Z buffer read from exr file is all ones

Question

I render RGB images and their depth maps as well for a series of frames (using cycles). Specifically, I need not a B/W depth png/jpg image, rather the actual depth values itself, which I can later read off in python. I found the exr to be the oft suggested format in online forums, as I want to store float values as such.

So, I stored the rgb image as well as the z buffer together in a .exr file. In python, while I could reassemble the rgb image perfectly, I discovered the Z channel info to be a matrix of ones. It should actually contain the float depth values.
What could be going wrong here? I have checked the 'zbuf' option in output properties.

For reference, this is the code I use to separate rgb and z values from the exr file:

import sys
import numpy as np
import OpenEXR as exr
import Imath

def readEXR(filename):
    """Read color + depth data from EXR image file.
    
    Parameters
    ----------
    filename : str
        File path.
        
    Returns
    -------
    img : RGB or RGBA image in float32 format. Each color channel
          lies within the interval [0, 1].
          Color conversion from linear RGB to standard RGB is performed
          internally. See https://en.wikipedia.org/wiki/SRGB#The_forward_transformation_(CIE_XYZ_to_sRGB)
          for more information.
          
    Z : Depth buffer in float32 format or None if the EXR file has no Z channel.
    """
    print(filename)
    filename = str(filename)
    exrfile = exr.InputFile(filename)
    header = exrfile.header()
    
    dw = header['dataWindow']
    isize = (dw.max.y - dw.min.y + 1, dw.max.x - dw.min.x + 1)
    
    channelData = dict()
    
    # convert all channels in the image to numpy arrays
    for c in header['channels']:
        C = exrfile.channel(c, Imath.PixelType(Imath.PixelType.FLOAT))
        C = np.fromstring(C, dtype=np.float32)
        C = np.reshape(C, isize)
        
        channelData[c] = C
    
    colorChannels = ['R', 'G', 'B', 'A'] if 'A' in header['channels'] else ['R', 'G', 'B']
    img = np.concatenate([channelData[c][...,np.newaxis] for c in colorChannels], axis=2)
    
    # linear to standard RGB
    img[..., :3] = np.where(img[..., :3] <= 0.0031308,
                            12.92 * img[..., :3],
                            1.055 * np.power(img[..., :3], 1 / 2.4) - 0.055)
    
    # sanitize image to be in range [0, 1]
    img = np.where(img < 0.0, 0.0, np.where(img > 1.0, 1, img))
    
    Z = None if 'Z' not in header['channels'] else channelData['Z']
    
    return img, Z

def main(fpth):
    img, depth = readEXR(fpth[-1])
    print(np.max(depth))
    print(np.min(depth))
    
    return 

if __name__ == '__main__':
    
    main(sys.argv[1:]) 

Answer 1

You just need to disable the compositor in Output Properties > Post Processing > Pipeline > Compositing. Basically if compositing is enabled only the Image pass gets forwarded to the composite node (even if Nodes weren't enabled) and the Depth defaults to Z=1.000

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If you do need to use the compositor, make sure you also link Depth to the composite node to get an exr file with a correct Z channel:

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