How to convert an RGBA image to Grayscale in python?

Question

I have array of shape (height, width, 4) , ie, RGBA format, and I want to convert this to grayscale. The original array has RGB values as 0 and the picture is rendered completely based on the alpha values over a white background, hence the traditional ways of turning this into grayscale fail (eg, cv2.cvtColor(img,cv2.COLOR_RGBA2GRAY) ).

Source image:

Answer 1

Assumptions:

• Your image is always black-n-white;

• White is defined as a transparency;

If these things are true, then you can use the Alpha channel directly :

import cv2
import matplotlib.pyplot as plt

img = cv2.imread('image.png', cv2.IMREAD_UNCHANGED)
img_gray = 255 - img[:, :, 3]

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

If you could also have colored images, then I would ask you to provide one.