I'm making some facial expressions for a stylized character, and I've run into an issue where, in order for the eyebrows to adhere to the surface of the face, I need to have a different shape for the negative and positive values of a shape key.
I could split these shapes into multiple keys, but that would be hard to animate, as I would have to make sure that one key always transitions linearly into the other. Alternatively, I could use drivers to control the two keys with a bone, but this would be inconvenient, as no other facial expressions are controlled by bones. I'd really like to keep this expression controlled by just one slider.