Blendshapes are duplicates of a shape that have been changed. They are used for various uses but in this case they will drive the facial animation.
Meryl will need several mouth and eye movements as part of her animation. To do this I need to duplicate the original mesh. On this new mesh I will change it to give her single facial movement, for example, angry eyes.
I then select the duplicate and orginal and assign it as a blendshape. I can then control with a slider the blend: for example the eyebrows will move from the orginal position to the position I put them on on the duplicate. I do this process for each and every movement on her face, including raised eyebrows, a smile, a frown, surprised looks and other expressions. With the sliders I can then mix them up to give full facial animation. Below are the amount of these blendshapes I had to use for Meryl.
These blendshapes are now part of the rig meaning I now nearly have full body and face animation, all that’s left is the eyes and mouth.