Viewing a numpy array as an image-CodePudding

I have generated a 2D numpy array which creates polygon perimeters to look something like:

0 0 0 0  0    0   0   0   0 0 0
0 0 0 0 256  256 256 256  0 0 0
0 0 0 0 256   0   0  256  0 0 0 
0 0 0 0 256   0   0  256  0 0 0 
0 0 0 0 256   0   0  256  0 0 0 
0 0 0 0 256   0   0  256  0 0 0 
0 0 0 0 256  256 256 256  0 0 0
0 0 0 0  0    0   0   0   0 0 0

when I use:

img = Image.fromarray(array, 'L') # from PIL library
img.save('test'.png)

I expect to open the image and see a white rectangle outline in an otherwise black backdrop but instead I get weird pseudorandom lines. I have tried replacing all the zeros with 1s and this simply changes the image to 3 straight vertical lines.
Any thoughts on what I am doing wrong?

CodePudding user response：

Your problem is that uint8 (what used with PIL in this case) is up to 255 (not 256). This code produces a correct result:

from PIL import Image
import matplotlib.pyplot as plt
import numpy as np

im_arr = np.array(
[[0, 0, 0, 0,  0 ,   0 ,  0 ,  0 ,  0, 0 ,0],
[0, 0, 0, 0, 255,  255, 255, 255,  0, 0 ,0],
[0, 0, 0, 0, 255,   0 ,  0 , 255,  0, 0 ,0],
[0, 0, 0, 0, 255,   0 ,  0 , 255,  0, 0 ,0],
[0, 0, 0, 0, 255,   0 ,  0 , 255,  0, 0 ,0],
[0, 0, 0, 0, 255,   0 ,  0 , 255,  0, 0 ,0],
[0, 0, 0, 0, 255,  255, 255, 255,  0, 0 ,0],
[0, 0, 0, 0,  0 ,   0 ,  0 ,  0 ,  0, 0 ,0]])


im = Image.fromarray(np.uint8(im_arr))
plt.imshow(im)
plt.show()

CodePudding user response：

Fortunately matplotlib has a special function called matshow exactly for your use case.

import numpy as np
import matplotlib.pyplot as plt

arr = np.array([[  0,   0,   0,   0,   0,   0,   0,   0,   0,   0,   0],
       [  0,   0,   0,   0, 256, 256, 256, 256,   0,   0,   0],
       [  0,   0,   0,   0, 256,   0,   0, 256,   0,   0,   0],
       [  0,   0,   0,   0, 256,   0,   0, 256,   0,   0,   0],
       [  0,   0,   0,   0, 256,   0,   0, 256,   0,   0,   0],
       [  0,   0,   0,   0, 256,   0,   0, 256,   0,   0,   0],
       [  0,   0,   0,   0, 256, 256, 256, 256,   0,   0,   0],
       [  0,   0,   0,   0,   0,   0,   0,   0,   0,   0,   0]])

plt.matshow(arr, cmap='gray')