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

Question

I'm moving from MatLab to python and playing around with the imshow function.

I can't seem to get my head around why it doesn't show the value 128 as grey with I have chosen the cmap to be gray-scale.

It seems as it uses the grayscale for highest (128) and lowest values.. I want it to use the grayscale for [0:255]. How do I do that?

Answer 1

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.

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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()