[Image-SIG] PIL: Unexpected results with Image.paste on RGBA images with transparency.

[Alex Clark]

On 2012-10-29 15:05:28 +0000, Lars Ruoff said:

> Hi,
> 
> I'm new to this list, so excuse me if this is a known issue.
> I didn't find anything on it on the net so far but didn't have time to 
> search much.
> 
> I have observed what i consider a flaw in the Image.paste algorithm 
> when used with transparent images.
> 
> transparent-columns.png is an image with 3 vertical black columns with 
> alpha=0.5, 0.2 and transparent respectively.
> transparent-bars.png is an image with 2 horizontal black bars with 
> transparent and alpha=0.5 respectively.
> (images attached)
> 
> When these two images are transparently pasted one over the other, 
> using the following code:
> 
> from PIL import Image
> source = Image.open('transparent-columns.png')
> result = Image.open('transparent-bars.png')
> result.paste(source, (0, 0), source)
> result.save('transparent-columns-on-bars-pil.png')
> 
> The result is unexpectet to say the least.
> PIL seems to modify the transparency even in regions where only one of 
> the images has non-transparent values.
> Very wrong in my opinion.
> 
> Doing the same manipulation in GIMP 2.6 gives what i consider the 
> expected result.
> (attached for reference)
> 
> I guess the per-pixel transparency of the result should be result_alpha 
> = 1 - (1-alpha1)*(1-alpha2),
> where alpha1,alpha2 are the alpha channel values from the original images.


You are welcome to submit a patch to Pillow (the "friendly" PIL fork) here:

- https://github.com/python-imaging/Pillow


Speaking of that, can anyone review:

- https://github.com/python-imaging/Pillow


Also, I'm looking for a few folks willing to "stand up" and regularly 
review image code fixes. I know nothing about that code, and plan to 
stick with being the release manager only. I'll be doing a 1.7.8 
release soon (by the end of this month) then maybe Pillow 1.8 will 
contain support for Python 3.3 (and cut everything before 2.7).


> 
> Regards,
> Lars Ruoff
> 
> <image>
> <image>
> <image>
> <image>
> <image>
> <image>
> _______________________________________________
> Image-SIG maillist  -  Image-SIG at python.org
> http://mail.python.org/mailman/listinfo/image-sig


-- 
Alex Clark · https://www.gittip.com/aclark4life/