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colorsgimplevels

Colour isolation

I'm fairly new to GIMP

I have some black-and-white images in RGB mode. I want to highlight some areas in plain Red (ie, zero Blue+Green), some in plain Blue (ie, zero Red+Green), and the rest in plain Green (ie, zero Red+Blue)

I will be selecting several areas using Paths, though a simple rectangle would be fine for now

The final image will be Green, with 2+ selections of Red, and 2+ selections of Blue

I have experimented with multiple layers & multiple images, but I always get problems with the selection areas

Thanks in advance !

Solution

  • If what you want is keep the value of the R (or G or B) channel in the selection, while setting the other two components to 0:

    • Create a layer group above your image layer
    • Add a transparent layer in the group, name it "Green", bucket-fill with green
    • Add a transparent layer in the group, above "Green", name it "Red+Blue"
    • Set the group to "Multiply" mode. You should see your image in green.
    • To add red/blue, make a selection, make sure "Red+Blue" is the active layer, and bucket fill the selection with Red or Blue.

    Example

    Notes

    • With some selection tools, (path, ellipse...) you may have to use Select>Sharpen before the bucket-fill to make sure that there are no partially selected pixels
    • To see the original image when making selections, just make the group invisible.
    • You can also use two separate layers for Red and Blue (both above the Green one, in the layer group)
    • If what you want is just areas of #FF0000 and #0000FF over a #00FF00 background, then just hide the initial image layer when exporting.