selection by value threshold

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selection by value threshold

Mitti Mithai
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?
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Re: selection by value threshold

Carol Spears-2
On Mon, Jul 10, 2017 at 2:33 AM, Mitti Mithai <[hidden email]> wrote:

>
> 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?
>

Image -->Colors-->Components-->Decompose has options to decompose by HSV
and HSL.  Perhaps the Saturation layer would be helpful?

If you have an example image, after decomposing, you could select by the
color of gray that your saturation would be.

Interesting problem....

carol
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Re: selection by value threshold

Ofnuts-2
In reply to this post by Mitti Mithai
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"

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Re: selection by value threshold

Carol Spears-2
On Tue, Jul 11, 2017 at 6:45 PM, Ofnuts <[hidden email]> 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
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Re: selection by value threshold

Mitti Mithai
In reply to this post by Carol Spears-2
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.

On Tue, Jul 11, 2017 at 3:10 PM, Carol Spears <[hidden email]>
wrote:

>
>
> On Mon, Jul 10, 2017 at 2:33 AM, Mitti Mithai <[hidden email]>
> wrote:
>
>>
>> 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?
>>
>
> Image -->Colors-->Components-->Decompose has options to decompose by HSV
> and HSL.  Perhaps the Saturation layer would be helpful?
>
> If you have an example image, after decomposing, you could select by the
> color of gray that your saturation would be.
>
> Interesting problem....
>
> carol
>
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Re: selection by value threshold

Ofnuts-2
In reply to this post by Carol Spears-2
On 07/12/17 02:18, Carol Spears wrote:

>
>
> On Tue, Jul 11, 2017 at 6:45 PM, Ofnuts <[hidden email]
> <mailto:[hidden email]>> 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




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Re: selection by value threshold

Carol Spears-2
In reply to this post by Mitti Mithai
On Wed, Jul 12, 2017 at 3:08 AM, Mitti Mithai <[hidden email]> 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
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Re: selection by value threshold

Mitti Mithai
(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 <[hidden email]>
wrote:

>
>
> On Wed, Jul 12, 2017 at 3:08 AM, Mitti Mithai <[hidden email]>
> 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
>
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