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I have a layer that I want to merge and I want the dark colors in the layer to have more weight. How can I make it so that black pixels are 100% opacity, white pixels are 0%, and #7F7F7F pixels are 50%? I tried adjusting color curves, but that doesn't seem to be what I want.

  • Do you only want white, black and any gray to become transparent, or any color corresponding to its darkness? – Michael Schumacher Feb 24 '15 at 16:36
  • @MichaelSchumacher Any color, corresponding to its darkness. – aebabis Feb 24 '15 at 17:47
  • Ah, good. Thought my answer would be totally off when I reread you question and realized the achromatic examples. – Michael Schumacher Feb 24 '15 at 18:17
  • So, did one of the answers help to solve the task? – Michael Schumacher Feb 27 '15 at 10:02
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In short:

I'd decompose the image two times, once into RGBA and once into HSV or HSL, with the Decompose filter: http://docs.gimp.org/en/plug-in-decompose-registered.html

Then, I'd do a RGBA compose with the RGB components taken from the first, and the V or L component (inverted prior to the compose, probably) of the second decompose for the A component: http://docs.gimp.org/en/plug-in-compose.html

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  • The general idea is ok. Probably you need to simplify it just adding the same image as grayscale and use it as an alpha mask to the original colour one. – Rafael Feb 27 '15 at 17:05
  • This can work out to be the same, with a few minor differences (mostly about how you get there). My goal was to point out that there is something resembling darkness (or lightness) that can be used there - and in that regard, my answer should rather be expanded, because it just assumes that everyone knows what RGBA and HSV/HSL are - and something like the latter would also be part of a "grayscale copy to layer mask" approach, because there are different ways to convert an image or layer to greyscale. – Michael Schumacher Feb 27 '15 at 18:29
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An easy way to add transparency in Gimp is to convert a color to alpha by choosing Layer > Transparency > Color to Alpha.

In your example choosing white as the color to convert to alpha (i.e. is fully transparent) will lead to the desired result making black 100% opaque, white 0 % opaque (i.e. transparent), and giving all shades of grey a corresponding semi-transparency:

enter image description here

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