Saving a Numpy array as an image

Using PIL, save a NumPy array arr by doing:

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

See the docs for available data formats, including JPEG, PNG, and so on.

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')

With matplotlib:

import matplotlib.image

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

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

Arguments:
  *fname*:
    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.
  *arr*:
    An MxN (luminance), MxNx3 (RGB) or MxNx4 (RGBA) array.

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([
        b'\x89PNG\r\n\x1a\n',
        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:
    fh.write(data)

• Original source
• See also: Rust Port from this question.
• Example usage thanks to @Evgeni Sergeev: https://stackoverflow.com/a/21034111/432509