How do I generate circular thumbnails with PIL?

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The easiest way to do it is by using masks. Create a black and white mask with any shape you want. And use putalpha to put that shape as an alpha layer:

from PIL import Image, ImageOps

mask = Image.open('mask.png').convert('L')
im = Image.open('image.png')

output = ImageOps.fit(im, mask.size, centering=(0.5, 0.5))
output.putalpha(mask)

output.save('output.png')

Here is the mask I used:

alt text

If you want the thumbnail size to be variable you can use ImageDraw and draw the mask:

from PIL import Image, ImageOps, ImageDraw

size = (128, 128)
mask = Image.new('L', size, 0)
draw = ImageDraw.Draw(mask) 
draw.ellipse((0, 0) + size, fill=255)

im = Image.open('image.jpg')

output = ImageOps.fit(im, mask.size, centering=(0.5, 0.5))
output.putalpha(mask)

output.save('output.png')

If you want the output in GIF then you need to use the paste function instead of putalpha:

from PIL import Image, ImageOps, ImageDraw

size = (128, 128)
mask = Image.new('L', size, 255)
draw = ImageDraw.Draw(mask)
draw.ellipse((0, 0) + size, fill=0)

im = Image.open('image.jpg')

output = ImageOps.fit(im, mask.size, centering=(0.5, 0.5))
output.paste(0, mask=mask)
output.convert('P', palette=Image.ADAPTIVE)

output.save('output.gif', transparency=0)

Note that I did the following changes:

  • The mask is now inverted. The white
    was replaced with black and vice versa.
  • I’m converting into ‘P’ with an ‘adaptive’ palette. Otherwise, PIL will only use web-safe colors and the result will look bad.
  • I’m adding transparency info to the image.

Please note: There is a big issue with this approach. If the GIF image contained black parts, all of them will become transparent as well. You can work around this by choosing another color for the transparency.
I would strongly advise you to use PNG format for this. But if you can’t then that is the best you could do.

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