How to symmetrize shape keys? Basically I done all the facial animations on the +x axis but I want to make some shape keys symmetrical like when the character opens the mouth.

I tried using the mirror shape key and the new shape from mix like I'm doing with the eyes to make them blink at the same time but since the mouth has movement exactly in the middle vertices the mix ends up with the middle vertices in a different place.

I could do the mirror, reset the position on the middle vertices back to basic shape and do the mix, but is there a faster way?
If you want to mirror some shape keys without doubling the quantity of movement for the center line of vertices, you need to use at least one weightgroup with all vertex at 1 except for the middle ones which will be at 0.5.



Individual shape will have their movement on center line halved, but it's a common practice. Here I show one weightgroup (1-0.5-1) but you will often see two weightgroups (1-0.5-0) and (0-0.5-1).

You can watch this video tutorial from Angela Guenette for this process, and I highly invite you to watch the whole series.