• Cookbook
• Matplotlib
• LoadImage

Image processing often works on gray scale images that were stored as PNG files. How do we import / export that file into python?

• Here is a recipy to do this with Matplotlib using the imread function (your image is called lena.png).

     1 from pylab import imread, imshow, gray, mean
     2 a = imread('lena.png')
     3 #generates a RGB image, so do
     4 aa=mean(a,2) # to get a 2-D array
     5 imshow(aa)
     6 gray()
  

This permits to do some processing for further exporting such as for converting a matrix to a raster image. In the newest version of pylab (check that your pylab.matplotlib.__version__ is superior to '0.98.0') you get directly a 2D numpy array if the image is grayscale.

• to write an image, do

     1 import Image
     2 mode = 'L'
     3 size= (256, 256)
     4 imNew=Image.new(mode , size)
     5 mat = numpy.random.uniform(size = size)
     6 data = numpy.ravel(mat)
     7 data = numpy.floor(data * 256)
     8 
     9 imNew.putdata(data)
    10 imNew.save("rand.png")
  

• this kind of functions live also under scipy.misc, see for instance scipy.misc.imsave to create a color image:

     1 from scipy.misc import imsave
     2 import numpy
     3 a = numpy.zeros((4,4,3))
     4 a[0,0,:] = [128, 0 , 255]
     5 imsave('graybackground_with_a_greyish_blue_square_on_top.png',a)
  

• to define the range, use:

     1 from scipy.misc import toimage
     2 import numpy
     3 a = numpy.random.rand(25,50) #between 0. and 1.
     4 toimage(a, cmin=0., cmax=2.).save('low_contrast_snow.png')
  

  (adapted from http://telin.ugent.be/~slippens/drupal/scipy_unscaledimsave )

• there was another (more direct) method suggested by http://jehiah.cz/archive/creating-images-with-numpy

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