Saving a Numpy array as an image

Posted on

Question :

Saving a Numpy array as an image

I have a matrix in the type of a Numpy array. How would I write it to disk it as an image? Any format works (png, jpeg, bmp…). One important constraint is that PIL is not present.

Asked By: M456


Answer #1:

You can use PyPNG. It’s a pure Python (no dependencies) open source PNG encoder/decoder and it supports writing NumPy arrays as images.

Answered By: dF.

Answer #2:

An answer using PIL (just in case it’s useful).

given a numpy array “A”:

from PIL import Image
im = Image.fromarray(A)"your_file.jpeg")

you can replace “jpeg” with almost any format you want. More details about the formats here

Answered By: migas

Answer #3:

This uses PIL, but maybe some might find it useful:

import scipy.misc
scipy.misc.imsave('outfile.jpg', image_array)

EDIT: The current scipy version started to normalize all images so that min(data) become black and max(data) become white. This is unwanted if the data should be exact grey levels or exact RGB channels. The solution:

import scipy.misc
scipy.misc.toimage(image_array, cmin=0.0, cmax=...).save('outfile.jpg')
Answered By: Steve Tjoa

Answer #4:

With matplotlib:

import matplotlib

matplotlib.image.imsave('name.png', array)

Works with matplotlib 1.3.1, I don’t know about lower version. From the docstring:

    A string containing a path to a filename, or a Python file-like object.
    If *format* is *None* and *fname* is a string, the output
    format is deduced from the extension of the filename.
    An MxN (luminance), MxNx3 (RGB) or MxNx4 (RGBA) array.

enter image description here

Answered By: christianbrodbeck

Answer #5:

Pure Python (2 & 3), a snippet without 3rd party dependencies.

This function writes compressed, true-color (4 bytes per pixel) RGBA PNG’s.

def write_png(buf, width, height):
    """ buf: must be bytes or a bytearray in Python3.x,
        a regular string in Python2.x.
    import zlib, struct

    # reverse the vertical line order and add null bytes at the start
    width_byte_4 = width * 4
    raw_data = b''.join(
        b'x00' + buf[span:span + width_byte_4]
        for span in range((height - 1) * width_byte_4, -1, - width_byte_4)

    def png_pack(png_tag, data):
        chunk_head = png_tag + data
        return (struct.pack("!I", len(data)) +
                chunk_head +
                struct.pack("!I", 0xFFFFFFFF & zlib.crc32(chunk_head)))

    return b''.join([
        png_pack(b'IHDR', struct.pack("!2I5B", width, height, 8, 6, 0, 0, 0)),
        png_pack(b'IDAT', zlib.compress(raw_data, 9)),
        png_pack(b'IEND', b'')])

… The data should be written directly to a file opened as binary, as in:

data = write_png(buf, 64, 64)
with open("my_image.png", 'wb') as fh:

Answered By: ideasman42

Answer #6:

There’s opencv for python (documentation here).

import cv2
import numpy as np

img = ... # Your image as a numpy array 

cv2.imwrite("filename.png", img)

useful if you need to do more processing other than saving.

Answered By: ButterDog

Answer #7:

If you have matplotlib, you can do:

import matplotlib.pyplot as plt
plt.imshow(matrix) #Needs to be in row,col order

This will save the plot (not the images itself).
enter image description here

Answered By: DopplerShift

Answer #8:

You can use ‘skimage’ library in Python


from import imsave

Leave a Reply

Your email address will not be published.