将图片读取为灰度numpy数组，并保存回来

我尝试了以下操作，希望看到源图像的灰度版本：

from PIL import Image
import numpy as np
img = Image.open("img.png").convert('L')
arr = np.array(img.getdata())
field = np.resize(arr, (img.size[1], img.size[0]))
out = field
img = Image.fromarray(out, mode='L')
img.show()

但由于某种原因，整个图像几乎是由很多点组成，中间有黑色。为什么会发生这种情况？

When you are creating the numpy array using the image data from your Pillow object, be advised that the default precision of the array is int32. I'm assuming that your data is actually uint8 as most images seen in practice are this way. Therefore, you must explicitly ensure that the array is the same type as what was seen in your image. Simply put, ensure that the array is uint8 after you get the image data, so that would be the fourth line in your code¹.

arr = np.array(img.getdata(), dtype=np.uint8) # Note the dtype input

^{1. Take note that I've added two more lines in your code at the beginning to import the necessary packages for this code to work (albeit with an image offline).}