How to convert list of pixel into image using python

Question

I want to change specific pixel value in whole image.I have converted image into List using img.getdata() function.now after processing i want to convert that list into image format.please suggest me if you know any way to do so.

import cv2
import numpy as np

from PIL import Image
img = Image.open('test.jpg','r')
pix=list(img.getdata())
for i in pix:
    if i ==(254,0,0):
        print("found",i)
        pi=np.array(pix)
        pi=Image.fromarray(pi)
        cv2.imshow("img",pi)

Error is


Traceback (most recent call last):
  File "C:\Python38\pixels.py", line 19, in <module>
    cv2.imshow("img",pi)
TypeError: Expected Ptr<cv::UMat> for argument 'mat'

i have tried some other way also but not able to see image.

Why are you mixing PIL and OpenCV, you will confuse yourself! Why are you treating an image as a list and processing pixels in a for loop? That will be horribly slow and hard to debug - look at some other PIL/OpenCV questions on Stack Overflow and start thinking about using Numpy or OpenCV vectorised operations. — Mark Setchell
– Mark Setchell, Commented Feb 19, 2020 at 11:34
Also, remember JPEG is lossy so you are probably going wrong if you are looking for a specific RGB(254,0,0) value, because JPEG can and WILL change your values when you write to a file. If your values are that specific you need to use PNG or another lossless format. — Mark Setchell
– Mark Setchell, Commented Feb 19, 2020 at 11:36
Try replacing your last line with pi.show() - you can't display a PIL Image with OpenCV which expects a Numpy array. — Mark Setchell
– Mark Setchell, Commented Feb 19, 2020 at 11:37
What do you actually want to do in plain language, no code? E.g. "I want to change all pixels with RGB(254,0,0) to red and display the result" — Mark Setchell
– Mark Setchell, Commented Feb 19, 2020 at 11:42
with PIL you can compare full image using img1 == img2 and if they have channels in different order then they will be different. If you convert to numpy array then you can compare regions (img[y1:y2,x1:x2] == img[y3:y4,x3:x4]).all(axis=2).all() — furas
– furas, Commented Feb 19, 2020 at 12:21

Mark Setchell · Accepted Answer · 2020-02-19 12:31:21Z

2

This should help:

#!/usr/bin/env python3

import numpy as np
import cv2

# Open an image
im = cv2.imread('image.png')

# Count RGB(254,0 ,0) pixels
sought = [254,0,0]
total  = np.count_nonzero(np.all(im==sought,axis=2))
print(f'Total before: {total}')

# Draw a rectangle 10x10 that colour
im[0:10,0:10] = sought

# Count pixels that colour
total  = np.count_nonzero(np.all(im==sought,axis=2))
print(f'Total after: {total}')

# Save
cv2.imwrite('result.png',im)

Sample Output

Total before: 0
Total after: 100

answered Feb 19, 2020 at 12:31

Mark Setchell

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2 Comments

Swapnil Over a year ago

thank you it work.but you did not get my question.i want to compare two images.which have sequence of color

Mark Setchell Over a year ago

You need to do some work on your question so it is clear what you are trying to do. Your code shows you want to know if there is a pixel with RGB(254,0,0) in your image, so if you count the number of pixels with RGB(254,0,0) and it is greater than zero, you will have your answer, surely? If not, please clarify your question and show your images and what you are looking for.

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