# Re: [Gimp-user] Select by color

```On Sunday 06 November 2005 05:27 pm, Bruno Silva de Oliveira wrote:
> Well, that was exactly what I tought...   if the threshold is zero,
> only the selected colour will be selected.
> But what about a RGB with R=50, G=110 and B=220, and a threshold of
> 10? The colour range will be something like R=40-60, G=100-120 and
> B=210-230 ?
> I'm trying to implement an algorithm that does almost the same of
> the select-by-colour. I'm processing images taken from a microscope
> (I'm not sure about how do you spell this!) where are two kinds of
> structures, one painted of yellow and the other of red. I need to
> make a code for measuring the percentage of yellow, red and the
> space unpainted in the picture...
> I've already tried the steps I told you above, without success...
>
> Do you (or anybody!) have some idea?
>
> Thanks a lot!```
```

Hi Bruno - I do not remeber right now, but I even docuemtend it on a
GIMP material I wrote once. It is somethign pretty obvious, and I
remeber at the time asking on #GIMP if the algotithim hada  name - it
has - I will check for it as son as I can and forward to you (and to
the list)

JS
-><-
>
> []'s
>
> Bruno.
>
> On 11/6/05, michael chang <[EMAIL PROTECTED]> wrote:
> > On 11/2/05, Bruno Silva de Oliveira <[EMAIL PROTECTED]> wrote:
> > > Hi everyone!
> > >
> > > I'm new in the list, but I think a can ask some things...
> > >
> > > Does anybody knows how the select by color, or more precisely,
> > > the threshold works. How does the GIMP makes the selection
> > > using the threshold interval and how does this threshlold works
> > > in the tree channels (R,G and B)?
> >
> > I'm not sure I _fully_ understand it, but I believe I get the
> > gist of the function.  When you select-by-colour, the colour
> > under the pixel is selected, and all the pixels of the same
> > colour are selected.  If the threshold is zero (or something like
> > that), then only the pixels with the exact same RGB values are
> > selected, and as you increase the threshold, it will get further
> > away from that colour.
> >
> > For example, if you had a gradient that went from black to green,
> > and you clicked on the greenest part of the gradient with select
> > by colour, then notice how the amount you select increases as you
> > increase the threshold.
> >
> > I don't know about it from a mathimatical perspective.
> >
> > If you use windows, have you considered looking at the
> > gimpwin-users mailing list?
> >
> > --
> > ~Mike
> >  - Just my two cents
> >  - No man is an island, and no man is unable.
>
> --
> /**
>  *  Bruno Silva de Oliveira
>  *  http://bsoliveira.cjb.net/
>  */
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