Handler frame_change_pre doesn't work in render

Question

this works in viewport but not in render.

import bpy
from math import *

def frame_handler(scene):
    z = scene.objects['Lift'].location[2]
    scene.objects['Text'].data.body = str(floor(z / 4.0) - 2)

bpy.app.handlers.frame_change_pre.clear()
bpy.app.handlers.frame_change_pre.append(frame_handler)

How to get it working in render? I've read this answer and changed from "bpy.data.objects" to "scene.objects" as already seen in the script. What am I missing? Blender v. 2.81.16

Edit. Tried that depsgraph thing mentioned in comments and changed to frame_change_post:

import bpy
from bpy.app.handlers import persistent
from math import *

@persistent  # maybe not relevant but doesn't harm anything?
def frame_handler(scene, depsgraph):
    z = scene.objects['Lift'].evaluated_get(depsgraph).location[2]
    scene.objects['Text'].data.body = str(floor(z / 4.0) - 2)

bpy.app.handlers.frame_change_pre.clear()
bpy.app.handlers.frame_change_post.clear()
bpy.app.handlers.frame_change_post.append(frame_handler)

Now the Text doesn't show up at all in rendered (but works in viewport).

Answer 1

The answer is similar to this question. Though all this is new from 2.81.

So here are some little more information (as far as I understand all this, which seems to be new thing along the current versions).

To make your script work, you can change it to:

import bpy

def frame_handler(scene, depsgraph):
    #print(depsgraph)
    obj = scene.objects['Cube']
    obj = obj.evaluated_get(depsgraph)
    scene.objects['Text'].data.body = '{0:.2f}'.format(obj.location.z)

bpy.app.handlers.frame_change_post.clear()
bpy.app.handlers.frame_change_post.append(frame_handler)

Here frame_change_post is used. But following this documentation:

This can be used, for example, to alter scene prior it gets rendered.

So the result will be what you want to do.

In the script above I've changed the rounding for Z. That allows to confirm that 'post' is before rendering. If object Z position is 10 at frame 10, the text object will display 10 at the same frame.

Additionally, depsgraph is for dependencies graph. That means a data provided in the API to take into account the fact that during animation (or others) the object state varies due to what it depends on.

frame_change_pre has no depsgraph parameter in 2.82.

Answer 2

In sequence-view:

• click View menu item
• click Sequence Render animation

This will correctly export an animation with frame_handler