imshow(img, cmap=cm.gray) shows a white for 128 value

by

Use the vmin and vmax parameters:

plt.imshow(bg, cmap=plt.get_cmap('gray'), vmin=0, vmax=255)

Without specifying vmin and vmax, plt.imshow auto-adjusts its range to the min and max of the data.

I do not know of a way to set default vmin and vmax parameters for all imshow plots, but you could use functools.partial to prepare a custom imshow-like command with default parameters set:

import matplotlib.pyplot as plt
import numpy as np
import functools

bwimshow = functools.partial(plt.imshow, vmin=0, vmax=255,
                             cmap=plt.get_cmap('gray'))

dots = np.random.randn(10, 10)*255
bwimshow(dots)
cbar = plt.colorbar()

plt.show()

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