Tutorial — Expression Editor Fundamentals

Augmented Traveler local tutorial. Back to Maya — Tutorials · previous: Tutorial — Obtaining Initial Head DNA · sculpt pass: Tutorial — ZBrush No-Topology-Change Round Trip

The Expression Editor in MetaHuman for Maya changes a head DNA file at the expression level. This tutorial covers the ideas you need before touching a historical figure's face: sessions, modes, the order of work and how to get the result back into Unreal.

Draft. Written from Epic's MetaHuman 5.8 documentation. Validate on the first figure and correct button names here.

1. What it is for (and not for)

Use it to Don't use it to
Restore likeness details the auto-rig smoothed away (neutral pose) Fix a large likeness miss. Re-sculpt and resubmit through Mesh to MetaHuman instead
Fix interpenetration or unflattering shapes in specific expressions Rebuild the facial rig by hand
Push a figure's characteristic expressions (for example Mitchell's slight asymmetric smile, if the sources support it) Invent features the sources don't support
Adjust skin weights where the face tears or pinches Edit body DNA or joints-only rigs (not supported)

Limit: very different morphologies (for example extra eyes or mouths) are not supported. That doesn't affect us.

2. Two words you must keep apart

The golden rule: editing the scene does not change the DNA. You must update the DNA, which writes the scene into the DNA in memory. Exporting writes the DNA in memory, as it was at the last update, not what you see in the viewport.

This split exists because some operations (spreading changes to all LODs, automatic joint matching) take seconds to minutes.

3. The three editing modes

Mode Holds Key facts
Neutral Pose Vertex and joint positions at rest (the bind pose) Work on LOD0 only and let the plugin spread changes to lower LODs
Skin Weights How each vertex follows joints Each LOD has its own weights. Maximum influences per vertex: 12 (LOD0–2), 8 (LOD3–5), 4 (LOD6–7)
Expression Poses Each expression as joint animation plus per-vertex displacement Edited through a graph of expressions, in tier order (below)

4. The order of work

Each step builds on the ones before it, so work in this order:

  1. Get the starting DNA. See Tutorial — Obtaining Initial Head DNA.
  2. Get the best neutral pose.
  3. Check skin weights and adjust them where needed.
  4. Edit expression poses.
  5. Export for MetaHuman Creator and/or Unreal.

Going back to the neutral after editing expressions can undo earlier expression work. If you must, re-check every expression you edited.

5. Neutral Pose editing

  1. Switch the editor to Neutral Pose mode and select LOD0.
  2. Change the head shape, either directly in Maya or from a ZBrush sculpt:
    • Directly: edit the LOD0 mesh in the scene.
    • From a sculpt: import the sculpted mesh, which must have the same topology and vertex order (Tutorial — ZBrush No-Topology-Change Round Trip). Select the LOD0 mesh first, then the sculpt, and click Transfer Vertices.
  3. Click Update Vertices in the toolbar. This writes the scene to the DNA in memory and moves the surface joints and some volume joints to fit the new shape.
  4. Leave surface joints to automation. Place volume joints (eyes, mouth) by hand only if you moved the eyes or mouth.
  5. Save with automatic LOD spreading, then reload the DNA so the copy in memory matches.

6. Expression Poses: the graph and the tiers

Expressions sit in a graph. Edit them from the top tier down:

Tier What it is Order
Primary The most important single expressions Edit first
Secondary Combinations of primaries Next
Tertiary Shallow combinations Rarely
Isolated Independent sub-graphs As needed
Tweaker Technical fixes Last

Selecting: click or drag-select nodes. Right-click → Select Upstream Nodes / Select Downstream Nodes / Select Upstream and Downstream, to see what a change will affect.

Checking:

Utility toolbar:

Group Commands
Mirror Toggle active side · Mirror vertex positions · Mirror joint transforms
Flip Flip Vertices · Flip Joints
Mesh utilities Transfer Vertex Positions · Delete History
Revert Back to the neutral, or back to the DNA as loaded
Copy / paste Joint transforms
Joint matching Machine-learning joint matching

Symmetry: historical faces are not perfectly symmetrical. Mirror only when the neutral is symmetrical at that spot. Otherwise edit each side and record why in the likeness log.

7. Likeness level: MH1 for our figures

8. Getting the result out

Destination How Caveat
MetaHuman Creator (to keep editing skin, eyes and grooms) Creator → Head → Conform → Import DNA → turn on Import Whole Rig → Import The Head tools are then locked for that character. Removing the rig or making a new one loses the Expression Editor edits
Unreal directly (FBX + DNA) Export from the Expression Editor: Z-up (default) or Y-up; provide the body DNA or a compatible Maya .ma file This DNA works with RigLogic but not with the Expression Editor or Creator. Keep both files

Importing into Unreal:

9. Practice exercise (before touching a real figure)

  1. Load a test MetaHuman's DNA.
  2. In Neutral Pose, push the cheekbones out slightly. Transfer the vertices, then click Update Vertices.
  3. Save with LOD spreading and reload.
  4. In Expression Poses, find the smile primary and remove any lip-into-teeth interpenetration.
  5. Export to Creator and check that the edits survive.
  6. Record every step and snag here, and correct this tutorial.

Sources