make matplotlib png plot semi-transparent with non integer alpha value-CodePudding

I know that I can produce a png plot with a transparent background

In response to the comments,

plt.get_backend()

gives me 'MacOSX' and I am on

Python 3.9.4 (default, Apr 5 2021, 01:49:30) [Clang 12.0.0 (clang-1200.0.32.29)] on darwin

CodePudding user response：

Your code behaved correctly and nothing is wrong. I tried it on colab and here is the results (notice the ax.patch.set_alpha() value):

import matplotlib.pyplot as plt
import numpy as np

fig,ax=plt.subplots()
ax.plot ([1,2,3],[2,1,3]) 
ax.patch.set_facecolor('white')
ax.patch.set_alpha(1)
plt.savefig('test.png', facecolor=fig.get_facecolor(), edgecolor='none')

import matplotlib.pyplot as plt
import numpy as np

fig,ax=plt.subplots()
ax.plot ([1,2,3],[2,1,3]) 
ax.patch.set_facecolor('white')
ax.patch.set_alpha(0)
plt.savefig('test.png', facecolor=fig.get_facecolor(), edgecolor='none')

Updated: After saving the plot, you can load it with opencv then change its transparency like this:

"""
pip install opencv-python
"""
import cv2

im = cv2.imread('test.png',cv2.IMREAD_UNCHANGED)
im[:,:,3] = im[:,:,3]/2
cv2.imwrite('adjust.png',im)

Update 2: I think I found what you want:

import matplotlib.pyplot as plt
import numpy as np
import cv2

fig,ax=plt.subplots()
ax.plot ([1,2,3],[2,1,3]) 
ax.patch.set_facecolor('white')
plt.savefig('original.png', edgecolor='none')
plt.savefig('transparent.png', edgecolor='none',transparent=True)
#Then
im = cv2.imread('original.png',cv2.IMREAD_UNCHANGED)
im[:,:,3] = im[:,:,3]/2   120
cv2.imwrite('semi_transparent.png',im)

Here is the results I got (tested on MS word):