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 
http://git.gnome.org/browse/gegl/tree/operations/common/unsharp-mask.c
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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