Saving list as an image in python

Question

I am trying to save a 2D list as an image in python (greyscale image) so 0 values in the array would be black and 255 would be white. For example:

255 255 255
255  0  255
255  0  255
255  0  255
255 255 255

Would save an l like shape. I have tried the following code utilising the PIL library as suggested by other questions on stack overflow:

WIDTH, HEIGHT = img.size
imgData = list(img.getdata()) 
imgData = [imgData[offset:offset + WIDTH] for offset in range(0, WIDTH * HEIGHT, WIDTH)]
#to print the image 
for row in data:
    print(' '.join('{:3}'.format(value) for value in row))
imgData = np.array(imgData)
**IMG VALUES AUGMENTED HERE**
newimg = Image.new('L', (WIDTH, HEIGHT), 'white')
newimg.putdata(imgData)
newimg.save('C:/File/Name.png')

However the image this creates does not reflect the list at all. If I was to have the 0s and 255s in different positions the same image is created. Anyone know a solution?

What's the exact structure of imgData? is it [[255, 255, 255], [255, 0, 255], ..., or [255, 255, 255, 255, 0, 255, ...]`? — rassar
– rassar, Commented Mar 28, 2020 at 20:20
So for the code I am using it is a much bigger list around 40 x 60 and I am using the code: WIDTH, HEIGHT = img.size imgData = list(img.getdata()) imgData = [imgData[offset:offset + WIDTH] for offset in range(0, WIDTH * HEIGHT, WIDTH)] Does that help? — Jack Timber
– Jack Timber, Commented Mar 28, 2020 at 20:23
Can you edit that code into the question? And put the rest of the for loop? — rassar
– rassar, Commented Mar 28, 2020 at 20:24
Sorry had issues formatting the reply there. You're right, would be better to add this to the main question — Jack Timber
– Jack Timber, Commented Mar 28, 2020 at 20:27

Mark Setchell · Accepted Answer · 2020-03-30 19:44:10Z

1

As your example is lacking any input data, I have just typed it in as you describe, made the image and then enlarged it. I also artificially added a red border so you can see the extent of it on StackOverflow's white background:

#!/usr/bin/env python3

from PIL import Image
import numpy as np

pixels = [[255,255,255],
          [255,0,255],
          [255,0,255],
          [255,0,255],
          [255,255,255]]

# Make list of pixels into Image
im = Image.fromarray(np.array(pixels,dtype=np.uint8))
im.save('result.png')

answered Mar 30, 2020 at 19:44

Mark Setchell

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Comments

rassar · Accepted Answer · 2020-03-28 22:51:24Z

0

Instead of:

newimg.putdata(imgData)

you need the line:

newimg.putdata([j[0] for i in imgData for j in i])

The grayscale data is specified in a 1d list, not a 2d list.

This creates the list:

>>> [j[0] for i in imgData for j in i]
[255, 255, 255, 255, 0, 255, 255, 0, 255, 255, 0, 255, 255, 255, 255]

Which is:

[255, 255, 255,
 255,  0 , 255, 
 255,  0 , 255, 
 255,  0 , 255, 
 255, 255, 255]

EDIT The above solution works if you edit imgData with imgData[0][0] = [0, 0, 0, 255]. If you're editing imgData with imgData[0][0] = 0, then you'll need the line to be:

[j[0] if hasattr(j, '__iter__') else j for i in imgData for j in i]

or, you can make it nicer with:

imgData = np.array([[j[0] for j in i] for i in imgData])
imgData[0][0] = 0
newimg.putdata(imgData.flatten())

edited Mar 28, 2020 at 22:51

answered Mar 28, 2020 at 20:54

rassar

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3 Comments

Jack Timber Over a year ago

thanks for the suggestion although I am getting the error "IndexError: invalid index to scalar variable." for that new line. Any ideas on how to resolve that?

Jack Timber Over a year ago

I'm still encountering an error with 'imgData = np.array([[j[0] for j in i] for i in imgData])' line. Now the error is ''int object' is not subscriptable' Just to make sure I'm piecing the code together properly should it be in this order: gyazo.com/19eb248e7f68fb85cb36a5f6ebc4cbc3

rassar Over a year ago

@JackTimber Interesting, that code works on my system. It must be that our img.getdata outputs are different. How are you loading the image?

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