Re: [Gimp-user] selection by value threshold

[Mitti Mithai <mittimithai gmail com>]

(in a separate subthread, apologies for out of order response since I am
not subscribed)

ofnuts' suggestion is the right idea, I think that should work fine
(threshold a duplicate image). The units are normalized instead of pixel
values, it appears to work fine (the thresholding is working on the 16bit
values, easy to see with with how finely the stars erode).

I think there was a misunderstanding in the other thread with the term
"saturation".  In this context I am referring to pixels that aren't
measuring the light accuracy because they have been over-exposed (and hit
the maximum of the CCD sensor), not the usual connotation of chroma/hue
purity etc.


On Wed, Jul 12, 2017 at 6:43 AM, Carol Spears <carol spears gmail com>
wrote:

> On Wed, Jul 12, 2017 at 3:08 AM, Mitti Mithai <mittimithai gmail com>
> wrote:
>
> > Somehow I neglected to mention that I am trying to do this in gimp 2.9.
> >
> > Indeed I could do something like convert to grayscale and then select,
> > but right now I can't numerically specify the range of 16-bit values that I
> > want to be selected.
> Select By Color can be used, but I can't tell how the selection is working
> > in gimp since the threshold appears to suggest it is working on the 8-bit
> > image on screen.  I can't quite suss out if 16-bit is fully supported from
> > python-fu in 2.9...if someone is confident it is I could write this as a
> > simple plugin.
> >
> > I confess, I don't understand the differences between 16-bit and 8-bit
> images other than certainly one has a lot more information than the
> other....
>
> I do know something about gimp-2.9.  Neither script-fu nor python-fu are
> being "maintained".  An example of this is that it is no longer possible to
> convert an image from RGB to GRAY due to an extra color profile parameter
> required by GIMP.  And I read of similar problems with script-fu (although
> that might be a new notation for layer modes).
>
> On the layer modes!  That is where gimp-2.9 is crazy different than all
> other GIMP!  Color erase has performed miracles for me.  As you work
> through solving this problem of yours, take a look at these new layer
> modes.  The answer you need might be in there.
>
> carol