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I have a grey scale image, and I would like to map it to a specific color channel (red in may case), in gimp. That is, I would like to produce an image with only red channel, and that the red and non-red parts will be determined according to the grey scale image (black- no red, white - red). Alternatively, I can have all 3 channels in my image, but the blue and green channels should be black (that is, no blue/ green light).

How can I do it?

I found this post, but it doesn't answer the question, as it affects all 3 channels.

How can I map the colors in a greyscale image to a specific color gradient?

Thanks!

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2 Answers 2

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  1. If you have a Greyscale image, click Image > Mode > RGB

  2. Choose black as the foreground colour

  3. In the Channels panel, deselect the red and alpha channel (if it exists), leaving both the green and blue selected.

  4. Do Edit > Fill with FG colour

Example

enter image description here

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  • Thanks for the answer. I now want to compose the 3 images, but they don't appear as an option in channel representation. How can I compose with them?
    – Tav
    Mar 25, 2020 at 11:17
  • @Tav doing this will not decompose the image in any way. Just enable the green and blue channels again once you have done it.
    – Billy Kerr
    Mar 25, 2020 at 11:59
  • Thanks! Sorry, I didn't explain myself well (or at all): I've applied the method you've suggested on 3 different images, one of them to get an image with only red channel, one for blue, and one for green. I now want to combine the 3 channels from the 3 different images into one image that will have one channel from each image. I've watched some tutorials on how to combine images but it's not exactly what I'm looking for.
    – Tav
    Mar 25, 2020 at 12:33
  • @Tav You can copy an image, then paste it into the channels in the same way as you can fill the channels with black. Ensure the channel you want to paste into is the only channel selected, then Paste the image you want, then hit Ctrl+H to anchor it. Repeat for the next channel and so on. Here's an example I made showing a different image pasted into each channel.
    – Billy Kerr
    Mar 25, 2020 at 15:02
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A very flexible way to do it:

For each color, create a layer group with two layers, the source image at the bottom, and above it a layer filled with the color to extract, set to Multiply mode.

Set the groups to Addition mode.

enter image description here

If you want to do color shifts, you can move the groups around. You can also edit the layers with the source images and see the result in real time (no need to redo the whole process)

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