Python render script different outcome when run in background When running my bpy script from command line with the argument -b (background mode) I get a different size of the image vector I want to render than without the argument. Did somebody encounter the same problem already?  For better explanation here is the code:

scn.render.resolution_percentage = 100
scene.use_nodes = True
tree = scene.node_tree
links = tree.links

# create input render layer node
rl = tree.nodes.new('CompositorNodeRLayers')    

# create output node
v = tree.nodes.new('CompositorNodeViewer')   
v.use_alpha = False

# Links
links.new(rl.outputs['Z'], v.inputs[0]) # link Z to output
scn.render.image_settings.color_mode = 'BW'
bpy.ops.render.render()

# get viewer pixels
pixels = bpy.data.images['Viewer Node'].pixels


Render settings are 520x424 pixels defined in the blend file. 
in the command line I use the following cmd: 

blender -b ...\Blender_Animation1.blend --python ...main.py

when I check the variable pixels i get different lengths depending whether I use the b or not. Without b I get  881920 values ( 1 pixel = RGBA -> 520*424*4 = 881920) which is correct. In the other case I get much less. Did somebody have the same problem already and knows what causes this error?
Rendering of viewer nodes and previews in background mode was disabled in 2013 for speed optimization (see here). However, you can easily enable it again by modifying 2 files of the blender sources and compiling blender yourself (as described here).

In the file source/blender/compositor/operations/COM_ViewerOperation.h, line ~58:

bool isOutputOperation(bool /*rendering*/) const { if (G.background) return false; return isActiveViewerOutput();


should be changed to

bool isOutputOperation(bool /*rendering*/) const {return isActiveViewerOutput(); }


and in file source/blender/compositor/operations/COM_PreviewOperation.h, line ~48:

bool isOutputOperation(bool /*rendering*/) const { return !G.background; }


should be changed to

bool isOutputOperation(bool /*rendering*/) const { return true; }


After these changes, the pixels array gets properly updated in background mode.