Expression Editor loads and saves DNA files. Maya scenes are of little importance in this process. DNA isn’t as much created in Expression Editor, as it’s calibrated to a likeness.
To that effect (calibration) obtaining an DNA file as close as possible to the target neutral pose is the best start possible. Poor neutral pose likeness or an expressive neutral pose always leads to poorer results.
Whether starting from a completely manually sculpted/matched Neutral pose, or refitting through Mesh to MetaHuman, two concepts need to be considered for good results: Semantic Significance of vertices, and Delta Minimization.
Semantic Significance
Semantic significance implies that specific vertices are coupled with specific facial features. The relationship between joints and vertices, how joints are mapped to expressions, and various region maps for the face, are what makes vertices semantically significant.
A vertex representing the edge of the eyelid should not be “rolled” over to be just outside, or just inside. The vertices used for the pinching point of the mouth shouldn’t be moved away to be used as a dimple, while other vertices slide in to do their job (in describing the volume), and so on.
Some features, especially in the most expressive and intricate areas like eyes and mouth corners, are extremely sensitive to the specificity of vertices. Some others move less (or not at all) and offer more leeway.
Delta Minimization
Mesh to MetaHuman performs a first local fit of features to the volume provided, then sends it to the MetaHuman Services for autorigging. During this second step an DNA is created that matches as closely as possible that volume, and (as much as possible of) the remaining difference between the find in the database and the volume is preserved as a delta.
What follows is meant to be read in the context of a first fit (Mesh to MetaHuman usually), and the Neutral Pose. It becomes inapplicable, and explanations will become more nuanced, in the Expression Poses Editing context. This Delta can be controlled and reduced in MetaHuman Creator using the Global Delta parameter in the Head Conform Controls, but at the cost of likeness. The Delta shows up as a gap between joint positioning and vertex positioning. Minimizing this delta away is key to expressions not carrying extra mass that’s too distant from the joints.
Using Mesh to MetaHuman
Because of the semantic significance of specific vertices, we strongly recommend you let our autorigging service (the backend part of the Mesh to MetaHuman feature) take care of a first pass fitting and placement for you.
This fitting “smooths” your mesh to some extent, as it will represent the volume with the best finds from a database of real heads. This smoothing can be pronounced if you have non-human features, for example pointy ears.
Results of fitting can be taken into Neutral Pose calibration to reintroduce features that were smoothed out, or the resulting mesh could be resculpted from the MetaHuman topologized mesh, then resubmit for autorigging with a strict constraint on vertex positions: Conforming from a Template Mesh is documented as part of MetaHuman Creator.
For Expression Editor users we recommend moving to Neutral Pose editing and selectively reintroducing the features there.
Loading MetaHuman DNA
Once a DNA file has been procured, open the Expression Editor window, go to the Home pane, and click on the folder icon by the path text field. Select the DNA file then click Open.
Once that’s done a copy of that DNA will have been loaded in memory and available for the session, but no visible changes to the Maya Scene will have taken place yet.
