How can I create a PNG image file from a list of pixel values in Python?

Question

I can generate a list of pixel values from an existing image file using a procedure like the following:

from PIL import Image
image = Image.open("test.png")
pixels = list(image.getdata())
width, height = image.size
pixels = [pixels[i * width:(i + 1) * width] for i in xrange(height)]

How could I convert this list of pixel values back to an image file?

Answer 1

Quick fix

First, you need to have your pixel tuples in a single un-nested list:

pixels_out = []
for row in pixels:
    for tup in row:
        pixels_out.append(tup)

Next, make a new image object, using properties of the input image, and put the data into it:

image_out = Image.new(image.mode,image.size)
image_out.putdata(pixels_out)

Finally, save it:

image_out.save('test_out.png')

Fundamental issue

Your list comprehension generates a list of lists, the latter being generated by the slicing (i*width:(i+1)*width). Your comprehension can be much easier: pixels = [pixel for pixel in pixels]. Obviously this outputs the same list, pixels, but you can use the idea to perform an operation on the pixels, e.g. pixels = [operation(pixel) for pixel in pixels].

Really, you overthought it. You don't have to manage the image dimensions. Getting the pixels in a list, and then putting them into an equal-sized image with putdata keeps the in order because they are linearized the same way by PIL.

In short, this is what your original snippet should have been:

from PIL import Image
image = Image.open("test.png")
image_out = Image.new(image.mode,image.size)

pixels = list(image.getdata())
image_out.putdata(pixels)
image_out.save('test_out.png')