Saving a Numpy array as an image

Question

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

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The world probably doesn t need yet another package for writing a numpy array to a PNG file  but for those who can t get enough  I recently put up numpngw on github   https   github com WarrenWeckesser numpngw  and on pypi  https   pypi python org pypi numpngw   The only external dependency is numpy   Here s the first example from the examples directory of the repository   The essential line is simply  write png  example1 png   img    where img is a numpy array   All the code before that line is import statements and code to create img   import numpy as np from numpngw import write png     Example 1     Create an 8-bit RGB image   img   np zeros  80  128  3   dtype np uint8   grad   np linspace 0  255  img shape 1    img  16          127 img 16 32     0    grad img 32 48     1    grad   -1  img 48 64     2    grad img 64           127  write png  example1 png   img    Here s the PNG file that it creates

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Assuming you want a grayscale image   im   Image new  L    width  height   im putdata an array flatten   tolist    im save  image tiff

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With pygame so this should work as I tested  you have to have pygame installed if you do not have pygame install it by using pip - gt  pip install pygame  that sometimes does not work so in that case you will have to download the wheel or sth but that you can look up on google    import pygame   pygame init   win   pygame display set mode  128  128   pygame surfarray blit array win  yourarray  pygame display update   pygame image save win   yourfilename png    just remember to change display width and height according to your array here is an example  run this code  import pygame from numpy import zeros   pygame init   win   pygame display set mode  128  128   striped   zeros  128  128  3   striped       255  0  0  striped      3     0  255  255  pygame surfarray blit array win  striped  pygame display update   pygame image save win   yourfilename png

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If you have matplotlib  you can do   import matplotlib pyplot as plt plt imshow matrix   Needs to be in row col order plt savefig filename    This will save the plot  not the images itself

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matplotlib svn has a new function to save images as just an image -- no axes etc   it s a very simple function to backport too  if you don t want to install svn  copied straight from image py in matplotlib svn  removed the docstring for brevity    def imsave fname  arr  vmin None  vmax None  cmap None  format None  origin None       from matplotlib backends backend agg import FigureCanvasAgg as FigureCanvas     from matplotlib figure import Figure      fig   Figure figsize arr shape   -1   dpi 1  frameon False      canvas   FigureCanvas fig      fig figimage arr  cmap cmap  vmin vmin  vmax vmax  origin origin      fig savefig fname  dpi 1  format format

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

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Use cv2 imwrite   import cv2 assert mat shape 2     1 or mat shape 2     3   the third dim should be channel  cv2 imwrite path  mat    note the form of data should be height - width - channel

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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   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

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If you are working in python environment Spyder  then it cannot get more easier than to just right click the array in variable explorer  and then choose Show Image option     This will ask you to save image to dsik  mostly in PNG format   PIL library will not be needed in this case

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Imageio is a Python library that provides an easy interface to read and write a wide range of image data  including animated images  video  volumetric data  and scientific formats  It is cross-platform  runs on Python 2 7 and 3 4   and is easy to install   This is example for grayscale image   import numpy as np import imageio    data is numpy array with grayscale value for each pixel  data   np array  70 80 82 72 58 58 60 63 54 58 60 48 89 115 121 119      16 pixels can be converted into square of 4x4 or 2x8 or 8x2 data   data reshape  4  4   astype  uint8      save image imageio imwrite  pic jpg   data

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If you happen to use  Py Qt already  you may be interested in qimage2ndarray   Starting with version 1 4  just released   PySide is supported as well  and there will be a tiny imsave filename  array  function similar to scipy s  but using Qt instead of PIL   With 1 3  just use something like the following   qImage   array2qimage image  normalize   False    create QImage from ndarray success   qImage save filename    use Qt s image IO functions for saving PNG JPG       Another advantage of 1 4 is that it is a pure python solution  which makes this even more lightweight

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There s opencv for python  documentation here   import cv2 import numpy as np  img         Your image as a numpy array   cv2 imwrite  quot filename png quot   img   useful if you need to do more processing other than saving

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Addendum to  ideasman42 s answer   def saveAsPNG array  filename       import struct     if any  len row     len array 0   for row in array            raise ValueError   Array should have elements of equal size                                    First row becomes top row of image      flat       map flat extend  reversed array                                     Big-endian  unsigned 32-byte integer      buf   b   join  struct pack   gt I     0xffFFff  amp  i32  lt  lt 8   i32 gt  gt 24                        for i32 in flat      Rotate from ARGB to RGBA       data   write png buf  len array 0    len array       f   open filename   wb       f write data      f close     So you can do   saveAsPNG   0xffFF0000  0xffFFFF00               0xff00aa77  0xff333333     test grid png     Producing test grid png      Transparency also works  by reducing the high byte from 0xff

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scipy misc gives deprecation warning about imsave function and suggests usage of imageio instead   import imageio imageio imwrite  image name png   img

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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

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An answer using PIL  just in case it s useful    given a numpy array  A    from PIL import Image im   Image fromarray A  im save  your file jpeg     you can replace  jpeg  with almost any format you want  More details about the formats here

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Pure Python  2  amp  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  amp  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

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For those looking for a direct fully working example   from PIL import Image import numpy  w h   200 100 img   numpy zeros  h w 3  dtype numpy uint8    has to be unsigned bytes  img       0 0 255    fill blue  x y   40 20 img y y 30  x x 50     255 0 0    50x30 red box  Image fromarray img  convert  RGB   save  art png     don t need to convert   also  if you want high quality jpeg s  save file  subsampling 0  quality 100

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You can use  skimage  library in Python  Example   from skimage io import imsave imsave  Path to your folder File name jpg  your array

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for saving a numpy array as image  U have several choices   1  best of other  OpenCV    import cv2     cv2 imwrite  file name with extension like  jpg    numpy array     2  Matplotlib     from matplotlib import pyplot as plt   plt imsave  file name with extension like  jpg    numpy array     3  PIL     from PIL import Image   image   Image fromarray numpy array    image save  file name with extension like  jpg       4

[python] Saving a Numpy array as an image

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