I have no problem to leave the examples out, now
that we all know what was meant.
Thanks Benoit, good stuff!
Dieter
> -----Original Message-----
> From: Benoit Bezaire [mailto:]
> Sent: Thursday, April 20, 2006 8:33 PM
> To:
> 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
> mulitiplications).
>
> 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:
>
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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
> >>alpha".
>
> > 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
> >>work.
>
>
>
>
