This section is based on years of experience at 3Lateral. It’s phrased assertively for clarity, but Expression Editor itself doesn’t lock you into this specific series of operations. As you mature experience with editing, and knowledge of the rig, deviating might become desirable, you are free to do so (at your own risk.)
These recommendations best apply to human-looking characters. The further from human morphology the model, the more likely it is that you’ll have to develop your own editing sequence.
Order of Operations
Workflow moves from Neutral Pose editing, to Skin Weight editing, to Expression Pose shape editing (meshes), to Expression Pose joint editing. At this granularity there is no reason to change order of operations, and that order is partly built into the tool.
It’s within expression editing, and when needing to go back from expression editing to Skin Weights or Neutral Pose corrections, that things become more fluid in regards to what expressions you edit and how. Your choice of LODs can also affect the workflow significantly.
We strongly recommend always taking care of LOD 0 though, even if you don’t intend to use it. Much depends on it and matching the meshes in it.
Neutral Pose Editing
There are no particular recommendations to editing the Neutral Pose that haven’t been made in the workflow section.
Summarily speaking there are three things to keep in mind:
Minimize the distance between the mesh and the surface joints in soft areas (usually done automatically).
Ensure volume joints are properly oriented to the features they most affect.
Preserve the semantic significance of vertices (which vertex underpins what facial feature).
If you deviate from the topological semantics of humans, be ready to match that work in the skinning stages.
Skin Weights Editing
Skin Weights editing has to be independently authored for each LOD you want to use.
We don’t provide tools to transfer weights between LODs, but we try to leave the scene as “vanilla Maya” as possible so that your own tooling can be used to address these needs.
It’s worth noting that not all LODs joint lists are a direct subset of the higher resolution LOD; for example, LOD 1 is a mix of joints from LOD 0 and 2. More is explained in the Joints Matching section later, but it’s useful to know that LOD 4 has all volume joints, and only volume joints, and LOD 2 is the lowest resolution LOD that contains all eyelid joints. Those are almost always good starting points.
In general, Skin Weight fixes tend to only be necessary to improve the seal of eyelids, and far more rarely of lips. For human morphologies it’s extremely rare, even in the presence of very accented features, that anything else will be necessary.
The farther from human morphologies and the standard topological semantics one moves, the more extensive and more involved work will need to be done at the Skin Weight editing stage. As those are all unique situations it’s impossible for us to provide general guidance.
Expression Poses Mesh Editing
Mesh Editing, also referred to as Shape Editing, has to always take place before Joints Matching.
Joints Matching is performed to match the volume described by the Expression Pose meshes, and overwrites Joint Animation corrections that might have already been made.
Broadly speaking there are two types of edit: Creative, and Technical. There is no technical difference between the two, only the intent differs.
We do recommend having both performance as well as synthetic ROM animations at hand. A good synthetic ROM of all expressions will allow you to quickly identify any need for technical fixes, which you can execute on methodically. An example ROM (gui_technical_rom.fbx) is provided as part of the installed MetaHuman for Maya plugin.
Creative needs usually relate to likeness, either to an actor or actress (to improve solving in MetaHuman Animator with a higher-fidelity Identity, or simply higher fidelity to recognizable cast), or to the artistic vision. Switch back frequently to animation after correcting poses to see if the effect is what you want. Correcting poses in isolation, until a lot of experience has been matured, rarely looks as good as one expects; this is particularly true when exaggerating them.
What to edit is entirely up to your creative needs. Choosing what combinatorial expressions inherit changes from upstream is also a matter of experience.
We recommend you import meshes with the changes to sync to (instead of manipulating the mesh in place), and create copies of dependent expression meshes pre-edit, this way you’ll be able to experiment and learn quickly, safe in the knowledge you can fall back to the original state if you over-propagated a correction.
Next Up
Joints Matching
Recommendations when joints matching for human-looking characters