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

[Lofton Henderson <lofton@rockynet.com>]

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.