Python – Resize rectangular image to square, keeping ratio and fill background with black

image processingnumpyopencvpythonpython-imaging-library

I'm trying to resize a batch of grayscale images that are 256 x N pixels (N varies, but is always ≤256).

My intention is to downscale the images.

The resize would have to output a square (1:1) image, with:

resized image centered vertically
aspect ratio maintained
remaining pixels rendered black

Visually this would be the desired result:

I have tried creating a numpy zeroes matrix with the target size (e.g. 200 x 200) but have not been able to paste the resized image into its vertical center.

Any suggestions using cv2, PIL or numpy are welcome.

Best Answer

You can use Pillow to accomplish that:

Code:

from PIL import Image

def make_square(im, min_size=256, fill_color=(0, 0, 0, 0)):
    x, y = im.size
    size = max(min_size, x, y)
    new_im = Image.new('RGBA', (size, size), fill_color)
    new_im.paste(im, (int((size - x) / 2), int((size - y) / 2)))
    return new_im

Test Code:

test_image = Image.open('hLarp.png')
new_image = make_square(test_image)
new_image.show()

For a white background you can do:

new_image = make_square(test_image, fill_color=(255, 255, 255, 0))

Result:

Related Solutions

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).

The proper size is oldsize*ratio.

There is of course also a library method to do this: the method Image.thumbnail.
Below is an (edited) example from the PIL documentation.

import os, sys
import Image

size = 128, 128

for infile in sys.argv[1:]:
    outfile = os.path.splitext(infile)[0] + ".thumbnail"
    if infile != outfile:
        try:
            im = Image.open(infile)
            im.thumbnail(size, Image.ANTIALIAS)
            im.save(outfile, "JPEG")
        except IOError:
            print "cannot create thumbnail for '%s'" % infile

Python – How to resize an image with OpenCV2.0 and Python2.6

If you wish to use CV2, you need to use the resize function.

For example, this will resize both axes by half:

small = cv2.resize(image, (0,0), fx=0.5, fy=0.5)

and this will resize the image to have 100 cols (width) and 50 rows (height):

resized_image = cv2.resize(image, (100, 50))

Another option is to use scipy module, by using:

small = scipy.misc.imresize(image, 0.5)

There are obviously more options you can read in the documentation of those functions (cv2.resize, scipy.misc.imresize).

Update:
According to the SciPy documentation:

imresize is deprecated in SciPy 1.0.0, and will be removed in 1.2.0.
Use skimage.transform.resize instead.

Note that if you're looking to resize by a factor, you may actually want skimage.transform.rescale.