Random driver expression for automated blinking

Question

I have a character with eyes.

I have a blink shapekey. and I am using this expression as a driver to randomly blink on certain frames

noise.random() > 0.97

But the blinking is a bit abrupt. I wish I could do it over three frames instead of one. Is there some way to get random blinking in this manner?

Answer 1

You can add custom functions instead of simple expressions in your driver. There you can realise more complex behaviours.

Head to the Scripting tab and run this script (see docs):

import bpy
from mathutils import noise

def blink_longer():

    # still blinking
    if blink_longer.time > 0:
        blink_longer.time -= 1
        return True
    # new blink?
    elif noise.random() > 0.97:
        blink_longer.time = 3 # max blink time
        return True
    # not blinking
    else:
        return False

blink_longer.time = 0  # avoid a global to track blinking

# add blink function to the driver namespace
bpy.app.driver_namespace['blink_longer'] = blink_longer

Side note: I added an attribute to the function to avoid a global, but globals will work too.

Now in your driver expression, just call the function blink_longer() et voila, longer blinking.

Edit: Based on @HarryMcKenzie's comment, some notes about usage: because driver functions drive something (i.e. values) they must return the driven value. In the screenshot i added a custom function

def func():
    print("func")

to the driver namespace by running the script (note that this action works as func itself is valid python). But using this function in a driver results in ERROR: Invalid Python expression which my be confusing. By checking the error with Window > Toggle System Console one can see that "func" is correctly printed (thus func has been called) and after that a type error occures. The driven weight value of the shape key is a number but func returns implicitly None, hence the error. func needs to return something number compatible in this case. The blink_longer function returns True/False which is ok as True/False are coerced to $1/0$ respectively. After fixing errors, run the namespace script again to override the old function definition and then refresh the driver expression.