what is the error : "ValueError: assignment destination is read-only"?

Question

When I am opening a jpg file using cv2.imread() and it fails sometimes which is likely due to BGR format I used. So I switched to PLT to use RGB.

import matplotlib.pyplot as plt
import numpy as np

def rgb_to_gray(img):
        grayImage = np.zeros(img.shape)
        R = np.array(img[:, :, 0])
        G = np.array(img[:, :, 1])
        B = np.array(img[:, :, 2])

        R = (R *.299)
        G = (G *.587)
        B = (B *.114)

        Avg = (R+G+B)
        grayImage = img

        for i in range(3):
           grayImage[:,:,i] = Avg

        return grayImage       

image_file = 'C:\A.jpg';
img = plt.imread(image_file,0)
gray = rgb_to_gray(img).copy()

How ever I am getting an error when I convert the image to gray scale. : "ValueError: assignment destination is read-only" How could I change my code here to avoid it?

This code seems to work. The image_file contains an escape sequence “\A” you should use a raw string or a double \\ — Laurent LAPORTE, Mar 17 '19 at 07:05
Possible duplicate of [np arrays being immutable - "assignment destination is read-only"](https://stackoverflow.com/questions/39554660/np-arrays-being-immutable-assignment-destination-is-read-only) — Michael Veksler, Mar 17 '19 at 07:31
Sounds like `C:\A.jpg` is read-only (either permission or code level). — Dmitry Belaventsev, Mar 17 '19 at 07:32

score 0 · Answer 1 · answered Mar 17 '19 at 07:24

0

This line appears redundant and causes the error, remove it:

        grayImage = img

answered Mar 17 '19 at 07:24

Inon Peled

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score 0 · Answer 2 · answered Dec 14 '20 at 15:27

Not to sure about the PIL library but if it's based on numpy array try this (https://numpy.org/doc/stable/reference/generated/numpy.copy.html):

grayImage = img.copy()

This will create a fully copy by initializing a whole different instance rather than referencing it (you reference it by using the '=' operator)

https://pythonexamples.org/python-numpy-duplicate-copy-array/#:~:text=Following%20is%20the%20syntax%20to%20make%20a%20copy,of%20array1.%20Example%201%3A%20Copy%20Array%20using%20Numpy

what is the error : "ValueError: assignment destination is read-only"?

2 Answers2