On Tue, Mar 11, 2003 at 10:14:50PM +0100, Steinar H. Gunderson wrote:
> On Tue, Mar 11, 2003 at 11:41:30AM -0800, Daniel Rogers wrote:
> > Weight the pixel value by the alpha value, just like you do with any 
> > other operation on pixels.  This makes sense when alpha is defined to be 
> > the coverage.  If a pixel is only really half covered, their should half 
> > the impulse response on the convolution kernel.
> And so, if you're blurring with some transparent area, it's equivalent to
> blurring with black? Doesn't make sense to me -- or am I missing something?

No; when you combine pixels premultiplied alpha works OK.
On separated alpha it can be written (with weigths 1/2, for

  Alpha        = (Alpha1 + Alpha2)/2
  ColorChannel = (Alpha1*ColorChannel1 + Alpha2*ColorChannel2)/2/Alpha

This is equivalent to

  Alpha         = (Alpha1 + Alpha2)/2
  PremulChannel = (PremulChannel1 + PremulChannel2)/2

on premultiplied alpha.

Blurring with black only occurs when you don't combine
pixels and just increase alpha of a pixel.


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