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 brevity) 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. Yeti
