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There are corresponding mouth shapes for most sounds, but the "Th" sound is still unsupported. I would really like to have a tongue between the teeth added into the mouth shapes to be computed with the lip-sync.
The "th" used to be in in an earlier version, you can see it in the every other helpfile or an early template puppet. I suggested s.th. similar to your idea, albeit in a more open, general way here: https://character-animator.ideas.aha.io/ideas/CH-I-213
It would mean that you could adapt "th" or any other sound plus mouthshape into Ch individually.
It's true that Th is no longer its own viseme, but it does get recognized and mapped to the "L" viseme. We found that they were close enough in most puppets to not be worth distinguishing between. We do plan in the future to have the viseme editor allow custom mouths to manually invoke, and in the meantime you can do this with keyboard triggers.
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