Newby Question - Select by Color Tool

[Figgley]

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

[Ofnuts]

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.
  
  
  
  
  

[Figgley]

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?

[Ofnuts]

Calibrating against which reference?

[Figgley]

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?

[Ofnuts]

(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?

[Figgley]

Thanks for your help Ofnuts.