Steinar H. Gunderson wrote:
On Tue, Mar 11, 2003 at 10:15:51AM -0800, Daniel Rogers wrote:

Alpha is a measure of the amount of coverage of the pixel. (e.g. an alpha of .5 means half the pixel is covered). In particular, 0 alpha means that the pixel is not covered at all. This means that the pixel contributes NO color information. I think this should hold for blur as well. And from that point of view, no pixel with alpha zero should ever contribute color information.

How do you propose this being implemented, ie. what value would you plug into
the IIR filter GIMP's blur is based on, for a pixel with alpha != 255?

/* Steinar */

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.

-- Dan

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