不使用反斜杠时，cv2 imread 不返回任何值

Question

Hi I want to convert an image into grayscale based on alpha values but cv2 imread function returns following error:

TypeError: 'NoneType' object is not subscriptable

import cv2
import matplotlib.pyplot as plt

img = cv2.imread('C:/Users/sgunes/Downloads/Cyrillic/Ж/5a793d87b7ef9.png', cv2.IMREAD_UNCHANGED)
img_gray = 255 - img[:, :, 3]

plt.imshow(img_gray, cmap='gray', vmin=0, vmax=255)
plt.show()

And when I debug it, my img objects gets None. How do I fix it? Thanks for your attention.

Answer 1

According to OpenCV documentation :

The function imread loads an image from the specified file and returns it. If the image cannot be read (because of missing file, improper permissions, unsupported or invalid format), the function returns an empty matrix ( Mat::data==NULL ).

So that's mean that either you entered a wrong path, or that you don't have permissions, or that the image is not readable.

From your title, maybe it is a wrong path issue. If you want to use backslash, you can add an 'r' before your string (mean raw), or escaping each backslash (so '\\' become '\\\\'). Note that if you use raw string (r''), you still can't finish your string by a backslash.

img = cv2.imread(r'C:\Users\sgunes\Downloads\Cyrillic\Ж\5a793d87b7ef9.png', cv2.IMREAD_UNCHANGED)

or

img = cv2.imread('C:\\\\Users\\\\sgunes\\\\Downloads\\\\Cyrillic\\\\Ж\\\\5a793d87b7ef9.png', cv2.IMREAD_UNCHANGED)