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Subject: Re[3]: [cgmo-webcgm] Drawing model (take 2)

Hi Lofton,

My personal guess... It's because of the Porter-Duff paper:

Compositing Digital Images, Thomas Porter, Tom Duff, Computer Graphics
Project, Lucasfilm Ltd.

Computer Graphics Volume 18, Number 3 July 1984

That paper had a lot of influence in the computer graphics industry.
In the paper, Porter and Duff, introduce the concept of
pre-multiplied RGBA quadruple (mainly to save unnecessary

Implementations that support the Porter-Duff operators often refer to
premultiplied color values.

Yes,  Esc.45 is not premultiplied.

Note, if it were up to me, I wouldn't put any examples at all (it's
too much).

 Benoit   mailto:benoit@itedo.com

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Thursday, April 20, 2006, 11:30:53 AM, you wrote:

> At 10:51 AM 4/20/2006 -0400, Benoit Bezaire wrote:
>>BTW, I forgot to mention (again), that the examples are using a 'argb'
>>notation instead of 'rgba'.

> Right, I'm fine with that.

>>Any way, it doesn't seem to be the source of your question. As I said
>>in this version of the wording "all color values use premultiplied

> Okay, I missed the implication of that in the equations.  So "all color
> values" -- does that mean that Cr,Cg,Cb are also premultiplied by Ca, 
> before plugging into the equations?  (None of the examples answer that
> question.)

> Btw, just out of curiosity...

> I have come across "premultiplied alpha" before.  Why is that a popular way
> to express the equations?  From a mathematical and intuitive perspective,
> non-premultiplied Pr,Pg,Pb is clearer, at least to me:

> Cr' = (1 - Pa)*Cr + Pa*Pr

> It looks like a linear interpolant, of the Cr and the Pr at the an 
> intermediate point defined by the alpha.  (You can probably tell that my
> graphics history does not include writing a rasterizer!).

> Finally, Esc.45 says,

> result = alpha*foreground + (1-alpha)*background

> So the expression in Esc45 is NOT pre-multiplied, correct?

> -Lofton.

>>So fully transparent red is not (0,1,0,0) it is (0,0,0,0). That should

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