Drivers.md

I don't know a whole lot about these, and the docs seem to do a good job of covering most things.

Find neat tricks in TextEditor -> Templates -> Python -> Driver Functions

Driver from Python file

This is the long way, the scenic route. Using a python expression from a file. Sometimes you'll have expressions that are a bit more complicated than what you can fit onto 1 line comfortably.

• Make a script in the Text Editor with content similar to this, then run it.

  import math
  import bpy
  
  def driver_func(current_frame):
      frames = 120   # <-- period.
      theta = math.pi * 2 / frames
      final = math.sin(current_frame % frames * theta)
      return final
  
  # add function to driver_namespace
  bpy.app.driver_namespace['driver_func'] = driver_func

• Right-click the property you want to control (for instance the X rotation of a cube), choose Add Single Driver

• enable Auto run python scripts in User Preferences -> File.

• Open the Graph Editor, and switch to Drivers view

• select the driven property, and toggle the rightside panel
  

If your driver has a python error, you can correct it and run the code again to overwrite the namespace key 'driver_func`. You can name this function whatever you wish, and can have as many of them as you need in the .blend.

example 2

This might be obvious, but let's be explicit about the possibilities. We can use Empties as dummy objects, and give them custom properties, and assign a Driver to the custom property. Drivers can execute pretty much anything, that means also updating multiple other objects.

The following code randomizes the location of an object named 'Text' between frame 30 and 60. The return value of the driver function is never used, it's the function body we care about.

import math
import bpy
import random

def driver_func2(current_frame):
    
    obj = bpy.data.objects.get('Text')
    if not obj:
        return 0.0

    rnd = random.random
    if 30 < current_frame < 60:
        obj.location = rnd(), rnd(), rnd()
    else:
        if obj.location.length > 0.0001:
            obj.location = 0, 0, 0

    return 0.0

# add function to driver_namespace
bpy.app.driver_namespace['driver_func2'] = driver_func2

example 3

Change material depending on delta location of keyframed object

import bpy
from mathutils import Vector

def driver_delta_to_RED(frame):
    # triggered by a frame change, any code inside here gets run.

    p = bpy.data.objects['Plane']
    current_xyz = p.location
    fcurve = p.animation_data.action.fcurves

    x = fcurve[0].evaluate(frame-1)
    y = fcurve[1].evaluate(frame-1)
    z = fcurve[2].evaluate(frame-1)

    # may want to find the top speed first, and normalize
    # this value using that information
    delta = (current_xyz - Vector((x, y, z))).length

    nodes = bpy.data.materials[0].node_tree.nodes
    nodes[1].inputs[0].default_value = (delta, 0, 0, 1.0)

    # the return value is of no relevance and can be static.
    return 0.0

bpy.app.driver_namespace['driver_delta_to_RED'] = driver_delta_to_RED