Alembic for Grooms Specification

Describes guidelines for exporting Grooms as Alembic files for use with Unreal Engine

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The Hair Rendering and Simulation features are intended to provide a minimally viable implementation of strand-based hair importing, rendering, and simulation. This update does not provide a hair grooming solution. Hair shaping, or grooming, will need to be performed in an external DCC tool such as Maya by using their XGen tool or the plugin Shave and a Haircut . Other tools like Ornatrix , Yeti , or Houdini can be used as well.

This document outlines an informal schema for Alembic files in an attempt to standardize the import of hair groom and guide hair information into Unreal Engine 4 (UE4) for use with the Hair Rendering and Simulation features. The intent is that as you generate Hair Grooms, conforming to this schema will allow UE4 to import those grooms directly. Upon import, attributes that are supported by UE4 for both rendering and physics will be mapped to those attributes in the Alembic file.

Goals

  1. Define a groom-specific, informal, Alembic metadata schema to facilitate importing of hair into Unreal Engine.

  2. Examine the attributes that are supported by UE4 for hair, and map those attributes onto Alembic.

Curves

The general shape of the hair is read from the standard Alembic Curve schema:

Alembic::AbcGeom::ICurves

Width

Groom attributes exported using this specification enables the importer to retrieve and use them when building the groom. However, the width attribute does have a special behavior. For example, in a DCC application like Maya, width values are exported directly on the curves so there's no need to export a custom groom_width attribute; the importer will convert Maya's width values into the groom_width attribute. It is worth noting that if the groom_width attribute is present in your groom, it will not be overwritten during import. If no groom_width attribute is specified, or one couldn't be converted from width values, the builder in Unreal Engine falls back to using a value of 1 centimeter for the width value.

The curve width can be stored per-ICurve using:

(GeometryScope::kConstantScope)

Or it can be stored per-curve using:

(GeometryScope::kUniformScope)

Or it can be stored per-cv using:

(GeometryScope::kVertexScope)

Note that the importer supports kUniformScope, but this has not been tested.

Naming Conventions

Property names must be lowercase with no spaces or special characters.

All properties must start with the following prefix:

groom_<attr>

For example, groom_id, groom_color, or groom_width are all using the proper naming conventions.

Scoping and Precedence

Properties in this document have an assigned scope. The scope controls at which level the groom property can be defined.

The available scopes are:

Scope

Description

Constant

These parameters have a single value for all curves with an AbcGeom::ICurves Object.

Uniform

These parameters have a value per-curve in an AbcGeom::ICurves Object.

Vertex

These parameters have a value per-curve per-cv.

General Properties

This section outlines properties that apply to the groom as a whole. These properties should be added to the topmost node: IXForm or ICurves.

Name

Type

Optional

Description

Value

groom_version_major

int16

No

Used to identify which major version of the groom schema this file conforms to.

1

groom_version_minor

int16

No

Used to Identify which minor version of the schema this file conforms to.

5

groom_tool

string

Yes

The name and version of the tool that generated this file. Useful for debugging and tracking down issues.

groom_properties

string

Yes

String describing the parameters used to generate this groom. These are tool-specific options, used for debugging.

Geometry Parameters

All geometry parameters are optional but have internal behavior to fall back on if they are not present with your exported groom.

Name

Type

Scope

Optional

Description

Range

groom_ guide

int8/16/32

Constant/Uniform

Yes

Guides are generated from the imported strands and decimation settings.

0 = Not a guide / 1 = Guide

groom_group_id

int32

Constant/Uniform

Yes

All strands will be grouped together.

[0, INT_MAX]

groom_root_uv

float32/64[2]

Uniform

Yes

Root UVs are computed by projecting the root of the strand onto a sphere.

This attribute is optional. If not specified, a root UV is automatically generated in the engine using a spherical mapping.

[-FLOAT_MAX, FLOAT_MAX]

groom_id

int32

Uniform

Yes

Strands won't have IDs. This is intended for use in debugging and with groom_closest_guides attribute.

[0, INT_MAX]

groom_color

float32/64[3]

Vertex

Yes

Falls back to black.

[0, FLOAT_MAX]

groom_closest_guides

int32[3]

Uniform

Yes

Intended for specialized use where the interpolation data is computed outside of Unreal Engine.

[0, INT_MAX]

groom_guide_weights

float32[3]

Uniform

Yes

Intended for specialized use where the interpolation data is computed outside of Unreal Engine.

[0, FLOAT_MAX]

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