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+--- Thread: Newby Question - Select by Color Tool (/Thread-Newby-Question-Select-by-Color-Tool)
Newby Question - Select by Color Tool - Figgley - 03-04-2020
Hi All,
I've just joined the forum and had a quick search but couldn't find the answer to my question.
I am using the select by color tool to map light intensities from different photos. I can select regions of different brightness and can adjust the threshold level to widen or narrow the selection, all this I'm fine with.
When I make a selection, the luminance is plotted on a histogram which shows the range of values the current selection is in. My question is; is there a way of setting the luminance values to drive the selection? I want to copy the selected regions and compare them so I want to ensure I have the same luminance values for the photos I'm comparing.
[edit]: I am using 2.10.14
Thanks in advance
Mark
RE: Newby Question - Select by Color Tool - Ofnuts - 03-04-2020
Yes, what you want is more or less a "luminosity mask":
* Make a B&W copy of the image
* Use the Threshold tool to set to white the values that are within a given range and to black everything else (the Threshold tool has two handles, so you can define a range with it).
* Use the result as a selection
In practice,
- The script linked above will create the B&W copy of the image as a "Channel" ("Raw luminosity")
- You apply the threshold tool to said channel
- Right click the channel and "Channel to selection".
A slower way is to:
- Duplicate the layer
- Color>Desaturate
- Apply the threshold tool
- Open the Channels list
- Right click any of the R/GB channels and Channel to selection".
- In the Layers list, hide/delete the B&W layer.
RE: Newby Question - Select by Color Tool - Figgley - 03-04-2020
Thanks Ofnuts. I'll give it a go
Do you know if there is a way of calibrating the luminosity of each photo I am comparing?
RE: Newby Question - Select by Color Tool - Ofnuts - 03-04-2020
Calibrating against which reference?
RE: Newby Question - Select by Color Tool - Figgley - 03-05-2020
Either getting max & min luminance values for each picture or for setting scaling them so max value is the same on each picture. Is this possible?
RE: Newby Question - Select by Color Tool - Ofnuts - 03-05-2020
(03-05-2020, 09:00 AM)Figgley Wrote: Either getting max & min luminance values for each picture or for setting scaling them so max value is the same on each picture. Is this possible?
Colors>Auto>Strech contrast or Colors>Auto>Equalize?
RE: Newby Question - Select by Color Tool - Figgley - 03-07-2020
Thanks for your help Ofnuts.