Re: [Gimp-user] Despeckling photos

[Liam R E Quin <liam holoweb net>]

On Wed, 2019-12-04 at 09:32 -0700, Gary Aitken wrote:
> On 12/4/19 1:44 AM, Liam R E Quin wrote:
> > On Wed, 2019-12-04 at 03:39 +0100, Gimp_Noob wrote:
> > >
> > (2) select by colour on a white area.
> > (3) grow selection e.g. 2 pixels
> > )4_ shrink selectin by 2 pixels

> Can you explain what steps 3 and 4 acconplish?  It seems like they
> should cancel each other.

Consider a black speckle that's a square 4 pixels wide and high (for an
example).

The selection goes to the edge of the speckle, all round.

Growing the selection by 2 pixels will make the marching ants meet in
the middle of the speckle - but there are no pixels of it left outside
the selection, it'snow fully selected.

Shrinking the selection won't create holes, so the tiny speckles are
gone.

Experiment using 1 pixel instead of 2, or grow by 2, shrink by 1,
feather by 1 or 2, making sure you don't lose too much detail.

In practice i scan in greyscale, not black and white, so it also
depends on the "threshhold" paraneter in the select-by-colour tool as
to how close the the speckles the selection goes, but i use Curves
first to brighten the hilights somewhat and to darken the shadows. If
you are working with a film rather than something printed in a book,
that may not be appropriate - in that case, it helps to have the image
in Precision 16 integer of 32 floating point (from the Image/Precision
menu) so that you don't lose detail as you work.

Hope this helps,

slave