[Image-SIG] I'm confused by alpha blending.

[Ned Batchelder]

(I send this days ago, and was told it was subject to moderation because 
of a suspicious header).

I found the problem, thanks to the code you posted.  My destination 
image was "RGBA", and I was viewing it in a web browser.  Most of the 
image had an opaque alpha channel, but where I had drawn the drop 
shadows, it had the shadow's alpha values, so the white of the browser 
was showing through.  This isn't the most intuitive way for alpha 
drawing to work.  I guess I could see it both ways: if the destination 
is opaque, it should stay opaque, or drawing pixels onto a destination 
takes all the values from the source.

In any case, changing my destination image from RGBA to RGB solved the 
problem.

Thanks.

--Ned.

Fredrik Lundh wrote:
>>> Is there something I am doing wrong?
>>>       
>> sounds confusing, indeed.
>>
>> are you 100% sure
>>
>> what happens
>>
>> what happens
>>     
>
> did you get anywhere with this ? 
>
>
>
> _______________________________________________
> Image-SIG maillist  -  Image-SIG at python.org
> http://mail.python.org/mailman/listinfo/image-sig
>
>
>
>   

-- 
Ned Batchelder, http://nedbatchelder.com

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