In Python, how to convert array of bits to array of bytes?

Question

Python PIL library has Image.getdata() method, which return raw image data, writing:

list(im.convert("1").getdata())

will return an array of bits. Each bit corresponds to one image pixel - pixel can only be black or white because of "1" mode.

I would like to have an array which size is 8 times smaller. Each array element contains one byte.

My questions are:

1. Can I get such an array of bytes directly from PIL?
2. If not, how to convert array of bits returned by PIL to smaller array of bytes?

przemoc · Accepted Answer · 2013-12-12 15:01:28Z

3

I don't know a thing about PIL and I haven't used Python for some time, but here is my take at this bits to bytes conversion problem.

import itertools
# assuming that `bits` is your array of bits (0 or 1)
# ordered from LSB to MSB in consecutive bytes they represent
# e.g. bits = [0,0,0,0,0,0,0,1,1,1,1,1,1,1,1,1] will give you bytes = [128,255]
bytes = [sum([byte[b] << b for b in range(0,8)])
            for byte in zip(*(iter(bits),) * 8)
        ]
# warning! if len(bits) % 8 != 0, then these last bits will be lost

This code should be mostly self-explanatory.

edited Dec 12, 2013 at 15:01

answered Dec 12, 2013 at 14:18

przemoc

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2 Comments

przemoc Over a year ago

In case of bits array that uses values different than 0 and 1, fix byte[b] accordingly, e.g. use [0,1][byte[b]>0] instead.

przemoc Over a year ago

This is what you get, when you become dormant language user... The whole zip(*[itertools.islice(bits, i, None, 8) for i in range(0,8)]) can be obviously simplified into zip(*(iter(bits),) * 8). I'll fix it in the answer too.

James Mills · Accepted Answer · 2013-12-12 10:50:43Z

0

PIL's ImagingCore can easily be converted to bytes:

from PIL import Image


img = Image.open("python.jpg", "r")
data = img.convert("1").getdata()
bytes = bytearray(data)

This gives you a bytearray() list of bytes which can then be manipulated or written to a file. You could write this to a file by doing:

with open("filename", "w") as f:
    f.write(b"".join(map(bytes, data)))

I hope this helps!

It's probably worth noting too that Python in fact does not have a data type for bits. And that list(data) returns a list of int(s) and not "bits" as you thought?

Example:

>>> list(data)[:10]
[255, 255, 255, 255, 255, 255, 255, 255, 255, 255]
>>> set(list(data))
set([0, 255])
>>> set(map(type, list(data)))
set([<type 'int'>])

So in reality you are not saving any data by doing this.

See: http://docs.python.org/2/library/stdtypes.html for Python data types and operations. You can perform "bit" operations, but there is no "bit" data type.

edited Dec 12, 2013 at 10:50

answered Dec 12, 2013 at 10:43

James Mills

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8 Comments

mnowotka Over a year ago

No it doesn't help at all. You just didn't understand the question. Note: len(list(img.convert("1").getdata())) == len(bytearray(img.convert("1").getdata()))

James Mills Over a year ago

I beg your pardon? That expression evaluates to True. What exactly is your point?

mnowotka Over a year ago

Exactly - that expression evaluates to True. That's the problem. I want to get an array such as len(theArray) = len(bytearray(img.convert("1").getdata())) / 8. I want an array of bytes - what getdata gives me is an array of bits. They can be packed.

James Mills Over a year ago

I'm sorry I do not see that. You get an array (more precisely: a list of ints) not "bits".

James Mills Over a year ago

A list of int(s) is not an array of "bits".

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