Compositor viewer node output never updates when rendering via Python script

Question

Based on this answer, I am trying to extract the per-frame RGB and depth arrays of some animation scene. However, the compositor render layer and viewer node setup that I am programmatically building does not appear to work. I am connecting 2 outputs of the render layer to 2 inputs of the viewer node, but any call to render() does not actually change or update the pixels field of the viewer node -- the render process itself works (for example, it shows stuff when storing to an output file), nothing happens in the context of the compositor. All output values are always zero.

In fact, even the size/shape of pixels is incorrect (262144) and does not depend on whatever input resolution I pass to the render engine.

Here is a minimal example to reproduce the problem:

import bpy
import numpy as np

width = 640
height = 480

bpy.context.scene.render.engine = 'CYCLES'
bpy.context.scene.render.resolution_x = width
bpy.context.scene.render.resolution_y = height
bpy.context.scene.render.resolution_percentage = 100

bpy.context.scene.render.use_compositing = True
bpy.context.scene.use_nodes = True
tree = bpy.context.scene.node_tree
links = tree.links

for n in tree.nodes:
    tree.nodes.remove(n)
rl = tree.nodes.new('CompositorNodeRLayers')      

vl = tree.nodes.new('CompositorNodeViewer')   
vl.use_alpha = True
links.new(rl.outputs[0], vl.inputs[0])  # link Renger Image to Viewer Image
links.new(rl.outputs[2], vl.inputs[1])  # link Render Z to Viewer Alpha

# Render
bpy.ops.render.render()

# Get the pixels and put them into a numpy array
pixels = np.array(bpy.data.images['Viewer Node'].pixels)
print(len(pixels))

width = bpy.context.scene.render.resolution_x 
height = bpy.context.scene.render.resolution_y

# Reshaping into image array 4 channel (rgbz)
print('pixels:', pixels.shape)  # (262144, ) even though 640*480*4 = 1228800
print('values:', np.min(pixels), np.max(pixels))  # 0.0, 0.0
image = pixels.reshape((height, width, 4))  # Error

I am using Blender 2.83.6 LTS, and am running this command to execute the script:

blender --background --python .\minimal_blender.py

I feel like I am overlooking something simple (I am a beginner to Blender after all) but I cannot determine what?

Note: I am aware that --background could have something to do with it, but I am unable to disable that option, because doing so introduces a whole range of RuntimeError: Operator bpy.ops.wm.append.poll() failed, context is incorrect errors all over the place in my other script, which I have no idea how to fix. So I guess this question is essentially how to force the viewer node to update when running Blender in the background.

Answer 1

Solved -- my problem was actually related to Blender's hard-coded behavior in background mode, rather than to something going wrong with the render process itself. In order to get the Viewer Node to work in background mode, you need to modify some source code and recompile Blender as described in the link, because it is disabled by default for optimization reasons. Then run:

path\to\recompiled\blender --background --python my_render_script.py