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
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Alex Clark ยท https://www.gittip.com/aclark4life/
_______________________________________________
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