Expression Editor uses a workflow based on a “Home” location that contains the first and last common operations (opening a DNA file for the first time and exporting back to DNA), with all other operations hosted in task-specific “screens”. These task specific screens and how they configure the Maya session are called Editing Modes.
Neutral Pose
The Neutral Pose is both an Editing Mode of the UX, as well as the catch-all term to describe the rest state of a MetaHuman head.
The neutral pose is represented by the positions of the vertices at rest, and the position of the joints at rest. It establishes the bind pose for the rig.
If your DNA has been obtained by conforming the head to a MetaHuman Identity (and so through Mesh to MetaHuman), and you didn’t blend out the Global Delta parameter from MetaHuman Creator, that delta (or offset) will be baked into the neutral pose point positions and might be apparent as an observable gap between the surface joints and and the mesh.
Skin Weights
Skin Weight(s) is both an editing mode, and the envelope of the Soft Skinning deformer binding the mesh to joints, which is to say: the common use of the word in rigging. Skin weights exist independently for each LOD.
It’s usually worth preserving the Influence per Vertex limit maximum as it exists across the LODs.
LODs 0, 1, and 2 have a 12 joints influence cap.
LODs 3, 4, and 5 have an 8 joint influence cap.
LODs 6, and 7 have a 4 joints influence cap.
Expression Poses (Expressions)
Expression Poses (often just Expressions, if capitalized) represent a specific state of the face.
This is at the core of the rig’s design. The number and intent of these Expressions is predetermined, and editing the right subsets of expressions in the right order is key to pushing likeness in MetaHuman faces.
These poses comprise a joint animation that positions all joints, and per-vertex displacement (morphs) for further and finer detail; the latter only exists at LOD0.
A comprehensive list of expressions and what they might look like can be found in the MetaHuman Facial Description Standard.
Bear in mind that the MetaHuman Facial Description Standard are the shapes after they have been processed into sides and directions. Expression Editor uses a smaller set of Expressions to generate the full list used by the rig.
Phased Expressions
Expressions can be single phase, or multiphase. Multiphase expressions are those that have multiple, sequential steps encoded.
In the current rig definition only neck_swallow and mouth_lipsSticky_tgt are multiphase expressions.
Side Specific Expressions
In the final rig most expressions are available in their left and right components. During expression editing, however, many of those are merged into a single expression target, and per side weightmaps are used to split them at DNA computation time.
Some expressions, however, are significant and interact prominently along the symmetry axis of the face; these will have their right and left components available for individual editing, and Primary combination to edit their interaction; for example, mouth_corner_pull.
Expression Tiers
Expressions can be tiered by their impact when editing. It’s important to note, however, that these are common cases and groupings matured on realistic humans. Purely technical fixes, strong characterization, or departing from the common human face, can very easily offset where or how you operate in the graph.
We provide a full list of all expressions and their tier for reference. Consider the following as useful guidelines, don’t treat it as strict guidance, or constraints on Expression Editor’s operation.
Primary Expressions
Primary Expressions are the more significant and impactful Expressions. This significance comes from the breadth of the dependencies, and their contribution to the likeness of the head in movement. Primary Expressions exist that depend on other Primary Expressions, which means they aren’t always independent when it comes to order of editing.
Secondary Expressions
Secondary Expressions are combinations of Primary Expressions, or combinations of preceding Secondary Expressions; usually inspected and modified only after all their upstream Primaries have been treated.
Tertiary Expressions
Some combinations are part of shallow subgraphs, are exclusively combinations, and only combine from Secondary Expressions. These tend to be technical, infrequent edits, and are performed only after all Secondary Expressions have been treated.
Isolated Expressions
Some Expressions participate in smaller and more isolated subgraphs than Primary or Secondary ones, and can be treated separately. Isolated expressions can still participate in combinations, but they are usually treated last, if at all.
Tweaker Expressions
Tweaker Expressions come from the need for occasional “technical” manipulation of a face that’s not tied to specific muscles. These are very infrequent, technical edits, and usually requires everything else to be settled before being worth the time.
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Expressions by Tier
A full list of all expressions and their tier for reference.
UX Overview
An overview of the Expression Editor user interface and functionality.