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Shifting RGB values of a single color channel
#2
If you have never coded, this is will be an uphill battle. Otherwise any language with some image library will do. Within Gimp you have the choice between C, Script-fu, and Python. C is powerful but hardcore, Script-fu is a bit alien for most people, Python is a popular language. But maybe you can avoid scripting:
  • I assume that in a 18x18 weaving pattern you haven't got that many colors. So you can possibly make your pattern color-indexed and edit the relevant colors in the color map.
  • Or with the conventional tools, for instance to shift up a channel by N:
    • Start the Levels tool
    • Select the color channel at the top
    • Subtract N on the right end if the input
    • Add N to the left end of the output
  •    
    You can do something equivalent with Curves by selecting one channel and setting the output curve to a straight line which is N above the identity diagonal (shift the line up/down at one end, and set the other end to make the line parallel)
This said, adding linear values to 8-bit color channels has no real meaning because these channels are gamma corrected (they don't really represent a linear change for your eyes). In Gimp 2.10 you can work in linear or gamma-corrected spaces.
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RE: Shifting RGB values of a single color channel - by Ofnuts - 12-23-2018, 09:25 AM

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