Re: [Gimp-user] selection by value threshold

[Ofnuts <ofnuts gmx com>]

On 07/12/17 02:18, Carol Spears wrote:
> On Tue, Jul 11, 2017 at 6:45 PM, Ofnuts <ofnuts gmx com 
> <mailto:ofnuts gmx com>> wrote:
>
>     On 07/10/17 08:33, Mitti Mithai wrote:
>
>         Gimp 2.9 loads by 16-bit astrophotography files either TIF or
>         FITS just
>         great.
>
>         I've never had to to this with gimp yet, but I need to make a
>         selection by
>         value threshold (I want to "cut out" stars that are hitting
>         saturation in
>         my image with values of 64k)...I can't quite seem to figure
>         out how to do
>         it.  Does anyone have any advice?
>         _______________________________________________
>
>     Quick and dirty:
>
>     - open the channels list
>     - drag any of the RGB channels down to the lsti to below to make a
>     copy
>     - treshold the copy
>     - channel to selection
>
>     Slightly more accurate:
>
>     - Create grayscale copy of layer (Color>Desaturate (lightness)
>     - Threshold to your liking
>     - While it is visible, open the channel list, right click any
>     R,G,B channel and "Channel to selection"
>
>
> These methods are selecting stars of the same saturation?
>
> carol

Yes, if the threshold values you use bracket the required star 
brightness. A variant is to use the quickmask:

- create the initial selection using the grayscale version of the image 
by converting one of the RGB channel to the selection

- start the quickmask

- you can then use the threshold tool interactively to select which 
values will be selected, and at the same time visually check which stars 
will be covered by the selection