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List:       xfree-render
Subject:    Re: [Render] Saturation alpha blending vs premultiplied alphas
From:       Keith Packard <keithp () suse ! com>
Date:       2000-07-13 16:48:40
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> My point was that it's misleading (to casual readers) to say that the
> OpenGL blending operators can't be used with premultiplied-alpha
> operands.

In fact, the bulk of the operators generate useful results only when used 
with premultiplied alpha images.  The essential difference between the two 
modes of operation is that "normal" compositing operations are symmetrical 
when operating on premultiplied images and assymetrical when operating on 
non-premultiplied images; alpha values need a different function than 
color values which only SRC_ALPHA_SATURATE provides.

Yes, they're all just arithemetic operations, but in applying them in the 
expected environments demonstrates their biases.

keithp@suse.com		XFree86 Core Team	SuSE, Inc.

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