Color change a whole range based on a break point

[norseman-2]

Hello;
         I'm new to the Gimp forum. I took a quick look at the archives
and decided to try a more direct approach.  This forum.


Wanted:
         In Gimp, to add (SUM) the RGB values for a pixel and then
change all pixels who's RGB SUM is greater than a user stated value to
WHITE.
Second side of that is to change all RGB SUMs less than value given to
Black. BUT not doing both at same time, thus allowing user to "step" in
and see what got lost.

Anyone who has scanned old CAD/Architectural files and prepped them for
Raster to Vector conversion will know intuitively my intent and reason
for wanting this ability.


Yes - I can do this with another program but it means "sending" the file
back and forth and working "blind".  It would be nice to have it "in" GIMP.

While I can program, I have never tried using Gimp's language. The docs
I have seen on the subject are not very clear to someone new to the
language.  Nor did I find any "clearly the Gimp's verbs (functions) and
their syntax" anywhere.  Again, I scanned the 'HELP' and Gimp.org site
and then came here. At the very least someone should be able to point me
to the "real stuff" much faster than me trying to wade through HTMLs.
(Grep doesn't like them :)


Steve
norseman@...


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[Sven Neumann]

Hi,

On Tue, 2009-06-23 at 14:51 -0700, norseman wrote:

>          In Gimp, to add (SUM) the RGB values for a pixel and then
> change all pixels who's RGB SUM is greater than a user stated value to
> WHITE.
> Second side of that is to change all RGB SUMs less than value given to
> Black. BUT not doing both at same time, thus allowing user to "step" in
> and see what got lost.

Use the Levels tool. That is pretty much exactly what you achieve by
moving the Input levels in the Value channel (except that Value is not
defined as the sum but the max of the RGB values).


Sven


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[norseman-2]

Sven Neumann wrote:

> Hi,
>
> On Tue, 2009-06-23 at 14:51 -0700, norseman wrote:
>
>>          In Gimp, to add (SUM) the RGB values for a pixel and then
>> change all pixels who's RGB SUM is greater than a user stated value to
>> WHITE.
>> Second side of that is to change all RGB SUMs less than value given to
>> Black. BUT not doing both at same time, thus allowing user to "step" in
>> and see what got lost.
>
> Use the Levels tool. That is pretty much exactly what you achieve by
> moving the Input levels in the Value channel (except that Value is not
> defined as the sum but the max of the RGB values).
>
>
> Sven
>
>
>

======================
I have been trying exactly that 'macro'.  It does not work as expected.
I'm using Gimp 2.6.6 on Windows XP Pro.  Using a Twain32, Gimp makes a
great scanner input system. Most files DO clean up quite nicely with
Levels, but some need more direct control.  As it has been programmed,
Max is NOT SUM, unfortunately.  Question is how to get Gimp to do what
is needed?


Steve
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[saulgoode]

Quoting norseman <norseman@...>:

> Wanted:
>          In Gimp, to add (SUM) the RGB values for a pixel and then
> change all pixels who's RGB SUM is greater than a user stated value to
> WHITE.
> Second side of that is to change all RGB SUMs less than value given to
> Black. BUT not doing both at same time, thus allowing user to "step" in
> and see what got lost.

* Duplicate the layer.
* Desaturate the duplicate using the "Average" method.
* Add layermask to the duplicate using the
   "Initialize to: Grayscale Copy of Layer" option.
* Bucket fill the duplicate layer with white.
* Run "Colors->Threshold" on the layermask, setting
   the value appropriately.
* Bucket fill your original layer with black.



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