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I have a high-res scanned image that I want to edit and then reprint. The scans have a certain level of noise that I'd like to mimic in my edits so that the editing is not obvious. I'm using GIMP.

As an example, I'd like to paint over the white and yellow component on the left with a blue that matches the color and noise of the blue on the right:

original

Ideally I could do this with some kind of painting or brush tool (i.e. click and drag) so that these kinds of changes, of which I have to make many, could be done quickly. However, I couldn't find a setting or dynamic for the pencil or paintbrush or airbrush that reproduced this noise.

Another strategy I've tried is to apply a sample average blue uniformly within a selection of the part I want to paint over, and then try to apply the noise as a second step. And possibly mend the boundaries afterward as well. This is as close as I've gotten with an RGB noise followed by a gaussian blur (edited selection is on left side):

selection with noise and blur

This strategy is less appealing since I have to work with selections and two filters rather than a more flexible, quicker brush-style approach. Is there a way I can use a brush approach for this? Or another, better way than what I'm doing here?

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  • It looks like you should add noise blur and sharpen.
    – joojaa
    Feb 4, 2021 at 4:17
  • You mean stick with a selection-based approach, and do 3 steps (noise, blur, sharpen)?
    – puffer
    Feb 4, 2021 at 4:20
  • No selections just apply all over a simulation of the physics of the capture.
    – joojaa
    Feb 4, 2021 at 4:31
  • if I applied it to the entire image, wouldn't that result in the unedited portions being more noisy than the edited portions, given that the unedited portions will already have the original noise in them?
    – puffer
    Feb 4, 2021 at 5:57
  • 1
    If there's enough of the blue area visible, you could use the Clone Stamp Tool, with a nice soft edge brush so it blends in nicely.
    – Billy Kerr
    Feb 4, 2021 at 19:45

1 Answer 1

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  • Add a layer on top
  • Fill with Filters > Render > Noise > Solid noise
  • Set to Soft light blend mode
  • Reduce opacity to adjust noise level
  • Add a layer mask to disable/enable it in specific areas

enter image description here

If you want some chroma variation, you can use a similar technique but use a blurry plasma instead of solid noise.

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