Re: [Gimp-user] selection by value threshold

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

        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
    - 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?


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

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