oh, I made a mistake. Thanks a lot!

In your mail:
>From: "William Skaggs" <[EMAIL PROTECTED]>
>Reply-To: <[EMAIL PROTECTED]>
>To: <[EMAIL PROTECTED]>,
   bear <[EMAIL PROTECTED]>
>Subject: Re: [Gimp-developer] how layers actually get merged?
>
>
> Bear wrote:
> > Assume there are four layers, all of which are in Normal mode and grayscale.
> > for a specific pixel in each layer,
> > Their intensity and alpha values(mapped to [0,1]) are
> > i1,a1
> > i2,a2
> > i3,a3
> > i4(background)
> >
> > the merged intensity is ((i1*a1+i2*(1-a1))*a2+i3*(1-a2))*a3+i4*(1-a3)
> > am I correct? Thanks! 
> 
> I hope not.  The proper expression should be
> 
> a1*i1 + (1-a1) * (a2*i2 + (1-a2) * (a3*i3 + (1-a3) * i4)).
> 
> That is, you have the expression reversed.  With your expression, the
> background could show through even with opaque layers above it.
> 
>


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