdvancedGroom. With this setup in place, Hair_MyAdvancedGroom will move and deform together with SKM_MH_Character_Head. 

Houdini Project

The package folder contains a Houdini project called Advanced_Houdini_Groom_Template. It contains the .hip scene MH_Advanced_Groom_Template. This template scene includes an advanced groom graph with all the essential tools needed to generate and export a MetaHuman-compatible groom, as well as to re-import a groom Alembic file back into Houdini.

This template was built in Houdini 21.0.440.

Visit the MetaHuman website to download and install the MH Groom Tools, which are required for this scene to function properly.

To install and set up the tools, please follow instructions on the MetaHuman Groom Tools Installation page.

Once the tools are installed and the scene is reopened, the groom graph should look like this:

The scene contains several nodes: a Geo node named Skin, the MH Viewport Lights tool, MH Groom Export and MH Groom Import tool, as well as multiple MH Groom Workspaces named mh_guides_scalp, mh_scalp_top, mh_scalp_top_knot, mh_scalp_braids_cornrows, mh_scalp_short_sides, mh_scalp_sideburns, mh_scalp_pulledBack_ponytail, mh_scalp_ponytail and mh_assembly_groom. 

If MH Groom Tools are not installed, errors will appear:

These errors are expected to appear if the scene is opened before setting up MetaHuman Groom Tools in Houdini. The warnings are informing the user that the scene contains nodes with incomplete asset definition. In other words, nodes are empty since the HDAs were not embedded in the Houdini project.

MH Groom Head

Inside the Geo node named Skin, the MH Groom Head tool exists. This node contains an imported MetaHuman head geometry, which is from the Kelvin preset. It includes the head mesh and eye assets: eyeballs, eye shells, eye edges, cartilages. The tool contains a number of parameters for editing and visualizing the head geometry.  This head geometry was imported with the updated MH Groom Head which includes a DNA Importer for Metahumans.

Predefined regions can be used as grooming geometry for eyebrows, eyelashes, hair (scalp) and facial hair (face). Each of the regions has a separate output. All regions as well as a full head geo can be subdivided. This action is recommended in cases when skin attributes are painted onto the points. Head tool also provides VDB, with and without ears, which is necessary for a number of other tools to function, predominantly used as collision geometry. In the VDB Tab of the tool, voxel size can be set, the default works well in most cases.

For more information about exporting characters from UE MHC, see the Assembly from the MetaHuman Creator.

For more information about the tool, users can refer to the Houdini Help Cards:

The SCALP_OUT section contains Attribute Paint nodes, which is a Houdini native tool for painting attributes. In this case several Attribute Paint nodes were used to paint the density attribute values onto the scalp region points. A Houdini Color (color_mask_density) node is used to quickly view masks. This is provided in all workspaces for debugging and/or verifying masks.

MH Viewport Lights

This tool is a MH utility tool, which contains several rendering and lighting presets that can be used for viewport visualization. Additionally, the tool provides several background colors as well as lighting and camera settings parameters for preset customization. 

MH Groom Workspaces

Another MH utility tool is MH Groom Workspace. It is a useful container for the groom graphs. Workarea includes several inputs and outputs that can be used to reference geometry from other workspaces, such as Strands, Guides, Skin and Skin VDB. Output nodes can be referenced by other nodes in the graph or other MH Workspaces. 

Node offers choice to reference SOP geometry, MH Workspace and it can also initialize grooming head MH Groom Head. 

It offers several hair shader presets, for better visualization of the strands output of the groom graph. 

You can set the attributes to be applied to guides, strands and skin and review groom statistics. 

In the template scene, MH Groom Workspace is used several times for different purposes. 

MetaHuman Groom Workspaces

A deeper dive into the workspaces used by the Houdini Groom Advanced Kit.

MH Groom Export

In the MH Groom Export node, Source Mode is set to Groom Workspace, with Source Data configured to include both Strands and Guides. The Workspace input references the workspace named mh_assembly_groom.

Under the Groups tab, the final export contains five group_ids.  

0_scalp (group_id 0), 1_ponytail (group_id 1), 2_ponytail_braids (group_id 2), 3_fan_strays (group_id 3), 4_sideburn (group_id 4).

In the Output section, the file Hair_MyAdvGroom_v01.abc is exported to the designated export folder within the Houdini project directory.

For additional information see MH Groom Export.

MH Groom Import

This MH utility tool is used for importing Alembic files. It does the opposite operation from the export tool, it scales and transforms the groom back to be Z up and accommodate Houdini Units.

Additionally, the import tool will determine if there are any overlapping curves based on the Strands Threshold and Guides Threshold. Users can choose which action to take: Ignore, Mark or Repair. The MH Groom Import in conjunction with the MH Groom Workspace can be a useful tool in checking stats and debugging grooms.

The final Advanced Groom is loaded back into the MH_Groom_Import workspace to verify the five exported group_ids and review groom statistics using the MH Groom Workspace.

For additional information see MH Groom Import.