Python – How to convert a PIL Image into a numpy array

imagenumpynumpy-ndarraypythonpython-imaging-library

Alright, I'm toying around with converting a PIL image object back and forth to a numpy array so I can do some faster pixel by pixel transformations than PIL's PixelAccess object would allow. I've figured out how to place the pixel information in a useful 3D numpy array by way of:

pic = Image.open("foo.jpg")
pix = numpy.array(pic.getdata()).reshape(pic.size[0], pic.size[1], 3)

But I can't seem to figure out how to load it back into the PIL object after I've done all my awesome transforms. I'm aware of the putdata() method, but can't quite seem to get it to behave.

Best Answer

You're not saying how exactly putdata() is not behaving. I'm assuming you're doing

>>> pic.putdata(a)
Traceback (most recent call last):
  File "...blablabla.../PIL/Image.py", line 1185, in putdata
    self.im.putdata(data, scale, offset)
SystemError: new style getargs format but argument is not a tuple

This is because putdata expects a sequence of tuples and you're giving it a numpy array. This

>>> data = list(tuple(pixel) for pixel in pix)
>>> pic.putdata(data)

will work but it is very slow.

As of PIL 1.1.6, the "proper" way to convert between images and numpy arrays is simply

>>> pix = numpy.array(pic)

although the resulting array is in a different format than yours (3-d array or rows/columns/rgb in this case).

Then, after you make your changes to the array, you should be able to do either pic.putdata(pix) or create a new image with Image.fromarray(pix).

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Python – How to resize an image using PIL and maintain its aspect ratio

Define a maximum size. Then, compute a resize ratio by taking min(maxwidth/width, maxheight/height).

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Below is an (edited) example from the PIL documentation.

import os, sys
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size = 128, 128

for infile in sys.argv[1:]:
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