Writing binary images with pillow

Pillow is a fork of PIL.

It's a useful image manipulation tool, but it seems to have some bugs when reading or writing binary data.

This Python notebook is to serve as a guide to working around issues in pillow.



In [1]:

    
#For data manipulations
%pylab inline
from IPython.display import set_matplotlib_formats
from io import BytesIO
import numpy as np

#to compare to scipy's builtin conversions
from scipy.misc import imsave, toimage

#import pillow
from PIL import Image

set_cmap('Greys')









    



Populating the interactive namespace from numpy and matplotlib






    





<matplotlib.figure.Figure at 0x79b2b00>



In [2]:

    
checkerboard = np.mod(np.arange(49).reshape(7,7),2).astype('bool')



In [3]:

    
matshow(checkerboard)









    Out[3]:





<matplotlib.image.AxesImage at 0x7d41c50>



In [4]:

    
#whats the dtype?
checkerboard.dtype









    Out[4]:





dtype('bool')



In [5]:

    
#create a PIL image with binary mode
cb_img = Image.fromarray(checkerboard,mode='1')



In [6]:

    
#write the image
cb_img.save('cb_img.bmp')

Here's the image, admittedly its small: but you can see there's no white!



In [7]:

    
#read in the image
cb_img_read = Image.open('cb_img.bmp')

#convert to array
checkerboard_read = np.asarray(cb_img_read)

#display 
matshow(checkerboard_read)









    Out[7]:





<matplotlib.image.AxesImage at 0x838ff28>



In [8]:

    
cb_img2 = Image.fromarray((checkerboard*255).astype('uint8'),mode='L')

#write the image
cb_img2.convert('1').save('cb_img2.bmp')

Here's the new image: Now its a checkerboard!



In [23]:

    
a = array([True, False])



In [29]:

    
(a*255).dtype









    Out[29]:





dtype('int32')



In [9]:

    
#read in the image
cb_img2_read = Image.open('cb_img2.bmp')

#convert to array
checkerboard2_read = np.asarray(cb_img2_read)

#display 
matshow(checkerboard2_read)









    Out[9]:





<matplotlib.image.AxesImage at 0x86165c0>

To write a binary file you first need to convert binary data to uint8 datatype and cast to that data type as well.

Then you need to convert using the Image.convert(mode) method to binary before saving.



In [10]:

    
checkerboard2_read.dtype









    Out[10]:





dtype('bool')



In [ ]:

    
cb_img2.save()

Writing direct to zipfiles



In [11]:

    
import zipfile as zp



In [20]:

    
# this is admittedly not the most transferable
with zp.ZipFile("junk.zip","w") as zf:
    # generate an output byte buffer to save the data in, instead of a file
    output = BytesIO()
    # convert and save the image to the byte buffer with the correct format
    cb_img2.convert('1').save(output, "BMP")
    # wrtie the byte buffer to the zipfile directly
    zf.writestr("junk.bmp", output.getvalue())



In [ ]: