The imshow function of Matplotlib displays grayscale images. Vmin and vmax2 parameters should be set

matplotlib Of imshow Functions are very complex , Support a lot of people colormap, Change and so on . When using this function, you need to pay attention to the parameter settings , Especially the gray image .
imshow Yes 2 Parameters vmin and vmax. When displaying grayscale images , To set the upper vmin=0 as well as vmax=255. otherwise imshow Function will automatically adjust the value .
The values entered are ：X1 = [[1,2,3],[4,5,6]] and X2 = [[111,112,113],[114,115,116]]. If not set vmin and vmax Then the image is similar .

#coding:utf8

import matplotlib.pyplot as plt

X1 = [[1,2,3],[4,5,6]]
X2 = [[111,112,113],[114,115,116]]
fig = plt.figure()
ax = fig.add_subplot(121) 
im = ax.imshow(X1,cmap=plt.cm.gray) 
plt.colorbar(im, cax=None, ax=None,shrink=0.5)

ax = fig.add_subplot(122)
im = ax.imshow(X2, cmap=plt.cm.gray)
plt.colorbar(im, cax=None, ax=None,shrink=0.5)

plt.show()

If you set vmin=0,vmax=255, The situation will be different ：

The first 3 Zhang heti 4 The gray value setting of the image is [0,255], here X1 and X2 Because the value is close , Display as a near solid color rather than a color block with a large difference in grayscale . So in use matplotlib Of imshow Pay attention to this situation when using the function , Otherwise, you may not get the desired results , For example, there may be no significant change in the grayscale .

# coding:utf8

import matplotlib.pyplot as plt

X1 = [[1, 2, 3], [4, 5, 6]]
X2 = [[111, 112, 113], [114, 115, 116]]
fig = plt.figure()
ax = fig.add_subplot(221)
im = ax.imshow(X1, cmap=plt.cm.gray)
plt.colorbar(im, cax=None, ax=None, shrink=0.5)

ax = fig.add_subplot(222)
im = ax.imshow(X2, cmap=plt.cm.gray)
plt.colorbar(im, cax=None, ax=None, shrink=0.5)

ax = fig.add_subplot(223)
im = ax.imshow(X1, cmap=plt.cm.gray, vmin=0, vmax=255)
plt.colorbar(im, cax=None, ax=None, shrink=0.5)

ax = fig.add_subplot(224)
im = ax.imshow(X2, cmap=plt.cm.gray, vmin=0, vmax=255)
plt.colorbar(im, cax=None, ax=None, shrink=0.5)

plt.show()