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

[Lars Ruoff]

Any other comments?

Alex,

Pillow says:
Please report any non-packaging related issues here first:

   - https://bitbucket.org/effbot/pil-2009-raclette/issues


Should i report the issue there?

Lars



On Mon, Oct 29, 2012 at 4:33 PM, Alex Clark <aclark at aclark.net> wrote:

> 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<https://github.com/python-imaging/Pillow>
>
>
> Speaking of that, can anyone review:
>
> - https://github.com/python-**imaging/Pillow<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<http://mail.python.org/mailman/listinfo/image-sig>
>>
>
>
> --
> Alex Clark · https://www.gittip.com/**aclark4life/<https://www.gittip.com/aclark4life/>
>
>
> ______________________________**_________________
> Image-SIG maillist  -  Image-SIG at python.org
> http://mail.python.org/**mailman/listinfo/image-sig<http://mail.python.org/mailman/listinfo/image-sig>
>
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