Render event order

To execute some actions at different stages of rendering, the developers have provided a set of handlers in the Blender Python API – special functions for handling events.

.blend file on Patreon

We can get all available handlers by executing the following command:

print(bpy.app.handlers)

# bpy.app.handlers(frame_change_pre=[], frame_change_post=[], render_pre=[], render_post=[], ... )

print(bpy.app.handlers)

# bpy.app.handlers(frame_change_pre=[], frame_change_post=[], render_pre=[], render_post=[], ... )

The handlers that are directly related to the rendering process:

# on_render_init, on_render_pre, on_render_post, on_render_write, on_render_complete, on_render_cancel

1	# on_render_init, on_render_pre, on_render_post, on_render_write, on_render_complete, on_render_cancel

Each of these handlers is called at a certain moment, for example, the “on_render_init” handler is called when the render starts.

For the actions we need to be performed during the call to this handler, let’s define a function:

def on_render_init(*args):
    print('on redner init')

1 2	def on_render_init(*args): print('on redner init')

In our example, it just prints a text to the console, but we can fill our function with actions whatever we need.

Bind our function to the handler call:

bpy.app.handlers.render_init.append(on_render_init)

1	bpy.app.handlers.render_init.append(on_render_init)

Now, every time at the beginning of the render, the text will be printed to the system console.

Let’s define similar functions for each render event:

def on_render_pre(*args):
    print('on redner pre')

def on_render_post(*args):
    print('on redner post')

def on_render_write(*args):
    print('on render write')

def on_render_complete(*args):
    print('on redner complete')

def on_render_cancel(*args):
    print('on redner cancel')

def on_render_pre(*args):

print('on redner pre')

def on_render_post(*args):

print('on redner post')

def on_render_write(*args):

print('on render write')

def on_render_complete(*args):

print('on redner complete')

def on_render_cancel(*args):

print('on redner cancel')

And link them with the corresponding handlers:

bpy.app.handlers.render_init.append(on_render_init)
bpy.app.handlers.render_pre.append(on_render_pre)
bpy.app.handlers.render_post.append(on_render_post)
bpy.app.handlers.render_write.append(on_render_write)
bpy.app.handlers.render_complete.append(on_render_complete)
bpy.app.handlers.render_cancel.append(on_render_cancel)

bpy.app.handlers.render_init.append(on_render_init)

bpy.app.handlers.render_pre.append(on_render_pre)

bpy.app.handlers.render_post.append(on_render_post)

bpy.app.handlers.render_write.append(on_render_write)

bpy.app.handlers.render_complete.append(on_render_complete)

bpy.app.handlers.render_cancel.append(on_render_cancel)

Now, when rendering, we will get the following output to the system console, for a two-frame animation:

on redner init
on redner pre
Saved: 'C:\tmp\0001.png'
 Time: 00:00.32on render stats
 (Saving: 00:00.02)
on redner post
on render write
on redner pre
Saved: 'C:\tmp\0002.png'
 Time: 00:00.13on render stats
 (Saving: 00:00.02)
on redner post
on render write
on redner complete

on redner init

on redner pre

Saved: 'C:\tmp\0001.png'

Time: 00:00.32on render stats

(Saving: 00:00.02)

on redner post

on render write

on redner pre

Saved: 'C:\tmp\0002.png'

Time: 00:00.13on render stats

(Saving: 00:00.02)

on redner post

on render write

on redner complete

As we can see, rendering events are handled in the following order:

render_init – before rendering starts
render_pre – before rendering of each frame
render_post – after each frame has finished rendering
render_write – after saving each frame
render_complete – after all frames are rendered

The render_cancel event is fired when a render is canceled, for example, if the user presses the ESC button during a render.