On Wed, Nov 24, 2010 at 4:25 PM, Bill Skaggs <weska...@gmail.com> wrote:
> Maybe it would help to have a general explanation of the principle behind
> the filter.
> The basic idea of the Unsharp Mask is to enhance the difference between the
> original image
> and a blurred version of it.  The algorithm first blurs the image, then
> calculates the difference
> between the original image and the blurred version, and then alters the
> original image by
> moving each pixel farther away from its blurred value.
> The convolution is simply a way of blurring the image.  There are countless
> ways of
> doing a blur -- the filter is using a rather crude approximation of a
> Gaussian blur, which
> is the most commonly used blurring algorithm.

In GEGL unsharp mask is implemented at a higher level of abstraction
than low-level filters and reuses gaussian blur directly, thus speed
improvements to gaussian blur in GEGL will also benefit unsharp mask
directly. See 
this version of unsharp mask can already be used in GIMP through the
GEGL-tool, it would probably be good if the unsharp-mask in GIMP was
properly replaced with the GEGL one.

/Øyvind Kolås
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