Any way to make nice antialiased round corners for images in python?

Question

Is there any way to make nice round corners with python? Currently PIL and GD2 are used in my project. Both of them have an arc() method, that allows you to draw a quater-circle, but the quater-circle is not antialiased, so the image looks crispy.

Is there any neat way to make antialiased/smooth round corners?

Would you give an example of the corners you are getting? I added more details to my answer by the way. — Nadia Alramli, Dec 02 '09 at 13:01
The same as on the yellow rectangle in you answer :) They look ugly on photos, unfortunately. Check my answer at the bottom. And thank you for your help. — DataGreed, Dec 03 '09 at 14:12

score 16 · Answer 1 · edited Dec 01 '20 at 13:44

16

What I usually do is use an image as a mask, like this one for example:

border.png

alt text

border = Image.open('border.png')
source = border.convert('RGB')
img.paste(source, mask=border)

The mask can be resized to fit the dimensions you want. Just make sure to use the Image.ANTIALIAS method.

You can refer to the tutorial I wrote a while ago for more details (shameless plug)

If you want more control over the size and the radius then you need to use arc or pieslice. I don't know about you but this rounded rectangle I created with PIL looks smooth enough to me:

alt text

Here is the code I used to draw it

Maybe you should check phatch: http://photobatch.wikidot.com/tutorials It is written in Python and PIL and can apply round corners to photos.

alt text

Here is a link to the code used to apply round corners: http://bazaar.launchpad.net/~stani/phatch/trunk/annotate/head:/phatch/actions/round.py

That also looks smooth enough to me.

edited Dec 01 '20 at 13:44

Benoit Blanchon

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answered Dec 01 '09 at 19:48

Nadia Alramli

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Yellow rectangle on a white background—not the easiest colours to see how smooth it might be. – Craig McQueen Dec 02 '09 at 00:02
Thanks for the idea with copying over round corners. Yes, i need to control size and radius and these corners do not look smooth to me, because I apply them to photos. – DataGreed Dec 02 '09 at 12:10
I added more details to the answer – Nadia Alramli Dec 02 '09 at 13:00
Wow, thanx for phatch - i'll check it out. – DataGreed Dec 03 '09 at 12:05
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Two images and the links to the tutorial and code are dead. Please can you update the answer? – Hugo Apr 04 '16 at 15:31
I replaced the dead links with snapshots from archive.org – Benoit Blanchon Dec 01 '20 at 13:46

score 5 · Answer 2 · answered Dec 01 '09 at 19:59

5

You'll have to supersample: draw the arc to a larger image and scale it down.

eg. a white, smoothly semi-transparent bottom-right 16x16 border, with 4x4x square supersampling:

>>> import Image, ImageDraw
>>> im= Image.new('RGBA', (16*4, 16*4), (255, 255, 255, 0))
>>> dr= ImageDraw.ImageDraw(im)
>>> dr.pieslice((-64, -64, 64, 64), 0, 340, fill=(255,255,255,255))
>>> im= im.resize((16, 16), Image.ANTIALIAS)
>>> im.save('border-se.png')

answered Dec 01 '09 at 19:59

bobince

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That's what i use now, but this method makes the modified image look blurry :( – DataGreed Dec 02 '09 at 10:38
I apply corners to photos and the photos should look like they used to + with smooth round corners – DataGreed Dec 02 '09 at 12:12
OK, then swap the transparency round in the above (so you're painting transparent-white (255,255,255,0) onto opaque-white (255,255,255,255). Now you've got a corner stencil. You can than get the photo and use `photoim.paste(cornerim, position, cornerim)` to put the white corner stencil on top of the bottom-right part of the photo. Flip the stencil with `transpose()` and do again for each of the corners. (If you need to make the output transparent that's a little bit more work, you'd have to make a separate complete mask image the same size as the photo then paste the photo and mask together.) – bobince Dec 02 '09 at 16:52
Nice solution, though i need transparent corners :) But thank you, anyway – DataGreed Dec 03 '09 at 14:06

DataGreed · Accepted Answer · 2009-12-08T17:29:06.270

So, okay, i found the solution.

I used the supersampling method bobince proposed before, but i found that the images become blurry. So, I would not provide the full code, but i'll explain my solution.

It may seem kind of idiotistic, but i haven't found any other way to do it without involving any other libraries.

With PIL you draw white (or whatever color you need, involving transparent ones) corners on the image. Then you copy it. You should now have two images with crispy non-antialiased white round corners.

Now you should use scale-up -> scale-down supersampling (see bobince's answer above for details) method to make one of the images smooth (i used 8x scaling).

Now you have two images - one with crispy corners and another one with smooth corners, but with blurry picture.

You have now to make corners on crispy image transparent and paste the whole image on the smooth one. I used the Image.composite(crispy_image, smooth_image, crispy_image) method in PIL library to do this.

Voila!

A possible improvement: Use a supersampled grayscale mask. There you draw a white image and then you can downscale it i.e. with a BICUBIC interpolation. This should give you round corners with grey pixels. Pasting this mask should give you smooth corners in the final image as well. — allo, Feb 22 '17 at 22:27

score 0 · Answer 4 · answered Sep 01 '12 at 07:30

Use Xiaolin Wu's circle algorithm as described in Graphics Gems II. A concise summary and example (in php) han be found here: http://create.stephan-brumme.com/antialiased-circle/

As for drawing an arc (partial circle), you must determine if a given point is within the arc's angular range before plotting it.

Any way to make nice antialiased round corners for images in python?

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