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

Ned Batchelder ned at nedbatchelder.com
Wed Mar 22 00:31:36 CET 2006

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 



Fredrik Lundh wrote:
>> are you 100% sure that the source pixels are black ?
> a quick way to find out is to do
>     print shadow.convert("RGB").getcolors()
> </F>
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Ned Batchelder, http://nedbatchelder.com

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