Question

Is the Unsharp mask algorithm the same for RGB images as for grayscale?
Do I just apply it to all channels?

Or maybe I could convert RGB -> HSL and apply it, then go back to RGB?

Thanks

Answer 1

From the online GIMP documentation:

To prevent color distortion while sharpening, Decompose your image to HSV and work only on Value. Then Compose the image to HSV. Go to
Colors → Components → Decompose…. Make sure the Decompose to Layers
box is checked. Choose HSV and click OK. You will get a new grey-level
image with three layers, one for Hue, one for Saturation, and one for
Value. (Close the original image so you won’t get confused). Select
the Value layer and apply your sharpening to it. When you are done,
with that same layer selected, reverse the process. Go to Colors →
Components → Compose…. Again choose HSV and click OK. You will get
back your original image except that it will have been sharpened in
the Value component.