I got my object and I need to know all pixels that are displaying this object in my rendered picture.

I read about a smiliar question here.

update I tried this code from here:

def tocam(scene, ob):
    cam_vec = cam.matrix_world.to_3x3() * Vector((0, 0, -1))
    R = cam_vec.to_track_quat('-Z', 'Y').to_matrix().to_4x4()

    # scale based on resolution
    S = Matrix() # resX=resY
    # translate such that origin is middle point of image (and hence cam)
    T = Matrix.Translation((-0.5, -0.5, 0))

    ob.matrix_world = Matrix()
    for v in ob.data.vertices:
        vec = w2cv(scene, cam, v.co)
        v.co = vec.x, vec.y, 0

    ob.data.transform(S * T)

    ob.matrix_world = R
    angle_x = cam.data.angle_x
    x = (0.5 /  tan(angle_x / 2)) * cam_vec.normalized()
    ob.matrix_world.translation = cam.matrix_world.translation + x
    if cam.data.type == 'ORTHO':
        ob.scale *= cam.data.ortho_scale

    res_x = 640
    res_y = 640

    # 2d data printout:
    rnd = lambda i: round(i)

    for v in ob.data.vertices:
        print("{},{}".format(rnd(res_x*v.co.x), rnd(res_y*v.co.y)))

when I plot the points I get this:

enter image description here

but my rendered image is this:

enter image description here

So you can see, that it is not on the correct position and also the shape is not really correct.

Can anyone help me on this? Thanks a lot!

  • $\begingroup$ Related blender.stackexchange.com/questions/159914/… $\endgroup$
    – batFINGER
    Feb 3, 2020 at 15:21
  • $\begingroup$ Can you tell me more about what your use case is? Would it be viable to you, if the script was run from the cmd and you'd then get a numpy array that you can work with or does it have to be run from inside blenders scripting tab? $\endgroup$
    – WhatAMesh
    Feb 7, 2020 at 8:54
  • $\begingroup$ @WhatAMesh it would be good if it's inside the same script. I am rendering multiple images in my script. The goal is to use them as trainings data for a neural network to do segmentation. So to use the renered images as trainingsdata I need to know which pixels in the rendered Image belong to my object I rendered. At the moment I am starting blender with my script like this: ./blender first.blend --background --python render.py $\endgroup$ Feb 7, 2020 at 14:26

1 Answer 1


Basically one can just render the masks of the objects as images, import them and generate the arrays. This process is the similar for materials (a mask for every material) and depth. enter image description here enter image description here

The script can be (at least) run in 2.8x and 2.79b, just adjust the code with the commented code. If you use 2.79b, put blender in the same folder as the your python.exe.

Master script:

import matplotlib.pyplot as plt
import numpy as np
import glob, os
import PIL.Image as pil
import subprocess

pathToSlave = r'C:\Users\YourName\PycharmProjects\bse\slave.py'

pathToFile = r'C:\Users\YourName\Desktop\segmentation.blend'
fileName = 'segmentation.blend'

pathToRenderOutput = r'C:\Users\YourName\Desktop\trash\\'
pathToBlender = r'C:\Users\YourName\AppData\Local\Programs\Python\Python36\blender-2.79b-windows64\\'

subprocess.call('{}blender {} -b  --python {}'.format(pathToBlender, pathToFile, pathToSlave))

images = glob.glob(os.path.join(pathToRenderOutput, '*.png'))
for img in images:
    arr = np.asarray(pil.open(img))
    arr = arr.sum(axis=-1)
    arr = arr.squeeze()


Blender part: A mask is generated for every mesh-object

import bpy

bpy.context.scene.render.engine = 'CYCLES'
bpy.context.scene.use_nodes = True
bpy.context.scene.cycles.samples = 1

# For 2.8x
bpy.context.view_layer.use_pass_object_index = True
# bpy.context.scene.view_layers['ViewLayer'].use_pass_object_index = True

# For 2.79b
#bpy.context.scene.render.layers["RenderLayer"].use_pass_object_index = True

tree = bpy.context.scene.node_tree
#tree = bpy.data.scenes['Scene'].node_tree

nodes = tree.nodes
links = tree.links
indexOBOutput = tree.get('IndexOB Output')

idMaskList = []

outputPath = r'C:\Users\YourName\Desktop\trash\\'

fileOutput = nodes.new(type="CompositorNodeOutputFile")
fileOutput.base_path = outputPath

for index, obj in enumerate(bpy.context.scene.objects, start=1):
    if obj.type == 'MESH':
        obj.pass_index = index

        idNode = nodes.new(type='CompositorNodeIDMask')
        idNode.index = index

        links.new(nodes.get('Render Layers').outputs.get('IndexOB'), idNode.inputs[0])

        links.new(idNode.outputs[0], fileOutput.inputs[index - 1])

  • $\begingroup$ Thank you so much for your work. I will try this tomorrow evening when I am back at my pc. $\endgroup$ Feb 8, 2020 at 0:02
  • $\begingroup$ I get AttributeError: 'Scene' object has no attribute 'view_layers' could this be because I am using blender 2.79? Maybe I can do this easier anyway as I just want a mask from one obj? $\endgroup$ Feb 10, 2020 at 9:26
  • $\begingroup$ @blenderNewbie Adjusted the answer for 2.79b. Only you know how far it can be simplified, if you have a specific object you can change the line to "for obj in bpy.data.objects['your_object_name']", I just wrote for the more general case in 'segmentation': one image containing multiple classes (objects) $\endgroup$
    – WhatAMesh
    Feb 10, 2020 at 10:43
  • $\begingroup$ Thank you! You are awesome. That helped me big time. One more question: Do you know how I can manipulate the output name of the rendered mask? now it is something like object_11000.png $\endgroup$ Feb 10, 2020 at 11:01
  • $\begingroup$ At the moment the fileoutputname is generated with fileOutput.file_slots.new('Object_{}'.format(indexPass)), you can change that however you like. The issue seems to be, that the frame number e.g. 000, 001 will always be added to the filename, which seems to be a bug: blender.stackexchange.com/questions/46062/…. In the masterscript you can rename the files with stackoverflow.com/questions/2491222/… $\endgroup$
    – WhatAMesh
    Feb 10, 2020 at 11:41

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